The Effects of High-Power Microwaves on Comminution and Downstream Processing

THE EFFECTS OF HIGH-POWER MICROWAVES ON COMMINUTION AND DOWNSTREAM PROCESSING

Adam Edward Olmsted

A thesis submitted to the Department of Mining Engineering

In conformity with the requirements for

the degree of Master of Applied Science

Queen’s University

Kingston, Ontario, Canada

(August, 2021)

The incentive for this research was to assess the potential of microwave treatment to fracture ores and improve surface area to reduce comminution energy consumption and improve downstream recovery.

Pilot-scale microwave treatment was performed on two ores: a gold ore and a copper-nickel sulphide ore.

Three microwave tests were done for each ore: batch tests at low-power (BB) and high-power (BP), and a continuous belt test at high-power (CP). Treatment variables investigated were heating time, microwave power and particle size. Treated ore was then used to assess the impacts on comminution (ore competency and liberation). Additionally, impacts on leaching (gold ore) and roasting (sulphide ore) were studied.

Surface area measurements showed improvements for each gold ore treatment; between a 2.5% and 21% increase in m2/g. The sulphide ore reported marginal increases to surface area, although the CP test showed a 7% improvement. While the differential heating improved surface area, comminution energy consumption was unchanged apart from the CP test, which reported a 19% decrease in SAG work index, WSDT. The treatments did not weaken the ore enough to reduce the energy consumption, but still promoted grinding that enhanced surface area. Liberation analysis confirmed this, showing increases to value sulphide liberation, particularly for the high-power tests.

Cyanidation showed that enhanced surface area improved the gold recovery. Improvements to gold recovery were proportional to the surface area increases reported. After 6 hours, a 26% increase in gold recovery was reported for the BP test; a 16% increase was reported for the CP test. This confirmed that continuous high-power microwave treatment can improve gold recovery by creating rapid thermal stresses.

No significant trend was found between fracture and cyanide consumption.

Roasting of the sulphide ore showed no trend with surface area. A lower sulphur content after microwave heating occurred due to oxidation of the sample, prior to entering the roaster. This degree of oxidation from the treatment was proportional to higher heating rates, and showed that the roast of the BP sample was adversely affected. For an industry process with shorter residence times, oxidation from microwave treatment would be less impactful.

Acknowledgements

The work carried out in this thesis study was very challenging and demanding. I would like to thank my supervisors Dr. Chris Pickles and Dr. Boyd Davis, who provided necessary guidance and expertise to assist in this work. I am thankful for them always setting aside time to meet and discuss my research with me, allowing me to learn from them and always having constant reassurance as to the direction of the project. They provided me with an incredible opportunity to work on this project with them, pushing myself to expand my research and critical thinking skills. For this I am forever grateful.

I would like to give a special thank you to my parents for their continuous love and support. Thank you to my father, Paul, who provided endless words of encouragement and support. I also want to acknowledge my incredible mother, Sandra, whose love and support knows no bounds. Thank you to my brother William for always being there. I would also like to declare my appreciation for my grandfather,

Charlie, who is a big inspiration and role model for me.

Next, I would like to thank Dr. Erin Bobicki and the research team at the University of Toronto for so graciously welcoming me into the team, allowing me to spend a summer term in the Department of

Materials Science working on the CanMicro project. Dr. Darryel Boucher is thanked for countless hours of advice, discussion, and guidance throughout this thesis project. I would like to also give special thanks to my colleague, roommate and friend John Forster, who spent the last year side-by-side with me in the lab.

I am very thankful to have shared this experience with him and to have learned from him throughout our work. Additional thanks are extended to the entire CanMicro project team who I had the pleasure of working with over the last two years. Thank you to my lab mates Wendy Tian, Izzat Redza, Spencer

Gulbrandsen, and Byron Liang who I spent considerable time with, in the laboratory.

I wanted to also give thanks to the entire team at Sepro Mineral Systems and Sepro Laboratories, who provided the lab space for the CanMicro project, where I spent the entirety of the 2020 school year collecting data. Thank you to Andrew Gillis and Danny Kwok for the constant support throughout the project campaign. Larry Ratchev, Jonathan Tan and Aaron Bazzana are also thanked for their support and

ii advice both in and out of the laboratory. Special thanks to Scott Burgess and Mike McClarty for endless entertainment and warm conversation throughout the year.

Thank you to all my fellow mining graduate students whom I shared office and lab space with.

Thank you to Wanda Badger, Kate Cowperthwaite and Tina McKenna for all their help throughout my time as a graduate student. Additional thanks to Dr. Sadan Kelebek for everything I learned throughout his classes and discussion.

Kingston Process Metallurgy is thanked for generously helping with the roasting work in this thesis.

Thank you to Ron Hutcheon of Microwave Properties North for consistent chats and insight on the data acquired. Thank you to James Wei of Sepro Laboratories for assisting in the completion of the leaching work in this thesis. Elizabeth Whiteman and Mike Khouri are thanked for all the ICP, XRD, and

QEMSCAN analysis completed at XPS, Glencore.

NRCan and the Natural Sciences and Engineering Research Council of Canada are thanked for the providing the funding for this research and for providing the opportunity to compete in the Crush It!

Challenge. Thank you to Queen’s University for providing me with this opportunity to be a graduate student in the Department of Mining and complete this thesis.

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Statement of Originality I hereby certify that all of the work within this thesis is the original work of the author. Any published (or unpublished) ideas and/or techniques from the work of others are fully acknowledged in accordance with the standard referencing practices.

(Adam Olmsted)

(August, 2021)

Table of Contents Abstract ...... i Acknowledgements ...... ii Statement of Originality ...... iv List of Figures ...... viii List of Tables ...... xi List of Abbreviations ...... xiii Chapter 1 ...... 1 Introduction ...... 1 1.1 General Overview ...... 1 1.2 Project Motivation ...... 1 1.3 Research Scope and Objectives of Experimental Work...... 3 1.4 Organization of Thesis ...... 4 Chapter 2 ...... 5 Literature Review ...... 5 2.1 Overview ...... 5 2.2 Mineral Processing ...... 5 2.2.1 Comminution ...... 5 2.3 Downstream Processes ...... 8 2.3.1 Leaching ...... 9 2.3.2 Roasting ...... 10 2.4 Industry Needs ...... 11 Chapter 3 ...... 13 Microwave Heating & Microwave Treatment of Ore ...... 13 3.1 Introduction ...... 13 3.2 Microwave Fundamentals ...... 13 3.3 Microwave Absorption in Materials ...... 15 3.4 Characterization of Microwave Response of Minerals ...... 17 3.4.1 Heating Characteristics ...... 17 3.4.2 Dielectric Properties of Minerals ...... 19 3.5 Microwave Systems ...... 22 3.6 Microwave-Assisted Comminution ...... 23 3.6.1 Microwave Fracturing ...... 23 3.6.2 Reduction in Comminution Energy ...... 25

3.6.3 Liberation ...... 27 3.6.4 Additional Variables ...... 28 3.6.5 Current Potential for the Mining Industry ...... 30 3.7 Effects on Downstream Processing ...... 33 Chapter 4 ...... 39 Experimental ...... 39 4.1 Materials ...... 39 4.1.1 Gold Ore ...... 39 4.1.2 Copper-Nickel Sulphide Ore ...... 40 4.2 Ore Sample Preparation ...... 41 4.2.1 Size Reduction ...... 41 4.2.2 Particle Size Classification and Sample Splitting ...... 42 4.3 Characterization Methods ...... 42 4.3.1 Inductively Coupled Plasma Optical Emission Spectroscopy (ICP) ...... 42 4.3.2 X-ray Diffraction Analysis (XRD) ...... 42 4.3.3 X-ray Fluorescence (XRF) ...... 43 4.3.4 QEMSCAN ...... 43 4.3.5 TGA/DTA ...... 43 4.3.6 Permittivity ...... 43 4.4 Microwave Treatment Systems & Procedures ...... 43 4.4.1 Batch Low-Power Microwave System ...... 44 4.4.2 Pilot-Scale Microwave System ...... 44 4.4.3 Summary of Microwave Treatment Tests ...... 45 4.5 Comminution Assessment...... 46 4.6 Surface Area Analysis ...... 46 4.7 Downstream Test Work ...... 47 4.7.1 Cyanide Leaching ...... 47 4.7.2 Sulphide Oxidative Roasting ...... 48 Chapter 5 ...... 49 Results and Discussion ...... 49 5.1 TGA and Permittivity ...... 49 5.2 Influence of Microwave Treatment on Selected Ores ...... 53 5.2.1 Ore Response to Microwave Treatment ...... 53 5.2.2 Effect of Process Parameters on Microwave Response ...... 60 5.3 Effects on Select Unit Operations ...... 63 5.3.1 Surface Area Analysis ...... 64

5.3.2 Comminution Results ...... 67 5.3.3 Cyanide Leaching (Gold Ore) ...... 72 5.3.4 Roasting Tests (Sulphide Ore) ...... 78 Chapter 6 ...... 82 Conclusions and Recommendations ...... 82 6.1 Impact of Microwaves on Ores and Processing ...... 82 6.2 Impact of Microwave Treatment on Leaching ...... 83 6.3 Impact of Microwave Treatment on Roasting ...... 84 6.4 Recommendations ...... 85 References ...... 87 Appendix A ...... 96 Microwave Fundamentals ...... 96 Electromagnetic Spectrum ...... 96 Dielectric Heating ...... 97 Appendix B ...... 105 Microwave Heating Behaviour of Minerals ...... 105 Appendix C ...... 107 Microwave Systems ...... 107 Appendix D ...... 111 Assays and XRD Plots ...... 111 Appendix E ...... 115 Experimental Microwave Equipment Setup ...... 115 Appendix F...... 118 Batch Microwave Treatment Results ...... 118 Low-Power Bench-Scale Microwave Tests ...... 118 Microwave Heating Behaviour ...... 118 Low-Power Batch Microwave Treatment ...... 119 High-Power Batch Microwave Treatment ...... 120 Appendix G ...... 123 Particle Size Distributions ...... 123 Appendix H ...... 125 Selected Sample Photos ...... 125

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List of Figures

Figure 1: Historical copper ore grades from 1900 to 2010 for select countries, with future projections up to 2100 (Northey, et al., 2014)...... 1 Figure 2: Flowsheet of the proposed CanMicro high-power microwave treatment process...... 3 Figure 3: Estimated energy usage by various equipment in mining operations (Powell & Bye, 2009)...... 5 Figure 4: Strain of a crystal lattice as a result of tensile or compressive stresses (Wills, 2016)...... 6 Figure 5: Different fracture mechanisms: intergranular (left) and transgranular (right)...... 7 Figure 6: Relationship between reduction in particle size and required comminution energy input (Hukki, 1961)...... 7 Figure 7: Microwave heating mechanisms showing dipolar rotation (A), and ionic conduction (B)...... 13 Figure 8: Forms of microwave interactions with materials, described by Haque (1999)...... 15 Figure 9: Gold recovery as a function of leach time for both as-received and microwave treated gold ore (Amankwah & Ofori-Sarpong, 2011)...... 36 Figure 10: Sample preparation flowsheet for the preparation of bulk samples for microwave treatment experiments...... 41 Figure 11: Bottle roll setup used at Sepro Laboratories for cyanide leaching...... 48 Figure 12: Real and imaginary permittivities as a function of temperature for the frequencies 912 MHz and 2466 MHz with a bulk density of 2.06 g/cm3...... 50 Figure 13: TGA at a heating rate of 10°C/min and real permittivity (912 MHz) at a powder bulk density of 2.06 g/cm3 for the gold ore (left) and sulphide ore (right)...... 50 Figure 14: DTGA and the real and imaginary permittivity (912 MHz) from 25°C to 1000°C at a heating rate of 10°C/min and powder bulk density of 2.06 g/cm3 for the gold ore (left) and the sulphide ore (right)...... 51 Figure 15: Penetration depth versus temperature for the gold ore (left) and the sulphide ore (right) in an argon atmosphere at a heating rate of 10°C/min...... 53 Figure 16: Infrared surface images of the A) gold ore and B) sulphide ore immediately after microwave treatment...... 54 Figure 17: Power trend data for the CP treatment of the gold ore showing forward, reflected and absorbed power, shown with the sample treatment window...... 57 Figure 18: Power trend data for the CP treatment of the sulphide ore showing forward, reflected and absorbed power, shown with the sample treatment window...... 58 Figure 19: Visible macrofracture of a gold ore particle (about 5 cm in length). Circled portion indicates high localized heating, initiating a crack shown by the arrow...... 59

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Figure 20: Observed microfracture in a gold ore particle of -63.5+25.4 mm. Arrows point to visible intergranular and transgranular breakage...... 60 Figure 21: Pyrrhotite and total sulphide liberation via QEMSCAN for the ground -106 µm gold ore product of the various reference (RB, RP) and microwave treated (BB, BP and CP) samples...... 70 Figure 22: Chalcopyrite and pentlandite liberation via QEMSCAN for the ground -106 µm sulphide ore product of the various reference (RB, RP) and microwave treated (BB, BP, and CP) samples...... 71 Figure 23: Effect of gold recovery versus time for 300g, -106 µm samples, at 40% solids density from the RB, BB and BP tests. Kinetics of the first 6 hours are shown, at a NaCN concentration A) 1.0 g/L, and B) 0.5 g/L...... 73 Figure 24: Effect of gold recovery versus time for 300g, -106 µm samples, at 40% solids density from the RP and CP tests. Kinetics of the first 6 hours are shown, at a NaCN concentration A) 1.0 g/L, and B) 0.5 g/L...... 73 Figure 25: Percent gold recovery increase as a function of percent SSA increase for the microwave treated samples with respect to reference tests. SSA was measured by BET, and gold recovery improvements are from the 1.0 g/L NaCN tests after 6 hours at 40% solids density...... 74 Figure 26: Effect of surface area on cyanide consumption after 24 hours of leaching with 300 g of -106 µm gold ore, 40% solids density, at a cyanide concentration of both 0.5 g/L and 1.0 g/L...... 77 Figure 27: Initial sulphur content before roasting as a function of temperature change in microwave tests, measured by ELTRA elemental analyzer...... 80 Figure 28: An electromagnetic wave with both an electric and magnetic field...... 96 Figure 29: Dipole rotation mechanism showing material with charged dipoles attempting to realign under an increasing electric field...... 98 Figure 30: Ionic conduction mechanism showing ions moving through an electric field...... 98 Figure 31: (a) A standard commercially available magnetron; (b) the internal structure of a magnetron, from Ong & Akbarnezhad (2015)...... 107 Figure 32: Straight waveguide cross-section, photo from Ong & Akbarnezhad (2015)...... 108 Figure 33: Commercial microwave heating unit as shown by Haque (1999)...... 110 Figure 34: Qualitative XRD analysis results for the gold ore, with the main mineral phases given at the bottom, and their calculated wt. %...... 111 Figure 35: Qualitative XRD results for the sulphide ore, with the main mineral phases given at the bottom, and their calculated wt. %...... 112 Figure 36: Bench-scale microwave unit by Microwave Research and Applications, IL, USA. 3.2 kW power input at 2.45 GHz...... 115

Figure 37: Internal components of one microwave generator power supply (A); and HMI control panel (B)...... 116 Figure 38: Conveyor belt with packed ore sample trays (A); and inside of the microwave applicator (B)...... 116 Figure 39: Example plot for power ramp-up, showing microwave power and conveyor belt speed. Once the power reaches a certain threshold, the belt switches on for ore treatment...... 117 Figure 40: Infrared camera setup at exit of microwave applicator, with infrared depiction, for obtaining ore heat signature...... 117 Figure 41: Maximum bulk temperatures recorded for 50g and 100g gold and sulphide ore samples at a particle size range of -3.35+1.7 mm. A microwave power of 3.2 kW and frequency of 2.45 GHz was used...... 118 Figure 42: Surface infrared image of 10 kg batch pilot (BP) microwave treatments for A) gold ore and B) sulphide ore...... 121 Figure 43: Power trend for the 10 kg BP treatment for the gold (left) and sulphide ore (right)...... 121 Figure 44: Particle size distributions of the ground -106 µm material for the gold ore and sulphide ore batch tests, BB and BP. Reference, RB, is also shown...... 123 Figure 45: Particle size distributions of the ground -106 µm material for the gold ore and sulphide ore continuous tests. Reference, RP, is also shown...... 124 Figure 46: Polypropylene tray filled with gold ore on the microwave transparent conveyor belt prior to microwave treatment. Particle size of -63.5+25.4 mm with -12.7 mm material packed to fill void spacing...... 125 Figure 47: Polypropylene tray filled with sulphide ore on the microwave transparent conveyor belt prior to microwave treatment. Particle size of -63.5+25.4 mm with -12.7 mm material packed to fill void spacing...... 125 Figure 48: Gold ore particle after microwave treatment showing an area of high localized heating with subsequent burning...... 126 Figure 49: Rocks from the bulk gold ore sample during material handling and sample preparation stages...... 126 Figure 50: Rocks from the bulk sulphide ore sample during material handling and sample preparation stages...... 127

List of Tables

Table 1: Dielectric properties of common minerals at different frequencies collected from literature. Table adapted from Zheng et al. (2020)...... 21 Table 2: Grouping of minerals based on dielectric properties (Zheng, et al., 2020)...... 22 Table 3: Microwave-assisted comminution, effects on Work Index reduction from relevant literature. ... 26 Table 4: Effects of microwave treatment on liberation, recovery and grade from relevant literature...... 29 Table 5: Summary of surface area improvements from microwave treated ores and minerals in the literature...... 33 Table 6: Summary of microwave treatment and subsequent leaching tests from the relevant literature, highlighting input energy dose and showing changes to recovery...... 35 Table 7: Mineralogy of the as-received gold ore using QEMSCAN...... 39 Table 8: Mineralogy of the as-received sulphide ore using QEMSCAN...... 40 Table 9: Summary of microwave treatment methods used in these experiments for both the gold ore and the sulphide ore...... 45 Table 10: Summary of comminution test work completed on each as-received and microwave treated ore...... 46 Table 11: Test parameters for the microwave treatment of the gold and sulphide ores for the continuous pilot (CP) tests...... 54 Table 12: Average heating rates for the microwave treatments, calculated from the average of temperatures observed over the irradiation time...... 55 Table 13: Microwave power absorption efficiency in the ore samples, calculated based on both the total forward and total electrical power for the continuous tests...... 58 Table 14: Summary of microwave treatment tests completed on each ore, with energy inputs and estimated power densities...... 62 Table 15: BET analysis results for the gold ore showing specific surface area, pore volume, and pore diameter. Treatments are italicized...... 64 Table 16: BET analysis results for the sulphide ore showing specific surface area, pore volume, and pore diameter. Treatments are italicized...... 65 Table 17: Summary of microwave treatment variables power input, energy input and particle size, with measured SSA and percentage increases. Treatments are italicized...... 66 Table 18: Comminution results of the batch microwave tests at -3.35+1.7 mm, showing microwave power and energy inputs with resulting Bond Work Indices...... 68

Table 19: Comminution results of the continuous pilot tests at -63.5+25.4 mm, showing microwave energy inputs with resulting SAG and Bond work indices...... 69 Table 20: Microwave treatment parameters shown with reported increases in specific surface area (from BET) and gold recovery after 6 hours at 1.0 g/L NaCN and 40% solids density for comparison...... 74 Table 21: Comparison between the gold recovery after 6 hours at 1.0 g/L NaCN and 40% solids density for the gold ore, and the relevant literature...... 76 Table 22: Cyanide consumption from the various leach tests at 1.0 g/L and 0.5 g/L NaCN concentrations for 300 g samples at a 40% solids density. Consumption was calculated after 24 hours of cyanidation. .. 77 Table 23: Summary of roasting tests comparing sulphur loss of reference and microwave treated sulphide ore with surface area. Samples of 50 g were used, roasted for 15 minutes at both 1050°C and 1100°C, and sulphur content was measured by ELTRA...... 79 Table 24: Heating rates of selected minerals at 1.6 kW of microwave power at a frequency of 2.45 GHz. (B) signifies a violent reaction - MnO was unable to be recorded (Ford & Pei, 1967)...... 105 Table 25: Interpretation of mineral groupings based on microwave heating response (Chen, et al., 1984)...... 105 Table 26: Maximum bulk temperature recorded after microwave irradiation at 1 kW of microwave power and a frequency of 2.45 GHz (Walkiewicz, et al., 1988)...... 106 Table 27: Elemental analysis of the gold ore using ICP...... 113 Table 28: Common mineralogical phases identified in the gold ore using qualitative XRD...... 113 Table 29: Tabulated XRF results reported for the gold ore...... 113 Table 30: Elemental analysis of the sulfide ore using ICP...... 113 Table 31: Common mineralogical phases identified in the sulphide ore using qualitative XRD...... 114 Table 32: Tabulated XRF results reported for the sulphide ore...... 114 Table 33: Summary of lab-scale treatment results for both ores, with resultant average temperature and mass loss...... 119 Table 34: Input parameters for treatment of the gold and sulphide ores for the batch pilot (BP) tests. ... 120 Table 35: Percentage of power absorbed in ore sample of the total forward and electrical power for the batch pilot tests...... 122

Table 36: F80 and P80 values for all as-received and microwave treated ground ore for the Bond ball mill...... 123

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List of Abbreviations

BET Brunauer-Emmett-Teller Theory

BWI Bond Work Index

DTA Differential Thermal Analysis

ICP-OES Inductively Coupled Plasma Optical Emission Spectroscopy

QEMSCAN Quantitative Evaluation of Minerals with Scanning Electron Microscopy

SAG Semi-autogenous Grinding Mill

SEM Scanning Electron Microscope

TGA Thermogravimetric Analysis

XRD X-ray Diffraction

XRF X-ray Fluorescence

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Chapter 1

Introduction 1.1 General Overview In this thesis, the effects of high-power microwave treatment of ores was investigated. The incentive for this project was to develop and advance a technology that has shown promising results in the literature, however, has not been scaled to industry. Current ore processing techniques require significant amounts of energy to maintain commercial throughputs. Treating ores with microwaves has been proposed as a method to reduce these high energy demands and improve process efficiency. The subsequent sections in this chapter will discuss the main project drivers, the scope and objectives of the present research, and the organization of this thesis.

1.2 Project Motivation In the mining and minerals industry, two main focuses for innovation are increasing the productivity of assets and improving the extraction processes of ores from increasingly complex and low- grade ore deposits. Increases to per capita consumption of refined metal will accelerate demand, and has been documented in numerous projections (Mateus & Martins, 2019). Ore grade models developed by

Northey et al. (2014) predict that copper deposits will continue to decline in grade, as shown in Figure 1.

Figure 1: Historical copper ore grades from 1900 to 2010 for select countries, with future projections up to 2100 (Northey, et al., 2014).

With decreasing ore grades, higher tonnages in mineral processing will be necessary to maintain the same metal production. Mateus and Martins (2019) suggested four important supplementary courses of action that should be addressed in the coming years to approach this problem; the most notable of these involved the processing of lower grades or more complex mineral matrices that are more abundant than ore bodies processed in the past. Perhaps declining ore grades should not be viewed as a sign of depletion or an indicator of resource availability, but rather an opportunity for other factors such as innovation and improvements to processing and extractive technologies to extend mine life and process previously uneconomic deposits (Calvo, et al., 2016).

It is well established that comminution is the most energy intensive part of a mineral processing operation (Tromans, 2008; Batterham, 2011). The high energy requirements associated with comminution are due to high strength and hardness of ores, the large volumes of ore processed and fine grinding.

Batchelor et al. (2015) suggested the following strategies which aim to tackle energy efficiency in comminution:

 Reducing ore competency,  Improving liberation,  Pre-concentration or barren waste rejection (via sorting) and,  Indirectly reducing the burden on energy and materials from the production and consumption of consumables, such as water, grinding media and wear liners.

One technique that has shown promise to address these areas is the utilization of microwaves to treat ores (Walkiewicz et al., 1988; Walkiewicz et al., 1991; Kingman & Rowson, 1998). Heating ores with microwaves has shown to initiate a temperature response for various minerals leading to fracture, resulting in reduced ore competency and improved mineral liberation, as well as other benefits.

The project team CanMicro, led by Dr. Erin Bobicki of the University of Alberta and University of

Toronto, is one of six semi-finalists entered in the CrushIt! Challenge sponsored by Natural Resources

Canada. The goal of the team is to demonstrate that the high-power microwave treatment of ore and subsequent infrared sorting can result in energy savings at the pilot-scale; winning this competition will

2 provide the team with further funding to scale-up to commercialization. The proposed CanMicro process can be seen in Figure 2. The experimental work in this thesis was completed in parallel with CanMicro, although the application of microwave sorting was not included in this work.

Figure 2: Flowsheet of the proposed CanMicro high-power microwave treatment process.

This thesis extends the scope of the CanMicro project team by including hydrometallurgical and pyrometallurgical investigations in the overall assessment of the microwave treatment. Literature on microwave treatment impacts on chemical processing is limited to bench-scale studies, and are non-existent at the pilot level. Perhaps added benefits to extractive processes may well incentivize scale-up if demonstrated at the pilot-scale. The impacts of microwaves on downstream processing is the primary subject of this thesis.

1.3 Research Scope and Objectives of Experimental Work Microwave treatment of ores is a relatively new research application, and there exists the possibility that its incorporation into ore processing can greatly improve process economics if more studies can demonstrate the potential at pilot-scale. The focus in the published literature is primarily concerned with its effects on comminution. Moreover, very few studies have linked the possibility of the cumulative benefits that can be realized further downstream as a result of microwave treatment. Therefore, it would be advantageous to combine these two research needs in an attempt to fill the void in the available literature: scale-up to high-power microwave treatment of ores, and investigate its effects on comminution and downstream processes. The primary research objectives are as follows:

1) Complete an in-depth literature review of relevant information regarding the microwave processing

of ores at bench and pilot scale, the implications of microwave heating on ore structure and

morphology, and their subsequent impacts to relevant unit operations in the mining industry.

2) Investigate the fundamental properties of two different ores to further understand their interaction

with microwaves.

3) Determine whether comminution energy requirements can be impacted by high-power microwave

treatment in terms of mechanical energy input and liberation.

4) Determine whether surface area can be improved by microwave treatment, and the downstream

impacts of this on leaching and roasting.

1.4 Organization of Thesis This thesis is organized into seven chapters, summarized as follows:

Chapter 2: This chapter consists of a brief literature review involving the basic principles of comminution, mineral liberation, and various principles related to extractive metallurgy. Industry trends are also discussed.

Chapter 3: This chapter reviews microwave fundamentals and the application of microwaves in mineral heating. This includes studies on the microwave heating of ores, scale-up requirements for the technology, and a review of the potential impacts in downstream processes.

Chapter 4: This chapter provides a description of the materials and methods used in the lab work for this study. This includes the preparation of sample, test plans and commissioning of the microwave system.

Characterization work for the ores is described. Comminution and downstream test work is also discussed.

Chapter 5: This chapter presents the results and discussion of this thesis. This chapter was split into three subsections and includes: fundamental characterization of the selected ores on their microwave response; the impact of microwave treatments on the ores; and the impact of the microwaves on selected downstream unit operations, namely leaching and roasting.

Chapter 6: Any significant findings from this research are reported in this concluding chapter.

Recommendations for future work are also discussed.

Chapter 2

Literature Review 2.1 Overview In this section, the fundamental mineral processing principles related to fracture, size reduction, and mineral liberation will be discussed. Additionally, basic hydrometallurgical and pyrometallurgical processes will be reviewed, specifically leaching and roasting. Lastly, current industry trends are discussed with an emphasis on ore treatment methods that have been proposed, and the role these developing technologies may play in the future of sustainable ore processing.

2.2 Mineral Processing

2.2.1 Comminution One main component of the mining value chain is comminution, a highly energy-intensive process defined as the breakage of coarse run-of-mine ore into smaller particles by crushing and grinding.

Comminution processes initiate and propagate fractures within pre-existing cracks to break the structural bonds between minerals, reducing the particle size as material progresses through a comminution circuit

(Whittles, et al., 2003). Comminution consumes the largest part energy used in mining operations, usually around 40% (Figure 3), with the total global consumption of energy in comminution processes estimated at

2-3% of the world’s total energy demand (Shuey, 2002; Tromans, 2008; Batterham, 2011).

Figure 3: Estimated energy usage by various equipment in mining operations (Powell & Bye, 2009).

Current comminution practices rely solely on mechanical energy to induce fractures and initiate breakage into smaller particles, however only 1-2% of all energy supplied is used to initiate breakage and create new particle surfaces (Haque, 1999; Batar, 2004; Wills, 2015). As a result of the high inefficiencies, the vast majority of the spent energy is lost in the form of heat generated in the material and equipment

(Haque, 1999).

2.2.1.1 Fracture Mechanisms To produce a mechanical fracture in an ore, the material must achieve a certain stress beyond a critical limit. Here, the local stress exceeds the strength of bonds within the material lattice (Singh, et al.,

2018). These stresses may be generated by tensile or compressive loading, as shown in Figure 4.

Figure 4: Strain of a crystal lattice as a result of tensile or compressive stresses (Wills, 2016).

Additionally, the distribution of stress depends on the mechanical properties of the individual minerals as well as pre-existing cracks or flaws. The number of pre-existing flaws in the rock structure increases the probability of its breakage (Parapari, et al., 2020). This leads to a lower required fracture strength and a resulting lower breakage pressure required (Wills, 2016; Singh, et al., 2018).

Mechanical breakage tends to create two types of fractures within an ore; intergranular and transgranular fractures, as shown in Figure 5. Intergranular fractures are when the breakage occurs along the grain boundaries between two separate phases, whereas transgranular breakage occurs through mineral grains, often producing a particle consisting of more than one mineral phase (Parapari, et al., 2020).

Figure 5: Different fracture mechanisms: intergranular (left) and transgranular (right).

Intergranular breakage is the more desirable of the two as it requires less mechanical energy to reduce particle size while achieving the same liberation. Transgranular fracturing is much more energy- intensive as it requires a finer grind size to achieve the same liberation; transgranular breakage is often the dominant mechanism in mineral comminution.

2.2.1.2 Energy and Size Reduction For equipment used in comminution processes, the specific energy consumption is defined as the energy consumed per unit mass of throughput (kWh/t), and is determined by a range of operating parameters such as ore properties, equipment parameters, and operating conditions (Klein, et al., 2018). Energy usage in comminution can be minimized by using optimum conditions for these parameters. A larger amount of energy is required to break finer particles, as shown in Figure 6, as there are fewer flaws (Hukki, 1961).

Figure 6: Relationship between reduction in particle size and required comminution energy input (Hukki, 1961).

The widely used relationship in the quantification of specific energy consumption in grinding was described in the third theory of comminution (Bond, 1952). Bond (1952) stated that the work input required

7 to reduce an ore from a feed size, F80, to a product size, P80, is dependent on the material specific Work

Index of an ore, WBWI, given by Equation 1.

10 10 [1] 푊 = 푊퐵푊퐼 ( − ) √푃80 √퐹80

The Bond Work Index equation cannot be used to adequately describe the full energy-size reduction relationship for coarser comminution practices. Other theories have been proposed to define specific energy consumption at coarser particle sizes; however, Bond’s third theory of comminution has remained the industry standard in comminution testing for the quantification of energy consumption.

2.2.1.3 Mineral Liberation The objective of comminution is to break down the ore to a particle size fine enough that the bonds between valuable minerals and other waste minerals are broken. Grinding is essentially a compromise between achieving a certain degree of liberation for downstream separation processes such as flotation, and the cost of mechanical energy to grind to a specific particle size required to achieve the degree of liberation

(Ure, 2017). Grinding coarser will require less energy, however the trade-off that must be considered is the loss of value to waste. The higher the liberation degree, the higher the percentage of value minerals recovered in separation processes.

Comminution processes that can promote intergranular breakage are ideal for maximizing the liberation potential of an ore. Additionally, more pre-existing flaws are desirable for comminution as mentioned, as the probability of breakage would increase. More selective methods of liberating minerals from surrounding gangue would be an important advance for comminution processes; however, the current technology relies on grinding to a critical particle size (Bradt, et al., 1995). To shift this critical value to a coarser size, either a new innovative form of comminution technology or a change to the structural make- up of the ore would be required.

2.3 Downstream Processes Innovative technologies that are suggested in literature to improve mineral processing circuits may have some implications to processes downstream. Downstream extractive metallurgy operations follow

8 the mineral processing stages, where the valuable minerals undergo a series of separation and reaction based steps to isolate the pay metal of interest. The steps taken in these processes are dependent on ore mineralogy and can be achieved through hydrometallurgy or pyrometallurgy. For this thesis, leaching and roasting are discussed.

2.3.1 Leaching Leaching reactions involve the dissolution of the target metals from either ores or concentrates.

Here, they are converted into soluble compounds for subsequent recovery by either a solvent or lixiviant.

One of the most important leaching processes in the mining industry is shown as Equation 2, where the dissolution of metallic gold into a soluble gold cyanide complex occurs. Cyanidation is widely used by most gold mining companies to dissolve gold into solution for subsequent extraction.

1 [2] 2퐴푢 + 4푁푎퐶푁 + 푂 + 퐻 푂 + 2푂퐻− → 2퐴푢(퐶푁)− + 4푁푎푂퐻 2 2 2 2 Leaching reactions can be studied at the bench-scale by various test work methods. One of the main methods for cyanidation is the bottle roll technique. During this process, fine solids are gently agitated in a slurry with water and cyanide in a bottle by slowly rotating for a set leach time. Different control parameters are used to optimize the test accordingly.

Parameters such as the availability of oxygen at the solid-liquid interface, cyanide concentration, particle size, surface area, mineralogy and mass transport rates are important in cyanidation. Most metal dissolution processes are controlled by the rate of diffusion of the leaching reagent to the mineral of interest.

This rate of mass transport is dependent upon whether the minerals of interest are bound to other waste minerals; liberation by grinding exposes these minerals to an attack by lixiviants, as it increases the amount of surface area available for a reaction (Gupta & Mukherjee, 1990).

The main focus of any leaching reaction is the reaction kinetics and the metal recovery achieved at the end of the leaching period. Gupta and Mukherjee (1990) stated that the kinetics of a general leaching reaction can be described by Equation 3, where k1 and k2 are reaction constants, [D] and [C] are the

9 concentration of leaching reagents, i.e., oxygen and cyanide in the case of gold leaching, and A is the total surface area of the solid in contact with the aqueous solution.

푘 푘 퐴[퐷][퐶] [3] 푅푎푡푒 표푓 퐷푖푠푠표푙푢푡푖표푛 = 1 2 푘1[퐷] + 푘2[퐶] Based on Equation 3, the rate of the leaching reaction is a function of the surface area of the suspended solids in the aqueous solution. One of the main disadvantages of hydrometallurgy is the time required to achieve high metal recovery, since these processes are usually carried out at low temperatures in comparison to pyrometallurgical processes. Therefore, a hydrometallurgical process is much more susceptible to chemical kinetics than to thermodynamic considerations (Gupta & Mukherjee, 1990); the leaching kinetics is directly tied to the amount of metal a plant can produce. The degree of influence of important parameters on the rate of gold dissolution are as follows: leaching time > cyanide concentration

> particle size and surface area > pH. While leaching time and cyanide concentration appear to have the largest effect on the reaction rate, these are difficult parameters to change in a commercial setting, as the residence time for such a reaction directly affects the achievable throughput. Cyanide also accounts for a large amount of reagent cost, and higher cyanide concentrations will produce more cyanide waste.

Therefore, improving the available surface area would be the most logical step to increasing metal recovery in leaching.

2.3.2 Roasting Roasting consists of thermal gas-solid reactions, with various applications in extractive metallurgy.

One important type of roasting process is known as an oxidizing roast, which is carried out to prepare a totally or partly oxidized product (Shamsuddin, 2016). Oxidizing roasts are useful as a high temperature treatment to convert sulphide minerals into oxides, which allows for easier metal extraction. In this process, ore or concentrate is treated with air. The sulphur atoms react with oxygen in the air to produce gaseous sulphur dioxide, with the resulting metal oxide or calcine left over as a result. A standard oxidizing roast reaction of a metal sulphide, MS, is shown in Equation 4.

2푀푆(푠) + 3푂2 (푔) → 2푀푂(푠) + 2푆푂2 (푔) [4]

Oxidative roasting is a surface reaction where the oxide layer is formed first and continues to remain porous. Oxygen continues to pass into the still unreacted inner sulphide portion of the particle as SO2 gas already formed dissipates. The required reaction occurs at the interface between previously produced solid product, MO, and unreacted sulphide, MS, if the gas ratio of pO2/pSO2 is locally higher than the equilibrium ratio for the reaction. Therefore, for Equation 4 to proceed to the right, it is essential that at least three molecules of oxygen reach the interface to form two molecules of sulphur dioxide. Key parameters that control the reaction rate of roasting include temperature, particle size, and porosity of the product phase. A decrease in particle size will result in faster reaction kinetics by improved gas-solid contact area. An increase in porosity will also result in improved gas-solid contact area.

There are several types of commercial roasters used in the mining industry, including multiple hearth roasters, rotary kilns, and fluidized-bed roasters. Fluidized-bed roasters are an attractive option due to the very large mass and heat transfer coefficients associated with the process (Dunne, et al., 2019).

Roasting is an exothermic reaction, and the heat generated from the process keeps the roaster at the required temperature so that the process can continue with little additional heat supplied (Thomas & Cole,

2016). Therefore, the energy costs to maintain the reaction are relatively low. The rate at which the ore is oxidized influences the calcine production rate, which ultimately drives the economics of the process. Thus, the reaction rate in roasting is of high importance, with direct implications for capital and operating costs.

Higher reaction rates and therefore increased productivity of a roaster will ultimately reduce the cost per tonne of calcine produced.

2.4 Industry Needs When addressing the low efficiencies in comminution, even the smallest improvement can result in considerable energy savings (Batar, 2004). The strategies to reduce energy consumption must be focused on avoiding size reduction where possible, and using more efficient and suitable comminution technology for selective breakage (Parapari, et al., 2020). Batterham (2013) suggested three areas where significant development is possible, all of which consider the reduction of energy requirements and costs of mineral

11 processing. Firstly, all the ore, including gangue minerals, is ground to the desired final particle size.

Secondly, the fact that current comminution practices are of the magnitude of 1-2% efficient, results in large amounts of wasted energy. Lastly, the energy demand in flotation is high and this process cannot accommodate coarse material. With decreasing ore grades, the energy demands associated with these practices will be impacted significantly. On the other hand, lower grade ores will not significantly increase the energy consumption of downstream metal extraction and refining stages, as a concentrate of fixed grade is produced for downstream processes irrespective of initial ore grade (Norgate & Haque, 2010). Therefore, the most drastic change by innovative technologies that could be realized likely lies within the mineral processing stages. Microwave treatment may form part of the solution by changing various ore characteristics to improve comminution efficiency. Furthermore, the structural impacts of the treatment may have additional benefits to metal extraction. This will be reviewed in the following chapter.

Chapter 3

Microwave Heating & Microwave Treatment of Ore 3.1 Introduction Microwave energy as a method of heating rose to prominence shortly after the end of the Second

World War, when substantial industrial use began (Ospechuk, 1984). During the surge in interest of microwaves after its invention, it became the subject of extensive research over numerous fields. One research field that attracted interest was the application of microwaves to rock, ore and minerals in an effort to induce breakage and fracture. The idea of heating rocks with microwaves has been explored since the

1950’s and 1960’s (Puschner, 1966; Ford & Pei, 1967; Santamarina, 1989). This section will review microwave heating fundamentals and the microwave heating of minerals and ore. Additionally, effects on comminution and potential downstream processes will be discussed.

3.2 Microwave Fundamentals Microwaves are a form of non-ionizing electromagnetic radiation with wavelengths between 1 mm

(300 MHz) and 1 m (300 GHz). Allotted frequency bands approved for industrial, scientific and medical use in microwave heating were chosen by international agreement and include frequencies close to 900

MHz, usually 915 MHz and close to 2450 MHz (Meredith, 1998). There are two primary mechanisms in which materials convert microwave energy into heat: dipole rotation and ionic conduction. Dipole rotation involves the short-range displacement of a charge through the formation and rotation of electric and magnetic dipoles as they attempt to realign (Figure 7 A), and ionic conduction requires the long-range migration of a charge (Figure 7 B) (Clark & Sutton, 1996).

A B

Figure 7: Microwave heating mechanisms showing dipolar rotation (A), and ionic conduction (B).

Important parameters used to describe microwave heating phenomena are the complex permittivity

(Equation 5); power density (Equation 6); heating rate (Equation 7); and penetration depth (Equation 8).

ε = ε0 · (ε′ − jε′′) [5] ퟐ 2 Pav = ퟐ훑퐟훆ퟎ훆′′퐄 + 2πfµ0µ′′H [6] dT 2πfε ε′′E2 푃 [7] = 0 r = 푎푣 dt (Cpρ) 퐶푝𝜌

1 − 2 ′′ 2 λ0 εr [8] Dp = {√[1 + ( ) ] − 1} ′ ′ 2π√2εr εr

-12 In Equation 5, ε0 is the permittivity of free space, defined as a constant 8.854x10 F/m. The relative dielectric constant or real permittivity, εr’, corresponds to the polarization ability of a material in a microwave field, and the relative dielectric loss factor or imaginary permittivity, εr’’, corresponds to the energy losses of a material, dissipated as heat (Meredith, 1998). Permittivities, ε’ and ε’’, are also referred to as the dielectric properties of a material. Generally, the degree to which any material will absorb microwave energy, is determined by the complex permittivity. The average power density, Equation 6, is the sum of the electric and magnetic losses, where E is the electric field intensity (V/m) and f is the frequency (Hz). Most materials are not magnetic, and consequently the interaction is mainly determined by the left part of Equation 6 (in bold) irrespective of permeability. The heating rate in Equation 7 is proportional to the power density, Pav, and inversely proportional to the specific heat capacity, Cp, and material density, ρ. Heating rate can be maximized by heating materials with high permittivities with a high electric field intensity. This is a key piece of information in the microwave treatment of ores, and will be discussed further. Equation 8, which describes penetration depth, is used to determine the distance at which the microwave power is decreased by half in a material, where λ0 is the wavelength of the microwaves. Materials that are similar in size to the penetration depth respond well to microwave heating

(Sun, et al., 2016). An in-depth review of the fundamentals of microwave heating, as well as important parameters that affect dielectric properties of materials is given in Appendix A. Further information related to the above-mentioned equations is also given in Appendix A.

3.3 Microwave Absorption in Materials When investigating the effectiveness of microwave heating, it is important to understand forward, reflected and absorbed power. Generally, materials can either absorb or reflect microwave energy, and the degree to which this occurs is based on the equations previously discussed. Absorbed power by definition, is the difference between the forward and reflected power, and is thus the energy that directly enters a material. Water is an ideal example of a material which has excellent microwave absorption properties, and has little reflected power. Other materials may only absorb a small portion of the forward power, with higher amounts of reflected power. To use microwave heating in an efficient manner, the reflected power of a material should be minimized as it can be considered wasted energy.

The extent to which a material absorbs microwave energy is dependent on its conductivity.

Materials with low conductivities, referred to as insulators, are ‘transparent’ to microwaves. The propagating electromagnetic waves pass through material without storing any energy or producing heat.

Materials with high conductivities such as metals will reflect microwaves, preventing any notable heat signature and often causing sparking or arcing, as commonly observed with metallic substances (Onol,

2007). Medium conductivity materials such as semiconductors, can absorb microwave energy at room temperature and produce heat as a result of the interaction between the waves and the material (Meredith,

1998). These microwave heating mechanisms were summarized by Haque (1999), given in Figure 8.

Figure 8: Forms of microwave interactions with materials, described by Haque (1999).

Conductive materials with high loss factors result in zero microwave penetration (small penetration depth), however absorbers with both medium loss factors and conductivities result in adequate penetration depth suitable for heating. Transparent materials with low loss factors result in large penetration depths with the propagating waves completely passing through them (Moravvej, et al., 2016).

The heating of a heterogeneous material with varying dielectric properties may cause non-uniform heating, with some parts heating faster than others (Haque, 1999). The variation in dielectric properties can actually be used to advantage depending on the application. For ores, microwaves may heat absorbing phases in a selective manner while passing through transparent phases (Clark, et al., 2000).

When microwaves are propagated through air before reaching the material, arcing may occur if the electric field strength exceeds the specific breakdown voltage of air (Ali & Coffey, 1980). This will occur if the material load in the microwave applicator cannot sufficiently absorb the microwave energy.

Additionally, the geometric shape of material in a can also be a factor in arc generation, with angular material fragments proving higher susceptibility to arc formation (Holmes, et al., 2020). Therefore, as coarser particles have a less ideal geometry in comparison to finer particles, coarser particles would be more susceptible to producing arcs and this phenomenon must be taken into consideration when heating ore.

Despite the advantages of microwave heating, the efficiency of a microwave process must include the initial energy conversion steps, the energy absorbed by the material, and heat losses. The conversion efficiency of electrical energy into microwaves is typically around 75-80% (Meredith, 1998). Additionally, the efficiency of converting source energy into electrical energy is also not 100% efficient. This coupled with the fact that any energy not absorbed by a material during irradiation is essentially wasted as reflected power, which also contributes to lower efficiencies regardless of the effectiveness of microwave energy delivery. The overall efficiency of a microwave process is estimated to be approximately 30% (Pickles &

Marzoughi, 2018). Therefore, the combined advantages of a microwave processes must outweigh the reduced efficiency of the process. Energy efficiency in comminution is generally low, and therefore there is scope to make use of microwave power that is more selective, regardless of the reduced efficiency.

3.4 Characterization of Microwave Response of Minerals At the beginning of the microwave research with minerals and ores, little was known about heating and the dielectric properties of minerals; i.e., why some material heats while others do not. As further research into the microwave treatment of minerals continued, an increasing amount of data related to these properties was produced. There are two approaches to characterize the microwave absorption capabilities of minerals: 1) heating rate tests using a laboratory microwave system or domestic microwave oven to heat the mineral sample, measuring the temperature increase over time, and 2) measuring the dielectric properties of minerals. The following subsections will discuss the relevant literature for both.

3.4.1 Heating Characteristics 3.4.1.1 Effect of Mineralogy The first notable heating study was by Ford and Pei (1967), who determined the heating behaviour of some naturally occurring minerals by microwave heating in a 2.45 GHz applicator with an input power of up to 1600 W. In general, darker coloured materials such as sulphides and some oxides appeared to heat rapidly to temperatures of about 1000°C, whereas lighter coloured materials required much longer heating times.

Chen et al. (1984) experimentally determined the heating rates of forty minerals using a 2.45 GHz,

800 W laboratory microwave. The authors also divided the minerals tested in the study into two separate groups: (1) little or no heat generated with mineral properties unaffected due to transparency or high reflectiveness, and (2) heat generated by minerals, either being thermally stable or dissociating rapidly. The authors realized that of these irradiated minerals, most silicates, carbonates, sulphates and certain oxides and sulphides such as sphalerite were part of the first group, whereas the majority of metal sulphides and some oxides were classified by the second. Also, it was concluded that the heating behaviours observed were also compositionally dependent; for instance, with a higher iron content in sphalerite, the sphalerite became much more responsive.

Wong (1975) characterized the microwave response of minerals in terms of four categories: hyperactive, active, difficult to heat, and inactive. Hyperactive was defined as a high susceptibility to

17 microwave heating, with heating rates ranging from 20°C to 200°C per second. Minerals in the active category, defined by their medium susceptibility, had heating rates in the range of 120°C to 400°C per minute. Difficult to heat minerals were in the heating rate range of 15°C to 80°C per minute, and could improve heating rates with increasing temperature. Inactive or transparent minerals were those that produced heating rates of no more than 5°C per minute. While it is difficult to directly compare the heating rates and temperature measurements between literature studies due to varying microwave conditions, value minerals mostly belong to the hyperactive and active categories and gangue minerals are either difficult to heat or inactive (Chen, et al., 1984).

Additional studies have been published investigating the heating of different minerals from various other researchers (Walkiewicz, et al, 1988; Chunpeng, et al., 1990; Harrison, 1997). The results of these studies are consistent with the earlier work of Ford and Pei (1967) and Chen et al. (1984), although vary depending on the microwave system, sample mass, and particle size (Kingman & Rowson, 1998; Haque,

1999). The relevant mineral heating data from these studies can be viewed in Appendix B. Clearly, the literature demonstrates a definite, selective relationship between value and gangue minerals.

3.4.1.2 Effect of Power Level McGill et al. (1988) experimentally evaluated the effect of power level on the microwave heating of selected compounds and minerals between 500 to 2000 W at a frequency of 2.45 GHz. The authors determined that heating rates increased as the power level increased in most instances. It was observed that some exceptions to this trend included very high-loss and very low-loss materials, which showed negligible changes with varying power. There certainly exists an ideal power input when heating minerals with microwaves. Too much power may result in high amounts of reflected power and too little will result in a small heat signature. Therefore, there would exist an optimal power requirement to achieve efficient heating of minerals and ores.

3.4.1.3 Effect of Frequency There are two permitted frequencies of 915 MHz and 2450 MHz for microwave heating in North

America (Meredith, 1998). Therefore, there is minimal room for changing this heating parameter outside

18 of the two accepted frequency bands. Generally, if frequency increases, the rate of increase of temperature increases and the penetration depth decreases (Santamarina, 1989). Lower frequencies exhibit a lower rate of temperature increase, with an increased penetration depth.

3.4.1.4 Effect of Particle Size While the interaction of microwaves with materials has been split into three categories of responses from the literature; i.e., reflection (metals and materials with high conductivity and dielectric properties), transparent (gangue minerals) and absorption (sulphides and some oxides), the responses have a high dependency on the particle size of materials. Standish et al. (1991) investigated the effect of particle size in the microwave heating of granular materials, namely magnetite (hyperactive) and alumina (difficult to heat). Their experimental work showed a particle size dependency with no definite relationship; finer particle sizes for alumina resulted in accelerated heating rates, whereas coarser particle sizes for magnetite displayed superior heating. Harrison (1997) experimentally determined the difference in heating rate of various minerals at different particle sizes and concluded that the effect of particle size on the heating rates of minerals could be divided into three relationships:

1) An increase in particle size causes a decrease in heating rates, 2) No discernible change in heating rate, 3) An increase in particle size causes an increase in heating rates.

Most minerals seem to conform to the general theory explained by the previous work by Standish et al.

(1991); an increase in heating rate is observed with decreasing particle size (galena, magnetite, pyrrhotite, hematite and quartz). There is however a group of minerals consisting of sphalerite, wolframite, gypsum and cassiterite that exhibit the opposite behaviour (Harrison, 1997).

3.4.2 Dielectric Properties of Minerals Historically, there has been a lack of data on the dielectric properties of most minerals as a function of temperature over the microwave frequency range. These properties are crucial in predicting and modelling microwave heating rates and behaviour.

The common techniques for measuring the dielectric properties of minerals include the short- circuited waveguide, transmission line, free space, coaxial probe, three terminal method and the resonant cavity perturbation technique. The most widely used methods are the resonant cavity perturbation technique and the short-circuited waveguide method (Zheng, et al., 2020). Table 1 summarizes the dielectric properties from studies of industry relevant minerals using different measurement techniques. The sulphides and oxides reported higher dielectric properties than the gangue minerals; most had a loss factor greater than 1. The gangue minerals show smaller dielectric constants (ε’), and much smaller loss factors

(ε’’). Due to the dependence of loss factor for heat generation, the dielectric data shown in Table 1 would indicate that the sulphides and oxides tested show a much greater potential for heating, aligned with the microwave heating studies previously discussed.

The data in Table 1 only includes minerals and not ores however. A recent study published by

Bobicki et al. (2020) investigated the high temperature permittivity measurements of selected industrially relevant ores, including bauxite, chromite, laterites, manganese carbonate, iron ore and ultramafic nickel, using the cavity perturbation technique. Permittivities were shown to increase with temperature, with some increasing slowly at first, and then exponentially after reaching a critical temperature. The authors also concluded that mineralogical and elemental composition has a large impact on the permittivities; the presence of sulphides and iron oxides in ores composed of low loss minerals was shown to increase the overall permittivities, thus improving microwave response.

Table 1: Dielectric properties of common minerals at different frequencies collected from literature. Table adapted from Zheng et al. (2020).

Frequency Dielectric Group Mineral Formula Measurement Technique Loss Factor, ε’’ Reference (GHz) Constant, ε’

Short-circuited waveguide 2.45 11.10 1.41 (Florek & Lovas, 1995) Galena PbS Coaxial probe 2.45 6.75 0.12 (Harrison, 1997) Sulphides Short-circuited waveguide 2.45 10.30 2.28 (Florek & Lovas, 1995) Chalcopyrite CuFeS2 Coaxial probe 2.45 4.75 0.26 (Harrison, 1997)

Pyrite FeS2 Coaxial probe 2.45 8.25 1.00 (Harrison, 1997)

Hematite Fe2O3 Short-circuited waveguide 2.45 18.3 2.23 (Nelson, et al., 1989) Short-circuited waveguide 2.45 61.9 10.7 (Nelson, et al., 1989) Metal Magnetite Fe3O4 Coaxial probe 2.45 14.50 2.50 (Harrison, 1997) Oxides Short-circuited waveguide 2.45 23.6 11.2 (Nelson, et al., 1989) Ilmenite FeTiO3 Resonant cavity 9.37 54.3 32.58 (Zheng, et al., 2005)

Carbonates Calcite CaCO3 Short-circuited waveguide 0.3-1 8.87 0.005 (Church, et al., 1988) Coaxial probe 0.3-1 2.82 0.02 (Harrison, 1997) Quartz SiO2 Resonant cavity 9.37 3.89 0.003 (Zheng, et al., 2005) Muscovite Short-circuited waveguide 2.45 8.69 0.091 (Nelson, et al., 1989) KAl2(Si3AlO10)(OH)2 (Mica) Three terminal method 0.001 7.30 0.204 (Olhoeft, 1979) Silicates Three terminal method 0.001 7.10-7.30 0.031-0.288 (Olhoeft, 1979) Olivine (Mg,Fe)2SiO4 Resonant cavity 9.37 8.05 0.058 (Zheng, et al., 2005) Diopside MgCaSi2O6 Resonant cavity 2.45 6.77 0.012 (Zheng, et al., 2020) (Pyroxene)

Similar to the groupings of minerals based on temperature response by researchers, Zheng et al.

(2020) categorized minerals based on the loss factor (ε’’). Here, a mineral can be either high loss (ε’’ ≥ 1),

medium loss (0.1 < ε’’ < 1) or low loss (ε’’< 0.1). These categories are shown in Table 2.

Table 2: Grouping of minerals based on dielectric properties (Zheng, et al., 2020).

Group Description Mineral Light-coloured minerals, ranging from white to pink. Olivine, diopside, quartz, Category I: Low Loss, ε’’< 0.1 Low microwave absorption calcite, albite, almandine properties. Light-coloured to dark-coloured Category II: Medium Loss, 0.1 <ε’’< minerals. Loss factors are 1-2 Feldspar, muscovite, augite, 1 magnitude higher than Category amphibole, biotite 1 minerals. Ore minerals which have a high Category III: High Loss, ε’’≥ 1 metal composition. Metal Galena, hematite, magnetite sulphides and oxides.

Based on the defined categories by Zheng et al. (2020) in Table 2, if the loss factor is less than 0.1

it is not a good candidate for microwave heating. For minerals with a loss factor greater than 1, rapid

heating is likely to occur. For any material with a loss factor much greater than 1, the penetration depth

would be quite small and non-uniform heating would likely occur (Bradshaw, 1999). The loss factors

compiled in Table 1 for the various minerals were grouped into Table 2. As expected, the sulphides and

metal oxides have high loss factors, while the gangue minerals have medium and low loss factors.

3.5 Microwave Systems Microwave systems are manufactured for a variety of applications, ranging from small scale

kitchen microwaves to industrial systems used in food processing. These systems are comprised of a few

main components: power supply, microwave power generator, waveguides, control circuitry and an

applicator. Other components such as circulators, directional couplers and stub tuners are also common.

There are many different configurations of these components depending on the application use. Microwave

heating applications generally operate by propagating the electromagnetic waves generated into a closed

system commonly referred to as an applicator or cavity. Here, microwave heating of material can occur in

a controlled environment to contain the energy and control the emissions. The two main applicator types

22 are single mode or monomode and multimode. A more in-depth review of the two microwave applicators and different microwave components is given in Appendix C.

3.6 Microwave-Assisted Comminution

Based on the previous discussion, in general, valuable minerals are good microwave absorbers and gangue minerals are poor absorbers. As a result of this varied response, differential heating will occur between mineral grains causing thermal stress. The stresses that arise from microwave heating within ore matrices can alter the physical and chemical structure of the ore, leading to a range of potential effects in processing circuits. It is these selective thermal stresses that are highly attractive to researchers for ore processing applications. The reported benefits of microwave treatment for comminution in the literature include reduced ore competency, improved liberation and other factors such as reduced mill wear and maintenance (Wills, et al., 1987; Kingman, et al., 2000; Orumwense, et al., 2004).

3.6.1 Microwave Fracturing

The basis for microwave treatment in mineral processing is the differential thermal expansion between minerals, which causes transgranular and intergranular breakage (Somani, et al., 2017).

Microwaves transmit energy to the centre and surface of the ore independent of thermal conductivity, overcoming the time required to heat materials in conventional processes (Fitzgibbon & Veasey, 1990).

The fracture arising from applied microwave energy often occurs at a microscopic level between associated minerals, and is often referred to as ‘microfracture’.

Preliminary work by Walkiewicz et al. (1988) showed thermal stress fracturing using a scanning electron microscope (SEM) after heating an iron ore with 3 kW of microwave power at a frequency of 2.45

GHz for 25 seconds. Bobicki et al. (2018) observed intergranular breakage by SEM, generated by thermal stresses due to differential heating in a nickel sulphide ore. The authors quantified the extent of fracturing and explained that the crack length per unit area increased substantially with treatment time. Amankwah and Ofori-Sarpong (2011) used SEM to observe microfractures after heating a gold ore to 735°C in a bench- scale microwave. A combination of both intergranular and transgranular fractures were observed.

For a simplified case, thermal stress is defined by Equation 9 where α is the thermal expansion coefficient, ∆T is the change in temperature, and E is the Young’s modulus (Didenko, et al., 2005). Given a higher ∆T between two mineral phases, Equation 9 will yield a higher thermal stress (σt). Mineral grains tend to have a high thermal expansion coefficient and Young’s modulus, and therefore rapid temperature changes can have a significant effect on fracturing (Santamarina, 1989; Rizmanoski, 2011).

𝜎푡 = 훼∆푇퐸 [9] Numerous modelling studies have been conducted to assess the most effective parameters to create fracture with microwaves. Salsman et al. (1996) investigated pulsed microwave treatment of ores using numerical modelling to predict the response of a single pyrite particle (good MW heater) in a calcite (poor

MW heater) matrix when subjected to microwave energy. Simulations yielded the highest stresses when short pulses of very high power density were used. Whittles et al. (2003) investigated the influence of power density on the strength reduction with the same pyrite-calcite matrix. The results described the use of higher power densities to maximize rapid stress generation within the ore, suggesting that there was less time for conductive heat transfer between active and transparent phases (Whittles, et al., 2003; Batchelor, et al., 2015). Jones et al. (2005) subjected the same simulations to a high electric field strength. The authors found that the maximum temperature attained increased linearly with power density, with power densities below 1 x 1010 W/m3 providing low thermal expansion and stresses. The modelling indicated that power density is a significant variable to generate temperatures required to fracture the material, and that the best results were achieved when heating times were minimized and power densities were high.

The mineral matrix selected in these studies however, is that of a good microwave absorber with a poor absorber, and is not representative of a more complex ore. It should be noted that adequate representation of fractures is not possible using this approach due to lack of ability to simulate irregularly- shaped particles found in ores (Ali & Bradshaw, 2010). The generation of these fractures would have various effects in an ore processing plant. The area which has gained the most interest is the ability to reduce comminution energy as a result of these fractures, discussed in the subsequent section.

3.6.2 Reduction in Comminution Energy

Reduction in comminution energy has been the main focus of microwave treatment applications.

As discussed, microwave treated ore can exhibit reduced competency as a result of thermal stress fractures.

For the mineral processing industry, these observed fractures could be used to address the high energy demands in current comminution processes.

This first paper related to microwave-assisted comminution was by Walkiewicz et al. (1991), who suggested the possibility for “microwave-assisted grinding”; one of the most highly referenced papers in the microwave and minerals literature. Iron ore samples were subjected to 3 kW of microwave power at

2.45 GHz with a laboratory-scale microwave oven, and temperatures reached up to 940°C. It was observed that the microwave treatment of the various iron ores reduced the Bond Work Index by 9.9% to 23.9% in comparison to untreated ore. The authors however, stated that the reported energy savings in comminution were not enough to justify the microwave energy costs.

Numerous studies assessing ore competency after microwave treatment followed Walkiewicz et al.

(1991). Table 3 outlines the relevant bench-scale microwave treatment results for 7 different ores: taconite

(Walkiewicz, et al., 1995); ilmenite (Harrison, 1997); clastic black (Gungor & Atalay, 1999); copper carbonatite (Vorster, et al., 2001; Sahyoun, et al., 2004; Kingman, et al., 2004); free-milling gold

(Amankwah, et al., 2005); copper-nickel (Henda, et al., 2005; Marion, et al., 2016); and lead-zinc (Ola-

Omole, et al., 2016). The input variables noted from each study include particle size, mass and microwave energy dosage, with the corresponding output of work index reduction. It should be noted that only two of these studies used a feasible microwave energy input at less than 2 kWh/t (Sahyoun, et al., 2004; Kingman, et al., 2004). While the data indicates that significant reductions in work index are possible using low powers, long treatment times and small samples, there is an absence of results in the literature using high- power microwaves – crucial if scaled up to a continuous process in the mining industry. Based on the previous discussion of fracture, maximizing power input and minimizing residence time has been suggested to maximize the effect of thermal stresses. This presents an opportunity to fill a void in the literature to replicate these enticing bench-scale results at the pilot-scale.

Table 3: Microwave-assisted comminution, effects on Work Index reduction from relevant literature.

MW Particle Energy WI Single mode/ Irradiation Mass Ore/Mineral Location MW Oven Power Size Input Reduction Reference Multimode Time (s) (kg) (kW) (mm) (kWh/t) (%) Minnesota, Cober (Walkiewicz, et Taconite Multimode 3.0 210 <3.36 0.35 500 18.2 USA Electronics al., 1995) Ilmenite Ore - - Multimode 2.6 30 - 0.25 87 84.0 (Harrison, 1997) Ҫayeki Gold Star MH- Clastic Black (Gungor & Deposit, 1075 Multimode 0.9 600 0.6 to 1.41 0.5 300 15.9 Copper Atalay, 1999) Turkey MD, Korea Copper Somincor, (Vorster, et al., - Multimode 2.6 90 - 0.5 130 70.0 Carbonatite Portugal 2001) Copper Palabora, Panasonic II (Sahyoun, et al., Multimode 2.6 30 - 1 21.7 65.1 Carbonatite South Africa 2600 2004) Copper Palabora, (Sahyoun, et al., Sairem, France Single mode 10 0.28 - 1 1.0 83 Carbonatite South Africa 2004) Copper (Kingman, et al., South Africa Sairem, France Single mode 15 0.2 - 1 1.0 >30% Carbonatite 2004) Conversion Free-milling Ghana, 0.212 to (Amankwah, et Technology Multimode 1.5 600 0.1 2500 18.5 gold ore West Africa 3.35 al., 2005) Corp., USA Microwave Copper- Sudbury, ON. Research (Henda, et al., Nickel Multimode 1.4 10 18 0.3 13.0 22% (RWI) Canada & Applications, 2005) Ore USA Copper- Voisey's Bay, (Marion, et al., Nickel Amana RC30 Multimode 3.0 60 <3.35 0.25 200 41.5 Canada 2016) Ore Galena- (Ola-Omole, et Sphalerite Itakpe, Nigeria Not Reported Multimode 0.75 90 0.6 0.3 62.5 34.3 al., 2016) Ore

3.6.3 Liberation

Aside from the above-mentioned benefits of reducing ore competency with microwave treatment, increased liberation has also been reported. Researchers claim that microwave induced fractures within an ore matrix selectively liberates minerals by intergranular thermal stresses (Batar, 2004). Enhanced liberation has been described as a very important part of any thermal treatment from a process economics viewpoint, and may be a requirement to make the process economically viable for most ores (Orumwense, et al., 2004). The effects of microwave treatment on liberation are reported as either an increase in liberated value minerals, a change in downstream recovery in either flotation or gravity concentration, or both.

Table 4 highlights the effects of microwave treatment on liberation in the literature for 7 different ores in the literature: lead-zinc (Orumwense, et al., 2004); copper carbonatite (Sahyoun, et al., 2004); copper-nickel (Henda, et al., 2005; Marion, et al., 2016); free-milling gold (Amankwah, et al., 2005); porphyry copper (Batchelor, et al., 2017); manganese (Singh, et al., 2017); and ultramafic nickel (Bobicki, et al., 2018). Input variables of particle size, mass and microwave energy dosage were reported. A study worth pointing out is by Henda et al. (2005), who reported an energy dose of 13 kWh/t from microwave treatment followed by grinding. However, the nickel and copper grades improved by 15% and 27% respectively, with a 26% increase in recovery. This would suggest that although reductions in ore competency alone may not be viable, downstream benefits may make the process economical. Generally, these studies concluded that differential heating between minerals promoted intergranular fracturing that improved liberation within the ore. As high-power, low treatment times have been suggested to maximize thermal gradients, it is reasonable to assume that the extent of intergranular fracturing from microwaves would further improve in a pilot-scale microwave system.

Liberation improvements as a result of microwave treatment can be used either to increase the liberation at the same grind size, or an equivalent liberation at a coarser grind size. This was reported by

Batchelor et al. (2017), who concluded in their study that the grind size could be increased by 50 to 60 µm while maintaining an equivalent liberation after microwaving a porphyry copper ore. Increasing the grind

27 size while maintaining an equivalent liberation would also impact the work index irrespective of changes to ore competency, due to an increased P80. This would also be advantageous as the required grind time would be reduced, and therefore the milling throughput could be increased (Ali & Bradshaw, 2009). On the other hand, maintaining the same grind size while increasing liberation would also be beneficial, as the improved recovery in the concentration steps would result in an increase in metal production while keeping process design parameters the same.

3.6.4 Additional Variables

Apart from a reduction in ore competency and improved liberation, additional benefits of microwave treatment have been reported by various researchers. Some of these include reduced wear to the mill, media and liner (Walkiewicz, et al., 1991). All of these items have significant impacts on operating costs in any mineral processing plant (Vorster, et al., 2001). Any number of these benefits would result in reduced capital and operating costs of mining equipment for current and developing mine projects.

Table 4: Effects of microwave treatment on liberation, recovery and grade from relevant literature.

Single Particle Ore Energy Reported Grade Location Microwave Power Time Recovery Mineral/Ore Mode Size Mass Dose Liberation Improve Authors of Ore Oven (kW) (s) Improvements /Multimode (mm) (kg) (kWh/t) Improvements ments New Cober (Orumwens 27.5% Pb-Zn Ore Brunswick Electronics - 3 240 3 2 100 85% increase 3.5% e, et al., (Zinc) , Canada Inc., USA 2004) Single Not (Sahyoun, 10 0.28 N/A 1 0.78 liberation Not reported Copper South mode reported et al., 2004) Sairem, France increased from carbonatite Africa Single Not (Kingman, 15 0.2 N/A 1 0.83 30% to 70% Not reported mode reported et al., 2004) Microwave Sudbury, Copper- Research and 15% Ni, (Henda, et ON, Multimode 1.4 10 18 0.3 13.02 Not reported 26% nickel ore Applications, 27% Cu al., 2005) Canada USA Conversion (Amankwah Ghana, Free-milling Technology .212 to Not 12% (Gravity & Ofori- West Multimode 1.5 600 0.1 2500 Not reported gold ore Corporation, 3.35 reported Separation) Sarpong, Africa USA 2011) 50 to 60 µm copper Porphyry Single 9.5 to coarser grain Not flotation (Batchelor, Chile Sairem, France 15 - - 2.1 copper mode 53 size (equivalent reported recovery of et al., 2017) liberation) 1% Voisey's Copper- Not (Marion, et Bay, Amana RC30 Multimode 0.8 120 <3.36 0.25 106.7 Not reported 33.6% nickel ore reported al., 2016) Canada Joda, Manganese LG Corp., Not (Singh, et Odisha, Multimode 0.9 300 <3.35 0.05 1500 Not reported 22.4% ore South Korea reported al., 2017) India Ultramafic Okanagan, Panasonic, .425 to 77.5% pn (up Not (Bobicki, et Multimode 1 900 0.1 2500 Not reported Ni ore WA, USA Japan 1 from 56%) reported al., 2018)

3.6.5 Current Potential for the Mining Industry 3.6.5.1 Scale-up and Commercialization Efforts In bench-scale tests reported in the literature, ores are subjected to long irradiation times (>> 1s, usually in the order of minutes) for small batch masses of ore (< 1kg) of a typical ball mill feed size, with high microwave energy inputs (>> 5 kWh/t). It is widely acknowledged in the literature that these high energy demands cannot be justified by the benefits of the treatment (Batchelor, et al., 2017). Further, these long residence times are not compatible with the high throughputs required in the mining industry (> 100 t/h). It has been reported that to effectively treat 100-1000 t/h of ore, a residence time of less than a second at high powers would be required to make the process commercially viable (Adewuyi, et al., 2020).

Therefore, the shift from low-power, high treatment time bench-scale studies to high-power, low treatment time pilot-scale studies is critical for successful scale-up.

To date, only a handful of continuous pilot plant studies for treatment of ore using a high-power microwave have been documented in the literature. While high-power microwave systems are used in the food industry, the application to ores has proved much difficult due to the heterogeneity of ores.

Preliminary modelling studies have suggested that microwave units operating at > 30 kW can induce significant grain boundary damage to ores (Jones, et al., 2007; Ali & Bradshaw, 2009; Bradshaw, et al.,

2011).

EMR Microwave Technology Corporation (EMR) operated a pilot-scale microwave system capable of 150 kW of input microwave power, designed to treat a refractory gold concentrate in a fluidized bed reactor to improve the liberation of gold (Tranquilla, et al., 1996). The study was designed to scale the process up to a 200 tonne per day operation, and the company suggested that their process could reduce the total operating costs from $47.32 USD/t to $8.60 USD/t. This technology was not used as a treatment for comminution, but to improve the gold recovery of a concentrate, however, it is one of the earliest high- power microwave ore application studies in the literature. Unfortunately, the company suffered many financial constraints and was not able to advance to full commercialization; thus the project ultimately folded.

The most recent and important progress made in this area of research is the work by the University of Nottingham, which involved the development and implementation of a continuous pilot plant microwave for the purposes of ore treatment (Batchelor, et al., 2017; Buttress, et al., 2017). The team demonstrated that the use of very high powers (100-200 kW) for short residence times can produce low energy doses of

0.7-1.3 kWh/t, thus maximizing the electric field intensity. By conducting various treatment tests using a porphyry copper ore, the authors concluded that the grind size could be increased by 40 to 70 µm while achieving the equivalent liberation as the untreated ore. Furthermore, a decrease of 3 to 9% in Bond work index was reported by the researchers, with a potential throughput increase of up to 10% at nominal plant grinds. These results are significant, as improvement in grind size and specific energy requirements in milling have direct implications for grinding mill throughput. The authors of these studies however, only published results testing porphyry copper ores, and it would be of interest to investigate the impacts on other ores. In this work a single mode applicator was used, and the authors suggested that any new applicator for further scale-up work should be designed to support a more homogenous electric field distribution, with the peak power density accounting for the majority of the applicator cross-sectional area; i.e., a multimode applicator.

More recently, Targeted Microwave Solutions (TMS), operated a continuous pilot plant, claiming to be capable of upgrading low rank coals using microwaves to remove moisture and improve the BTU of the dried coal. The company used a microwave system with eight 100 kW microwave generators together with a vertical chute for the coal to pass through. The control system designed for the process monitored moisture, belt speed and input microwave power, which could be adjusted for the processed material to meet the required upgrading specifications (Young & Lawson, 2017). Despite the process reaching optimization, similar to EMR, the process at TMS was never commercialized with financial constraints requiring the project to fold. Like EMR, this project was not proposed as a treatment in comminution, but it is an example of a reported high-power microwave application.

Despite some promising results from scale-up studies reported in the literature, the technology has not been adopted by the mining industry. The challenges that impair progress are the need for a further

31 understanding of microwave fundamentals, the need for multidisciplinary research teams, microwave engineering expertise and more pilot-scale demonstrations of the method for the most widely mined ores

(Kingman, 2018). The work from the University of Nottingham represents the furthest progression of the technology to date.

3.6.5.2 Economics of Microwave Treatment Despite the potential benefits of microwave treatment, the economics of the technology are still unproven. Unfortunately, most bench-scale microwave treatment studies conclude that the high energy requirements are not economically viable (Walkiewicz, et al., 1991; Henda, et al., 2005). In the low-power high treatment time experiments, the order of magnitude of input microwave energy, in kWh/t of ore, is often reported in the thousands, which is incredibly inefficient. While these conditions are suitable for producing positive effects at the bench-scale, they are not scalable from an industry standpoint. Bradshaw et al. (2007) suggested the economic range for microwave input energy is between 1 to 5 kWh/t after completing a techno-economic modelling assessment of the technology. Out of the reported bench-scale studies in the literature, three were in this economically feasible range with energy doses of equal to or less than 5 kWh/t (Sahyoun et al., 2004; Kingman et al., 2004; Batchelor et al., 2016). As previously discussed, modelling studies suggest that to improve the economics, higher power densities and electric field intensities are preferred to increasing exposure time, as throughput can be maximized (Bradshaw, et al.,

2011). Therefore it must be demonstrated that this shift in process parameters can still produce similar results seen in the bench-scale literature.

Perhaps the most drastic effect on process economics is the potential to improve liberation of value minerals, which would improve downstream processes by increasing the recovery of metals (Kingman &

Rowson, 1998). Despite this, a comprehensive study has not been conducted to assess the overall economics when considering a full cycle of metal production (Adewuyi, et al., 2020). A full consideration of economic viability of the process at all stages of development, not just the comminution circuit, is critical in understanding the true economic value of the technology (Bradshaw, et al., 2007).

3.7 Effects on Downstream Processing Due to the promotion of intergranular breakage, increased liberation as well as improved surface area of value minerals would be expected. Clearly an increase in surface area can have positive impacts on downstream processes, providing additional benefits from microwave treatment over and above reductions in energy consumption. Previous work in this area, in part, supports this idea of improved surface area after microwave treatment. Table 5 shows the relevant studies that reported improvements to surface area, in m2/g, after microwave treatment when compared to reference samples. Surface area for each of these studies was measured using the Brunauer-Emmett-Teller (BET) method.

Table 5: Summary of surface area improvements from microwave treated ores and minerals in the literature.

Particle Size in Energy Dose Surface Area Mineral/Ore Authors Treatment (kWh/t) Improvement Bornite 37% Chalcopyrite 844% <2 mm 347 (Harrison, 1997) Magnetite 172% Pyrite 74% Ultramafic 3.2x compared to Ni Ore #1 reference -1.0+0.45 mm 2500 (Bobicki, et al., 2018) Ultramafic 1.7x compared to Ni Ore #2 reference Chalcopyrite -150+38 µm 833 30% (da Silva, et al., 2018) 7% for +75 µm mill Manganese- product Not reported 444 (Huang & Liu, 2021) Iron Ore 18% for -75+38 µm mill product

Significant increases in surface area for different microwave treated minerals and ores can be observed from the literature studies. The largest improvement in surface area was 840% for treated chalcopyrite, observed by Harrison (1997), and attributed to the fracturing of the mineral surface and a decrease in particle size after treatment. In the work of da Silva et al. (2018), both surface area and pore width were reportedly higher after treatment. Also worth mentioning is the study by Huang and Liu (2021), reporting a treated manganese iron ore with an 18% improvement in surface area after grinding. The authors also reported increased pore volume and diameter, similarly to da Silva et al. (2018).

Clearly, surface area can be positively affected by microwave treatment. The fracturing and enhanced particle breakage, reported in the previous sections, effectively weakens the microstructure of ores, allowing for this benefit after grinding. The majority of these characterization studies however, are focused on copper bearing minerals/ores. Also, there are few studies related to any microwave ore application which report the surface area of treated samples. Therefore, the data suggests an opportunity to further investigate the influence of microwave treatment on surface area and the potential implications these effects would have in downstream processes.

In leaching processes, the mass transport of lixiviants from the bulk solution to the mineral surface is usually the rate determining step. Associated gangue minerals further reduce the rate of mass transport of lixiviant. Therefore, the liberation of value minerals by comminution is crucial in exposing the minerals to lixiviant attack. Where agreements by researchers have been made regarding the creation of further surface area by fracturing ore with microwave treatment, there would exist the potential for improved reaction rates, recovery and reduced reagent consumption in leaching processes. Previous studies on the kinetics and recovery of treated ore confirm this.

Table 6 shows a summary of relevant studies from the literature which reported an improvement in leaching after microwave treatment when compared to reference samples for 6 ores: free milling gold

(Amankwah & Ofori-Sarpong, 2011); copper sulphide (Schmuhl, et al., 2011; Moravvej, et al., 2016); copper oxide (Moravvej, et al., 2016); zinc (Charikinya & Bradshaw, 2016); and gold/silver (Seflek &

Bayat, 2018). Microwave conditions are provided with recoveries for reference and treated ores. These studies all show significant improvements in recovery after microwave treatment. The most significant of these was from Schmuhl et al. (2011), who reported a 42.6% increase in copper recovery after just 15 seconds of microwave treatment using a pulsing microwave field. The authors concluded that the treatment increased the porosity of the ore particles, exposing locked copper bearing minerals to the lixiviant.

Cyanidation of treated gold ores displayed improved recovery as well (Amankwah & Ofori-Sarpong, 2011;

Seflek & Bayat, 2018). Seflek and Bayat also reported a 9.1% improvement in silver recovery in addition to gold. Improved gold recovery was concluded due to improved mass transport from microfractures.

Table 6: Summary of microwave treatment and subsequent leaching tests from the relevant literature, highlighting input energy dose and showing changes to recovery.

Energy Increase in Power MW Oven & Mass Conventional MW Treated Ore Type Main Minerals Location Time (s) Dose Recovery Reference Notes (kW) Conditions (kg) Recovery (%) Recovery (%) (kWh/t) (%)

Magnetite, -Hinari Model (Amankwah & -Leached using bottle roll Free-milling hematite, Tarkwaian Rock MX702, 2.45 GHz Ofori- technique 0.7 300 0.01 5833 68.0 92.0 26.1 Gold Ore aluminosilicate, System, Ghana -Variable power input Sarpong, -26.09% increase after silica up to 800W 2011) leaching for 8 hours.

Sarcheshmemh -Study reported lower acid Alumina, silica, -Myson928 single- Copper Copper Mine, (Moravvej, et consumption in the treated ore vs. 4% sulphur, 5% 0.9 300 mode applicator, 0.105 714 6.1 8.2 26.0 Sulphide Kerman al., 2016) non-treated by 28.8%. iron 2.45 GHz, 900W province, Iran -Recovery increased by 2.5x.

-Myson928 single- Sarcheshmemh Alumina, silica, mode applicator, -Study reported lower acid Copper Mine, (Moravvej, et Copper Oxide 4% sulphur, 0.9 300 2.45 GHz 0.105 714 84.8 93.7 9.5 consumption. Kerman al., 2016) 6% iron -variable power input -Leached using bottle roll. province, Iran up to 900W

Silica, feldspar, -TE10n single-mode Disseminated muscovite; applicator, -Copper yields estimated from (Schmuhl, et Copper Chalcopyrite, Not reported 5.6 15 2.45 GHz 0.05 467 31.0 54.0 42.6 recovery graph. al., 2011) Sulphide Ore chalcocite, -Variable power input -Leached using bottle roll. bornite up to 3kW

Gamsberg Zinc (Charikinya & Sphalerite, Not Not -Single-mode Zinc Ore Mine, South 0.4 to 1 2 to 5 74.0 93.1 20.5 Bradshaw, -Bioleaching pyrite, quartz reported reported applicator, 2.45 GHz Africa 2016)

-Silver recovery increase from Gold/Silver Bolkardag, -Siemens HF24G241, (Seflek & Silica, hematite 0.6 1800 0.25 1200 74.9 80.6 7.1 80.9% to 88.3% Ore Turkey 2.45 GHz Bayat, 2018) -Leached by bottle roll.

Figure 9 shows the recovery versus cyanidation time curve from the Amankwah and Ofori-Sarpong

(2011) study. At just 12 hours, most of the gold was leached resulting in a recovery of 97%, whereas the as-received sample had a reported recovery of only 92% after 24 hours. Therefore not only was there an improved gold recovery, but the kinetics were largely improved as well. This figure shows just how important the potential downstream impacts of this technology are when assessing the economics. If a similar trend could be reproduced at the pilot level, there would be further incentive to implement this into existing processes. Leaching kinetics are directly tied to throughput and thus metal production; therefore throughput would also be able to be increased, resulting in huge increases in pay metal production.

Figure 9: Gold recovery as a function of leach time for both as-received and microwave treated gold ore (Amankwah & Ofori-Sarpong, 2011).

Only the work of Charikinya and Bradshaw (2016) reported economical energy inputs of 2 to 5 kWh/t, with a 20.52% increase in recovery when treating a zinc ore subjected to subsequent bioleaching.

The remaining studies used large microwave energy inputs similar to the comminution studies discussed previously. While uneconomical, there is a clear agreement between researchers that microwave treatment can not only reduce comminution energy, but also improve leaching processes for different ores. As these studies have not progressed further than the bench-scale, a more thorough investigation should be done on the feasibility of leaching ores treated at higher powers to confirm these results. It has been stated in the literature that the higher electric field intensities associated with pilot-scale microwaves should result in greater thermal gradients and stresses, which should ultimately improve the effects on comminution. Due

36 to the improvements in surface area shown, which has also been attributed to microfractures, it would be logical to assume that the bench-scale leaching results could be further improved as well.

There have been two studies that assess the impacts of microwave treatment on leaching reagent consumption found in the literature. While the number of studies is limited, they still demonstrated an improvement.

Moravvej et al. (2016) reported lower acid consumptions in leaching for microwave treated copper oxide and sulphide ores in comparison to conventional leaching. Copper oxide and sulphide ores with sample masses of 105 g were irradiated with a at 900 W and 2.45 GHz for 5 minutes. The oxide and sulphide ores were leached for 8 and 10 days, respectively. Sulphuric acid consumption was reported to be

37.69 kg H2SO4 and 52.78 kg H2SO4 per ton of sulphide ore for treated and as-received samples, respectively, and 128.22 kg H2SO4 and 143.31 kg H2SO4 per ton of oxide ore for treated and as-received sample respectively. It was concluded that microwave treatment could decrease the acid consumption of these sulphide and oxide ores, realizing a 28.8% and 10.5% decrease in acid consumption for the sulphide and oxide ore, respectively. The authors reported that due to the increased surface exposure of the copper phases after microwave treatment, sulphuric acid lixiviant could attack the minerals with increased surface exposure and react with them more rapidly.

Similarly to Moravvej et al. (2016), Seflek and Bayat (2018) found that microwave treated gold ore samples exhibited a decrease in cyanide consumption. Gold ore with a size range of -8 + 4.7 mm was subjected to microwave treatment at 600 W and 2.45 GHz for 30 minutes, then leached at a cyanide concentration of 2 g/L for 77 hours. The authors noted that the total cyanide consumption was 1.84 g/L for the as-received sample, whereas the microwave treated sample showed a total cyanide consumption of 1.62 g/L, a 12.0% decrease in total cyanide consumption. The authors concluded that the increase in microfractures after irradiation allowed for improved gold exposure when subjected to cyanidation.

With the outlined literature clearly suggesting that microwave treatment can be beneficial to leaching, there has been no research on the impact of microwave treatment on downstream roasting processes. While the use of microwave treatment technology in comminution is attractive due to the

37 benefits of breakage and fracturing of an ore, changes in roasting may be more related to chemical changes such as changes of phase rather than physical changes to the ore. Roasting reactions are rapid and exothermic; leaching reactions usually take hours or days to achieve adequate metal recovery, compared to roasting which takes minutes or seconds. It may be that structural changes to the ore such as improved surface area and porosity may have a positive effect on roasting, although it likely cannot be accurately measured with current techniques. For commodities such as copper and zinc which rely on roasting for extraction, the effects of microwave treatment should be studied.

The nature of long heating times at low powers with bench-scale microwave heating results in high temperatures and ultimately phase changes – notably the oxidation of sulphide phases when heated in air (Barbu, et al., 2020). Furthermore, microwaves have been used as an alternative to roasting by various researchers. Amankwah and Pickles (2009) reported lower specific energy consumptions when microwaving a refractory gold concentrate in a bench-scale unit when compared to conventional roasting.

More recently, Amankwah and Ofori-Sarpong (2020) reported better oxidation after microwaving a refractory gold concentrate, which resulted in higher gold recovery. While these are enticing results, these studies utilize the microwaves primarily for heat generation and phase change rather than fracture. Perhaps the absence of studies related to microwave treated ore on roasting is due to the lack of impact physical changes would have on the process. It also may be that the time required to generate enough treated ore to produce a concentrate for roasting has limited such studies. Due to the location of the microwave treatment proposed in this project however, and the desire to investigate the effects on a downstream solid-gas reaction in addition to a solid-liquid reaction, it was decided this was worthy of investigation.

Any potential effects that microwave treatment may have on roasting is merely speculative at this point. If a certain degree of oxidation occurs during the treatment, a reduced amount of sulphur would be expected in the roaster feed. For a reduced sulphur content in the feed, the required time to achieve the same oxidation degree would be reduced as well. This could result in throughput improvements. While no work has been done that would indicate this can occur, it is important to understand the full downstream implications of the preceding microwave process.

Chapter 4

Experimental 4.1 Materials

4.1.1 Gold Ore The gold ore used was obtained from the Moose River Gold Mine operated by Atlantic Gold, located in Nova Scotia, Canada. Mineralogy by Quantitative Evaluation of Minerals with Scanning

Electron Microscopy (QEMSCAN) is given in Table 7. An average gold grade of 1.04 g/t was determined by fire assay. The minerals associated with the gold are the sulphides, namely pyrrhotite (1.65%), pyrite

(0.39%) and arsenopyrite. Arsenopyrite was not detected, but about 0.25% arsenic was shown by ICP.

Table 7: Mineralogy of the as-received gold ore using QEMSCAN.

Mineral Assay (wt. %) Chalcopyrite 0.02 Pyrite 0.39 Pyrrhotite 1.65 Other Sulphides 0.34 Fe Oxides 0.65 Carbonates 4.23 Quartz 35.42 Chlorite 18.20 Micas 35.59 Feldspars 1.97 Amphibole 0.02 Rutile 0.86 Other 0.56 Sum 100.00

Each of the gold associated sulphide minerals are good microwave absorbers. On the other hand, the gangue minerals are mostly complex silicates, hydroxides and carbonates, which are either poor microwave absorbers or completely transparent. Therefore, this ore should be suitable for microwave treatment due to the difference in heating between value and gangue minerals. XRD revealed that the chlorites and micas present were clinochlore and muscovite respectively. The acquired XRD plot is given in Figure 34 of Appendix D. XRF and ICP results are also given in Appendix D.

4.1.2 Copper-Nickel Sulphide Ore The copper (low-nickel) sulphide ore used for this research was provided by Glencore, from the

Strathcona Mill in Sudbury, Canada. Table 8 gives the QEMSCAN mineralogy. The main sulphides are chalcopyrite (2.62%), pentlandite (0.64%) and pyrrhotite (0.30%), with about 4-5% total sulphides.

Table 8: Mineralogy of the as-received sulphide ore using QEMSCAN.

Mineral Assay (wt. %) Pentlandite 0.64 Chalcopyrite 2.62 Pyrrhotite 0.30 Pyrite 0.07 Magnetite/Ilmenite 1.87 Quartz 9.28 Feldspars 47.96 Micas 10.55 Chlorite 6.94 Pyroxene 0.48 Epidote 7.03 Amphibole 9.52 Titanite 1.41 Other 1.33 Sum 100.00

The value minerals in this ore include both chalcopyrite and pentlandite; these sulphide minerals are good microwave absorbing minerals and therefore selective heating should be promoted along these mineral grains. Additionally, there is 1.87% iron oxides, which are also excellent microwave absorbers.

Therefore in comparison to the gold ore, the sulphide ore would expect more rapid heating due to the higher percentage of microwave absorbing minerals. Feldspars make up the bulk of the gangue, which are effectively transparent to microwaves. Smaller amounts of complex silicates and hydroxides are also present. It has been suggested that 2-20 wt. % of good microwave absorbing minerals present with hard gangue such as quartz and feldspar is best suited for microwave treatment (Batchelor, et al., 2015). Based on this definition, this sulphide ore is an excellent candidate for this work. XRD showed that the micas and chlorites detected by QEMSCAN were annite and clinochlore respectively. The XRD plot is given in

Figure 35 of Appendix D. XRF and ICP results are also given in Appendix D. Selected sample photos for both the gold ore and the sulphide ore are given in Appendix H.

4.2 Ore Sample Preparation The ores were received as a bulk sample at Sepro Mineral Systems’ (British Columbia, Canada) facility, weighing on average several tonnes with varying particle sizes. Size reduction, sample splitting and particle size classification were performed to produce batches and subsamples for microwave treatment tests. Figure 10 shows the flowsheet followed for the bulk sample preparation of each ore.

Figure 10: Sample preparation flowsheet for the preparation of bulk samples for microwave treatment experiments. 4.2.1 Size Reduction The +63.5 mm received ore was crushed to 100% passing 63.5 mm using a 6” by 10” portable jaw crusher from Pavestone Creations Ltd. (Halifax, Canada). Twenty-six kilograms of -3.35+1.7 mm material was made by further crushing for bench-scale microwave tests.

Ground ore was produced using a laboratory Bond Ball Mill. The mill is a standard 12x12 inches with rounded corners and runs at approximately 70 rpm with a grinding media load of 285 balls weighing approximately 20 kg. The Bond Ball Mill was used in comminution test procedures as well as the generation of fine material for downstream tests. Pulverizing of milled ore samples for assay and other characterization work was conducted using a ring and puck mill for particle sizes finer than 75 µm.

4.2.2 Particle Size Classification and Sample Splitting

The crushed ore from the bulk samples was classified by scalping using a 25.4 mm screen and a

63.5 mm screen. Ore crushed to -25.4 mm was removed, and ore +63.5 mm was recycled back into the jaw crusher. The overall goal was to produce two separate size fractions: -25.4 mm and -63.5+25.4 mm.

The -63.5+25.4 mm material was then split into batches by cone and quartering, with each batch weighing approximately 35-45 kg. The quantity of batches produced was proportional to the amount of initial bulk sample received. For the two ores used in this study, four gold ore batches and thirty-two batches of copper sulphide ore were made.

Following splitting, the particle size distribution was determined to ensure representative batches were obtained. This was performed using a Gilson Testing Screen to vigorously shake and distribute the mass according to particle size.

Finer sample preparation of -3.35+1.7 mm ore was done using a sequence of both rotary and riffle splitting to make the following sample quantities: eighty 100 g sample bags, one 10 kg sample and 8 kg of leftover material. In addition, a handful of prepared 100 g samples were split into smaller 50 g subsamples.

For finer samples, either an 8-inch or 12-inch W.S. Tyler Ro-Tap sieve shaker was used for particle size distributions.

4.3 Characterization Methods

4.3.1 Inductively Coupled Plasma Optical Emission Spectroscopy (ICP) Inductively Coupled Plasma Optical Emission Spectroscopy (ICP) at XPS in Falconbridge, ON, was used to determine the chemistry of the bulk samples for elements: Al, As, Ca, Cu, Fe, Mg, Ni, S and

Si.

4.3.2 X-ray Diffraction Analysis (XRD) Qualitative X-ray Diffraction Analysis (XRD) from XPS was used to find the mineralogical phases in the ores.

4.3.3 X-ray Fluorescence (XRF) X-ray Fluorescence (XRF) analysis was done on each ore. The XRF used was a 9430 048 00111

Epsilon 1 model, manufactured by Malvern Panalytical. The XRF was calibrated using data from ICP.

4.3.4 QEMSCAN Fine ore from the grinding tests was analyzed using Quantitative Evaluation of Minerals with

Scanning Electron Microscopy (QEMSCAN) at XPS. The sample was split, mounted in epoxy, polished and then imaged. This method calculates mineral composition, mineral association, and the number and size of particles. The liberation degree of target minerals is also calculated, and the particles observed are grouped into liberation classes based on area percent values as follows: locked (<30%), low grade middling

(30 to 80%), high grade middling (80 to 95%), liberated (>95%) and free (100%).

4.3.5 TGA/DTA Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) was conducted at

McMaster University using a Netzch STA 449 machine to measure the mass loss and detect phase transformations. The process involved heating up a 40 mg pulverized sample to 1200°C at 10°C/min, under argon at 40 mL/min.

4.3.6 Permittivity The real and imaginary permittivities of the two ores selected for this study were measured by

Microwave Properties North using the cavity perturbation technique (Hutcheon, et al., 1992). Pulverized samples were compacted into 300 mg briquettes. Temperature was increased from ambient to 1000°C under argon, and the permittivities were measured at 25°C, 100°C and thereafter every 50°C until 1000°C.

4.4 Microwave Treatment Systems & Procedures Two systems were used to treat the ores in this study: a low-power bench-scale microwave unit, and a high-power pilot-scale microwave unit. The microwave systems are located at Sepro Laboratories, in Langley, B.C. A more detailed description of the two systems and the treatment processes used for each system is given in Appendix E. Bulk temperature was measured using a Type-K thermocouple for the batch treatments and surface temperature was measured using a handheld FLIR infrared camera for all tests.

4.4.1 Batch Low-Power Microwave System 4.4.1.1 Microwave Heating Behaviour

Microwave heating behaviour tests were performed using a set power for various irradiation times.

Both 50 g and 100 g prepared samples of -3.35+1.7 mm were placed into a quartz crucible and irradiated for 0.5, 1, 2, 4 and 8 minutes. After treatment, the quartz crucible was quickly removed and the temperature was measured. A temperature versus time plot was then generated.

4.4.1.2 Low-Power “Batch Bench” Microwave Treatments, BB

Low-power bench-scale treatment tests were done on each ore to complement the pilot-scale work and to compare with similar work reported in the literature. Once the microwave heating behaviour curve was made, an irradiation time was selected to generate enough material for grinding tests. Four minutes of microwave treatment was chosen for the eighty 100 g batch samples. Each sample was heated at maximum power, 3.2 kW, and the change in sample mass was determined after each test. The material was then combined for work index testing. These tests are described with the acronym BB.

4.4.2 Pilot-Scale Microwave System 4.4.2.1 High-Power “Batch Pilot” Microwave Treatments, BP

Batch pilot-scale treatment tests were done to provide a comparable result to the bench-scale work, but at a higher power and sample mass. The prepared 10 kg sample of -3.35+1.7 mm was placed in a polypropylene bucket, positioned at the bottom of the applicator below the conveyor belt with a set power of 75 kW. The gold ore was irradiated for 45 seconds and the sulphide ore for 30 seconds. Once the irradiation time expired, the microwave system was shut off and the sample bucket was removed for bulk temperature measurements. The sample was then set aside for work index testing. These tests are described with the acronym BP.

4.4.2.2 High-Power “Continuous Pilot” Microwave Treatments, CP

Ultimately, the continuous high-power treatments were the key tests in this study. In the continuous microwave treatment tests the sample trays, holding 35-45 kg of ore with a size range of -63.5+25.4 mm,

44 entered and exited the applicator while heating in a 1-4 second timeframe, treated at 75-150 kW. Additional

-12.7 mm ore was used for packing to reduce void spacing, as this helped to mitigate arcing (Holmes, et al.,

2020). Arc quantity was measured in the pilot system using a built-in arc sensor. Input variables including conveyor belt speed, conveyor belt acceleration and power input were selected. The operating parameters were then entered into the control panel, which created the conditions for the microwave test. During the treatments, forward and reflected power were measured using directional couplers – the data was displayed in the control panel, accessible after the test was finished. After treatment, surface temperature was measured and the sample was set aside for work index testing. These tests have the acronym CP.

4.4.3 Summary of Microwave Treatment Tests A summary of the treatment methods used in the experiments are outlined in Table 9, with the details of the reference or as-received material prepared for comparison with corresponding test labels. RB denotes the reference for the -3.35+1.7 mm BB and BP tests, and RP denotes the reference for the CP test, with a size range of -63.5+25.4 mm.

Table 9: Summary of microwave treatment methods used in these experiments for both the gold ore and the sulphide ore.

Input Microwave System Particle Treatment Frequency Treatment Power (Bench/Pilot) Size (mm) Time (s) (MHz) (kW) Reference Bench (RB) - -3.35+1.7 - - - Batch Bench (BB) Bench -3.35+1.7 3.2 240 2450 Batch Pilot (BP) Pilot -3.35+1.7 75 30-45 915 Reference Pilot (RP) - -63.5+25.4 - - - Continuous Pilot (CP) Pilot -63.5+25.4 75/150 1-4 915

It should be noted that only a single sample was produced of each ore for each microwave treatment condition. While it would be advantageous to assess the reproducibility of the potential benefits, limitations regarding sample quantity and timescales required for experimental work and analysis made this unfeasible.

4.5 Comminution Assessment The SAG mill test procedure, known as the SAGDesign test (Starkey & Associates Inc), was used for comminution assessment. The test procedure determines the grinding energy required for a SAG mill to comminute material of feed size -25 mm with an F80 of 19 mm, to a product size P80 of 1.7 mm. The end results provide a SAG mill work index, WSDT, expressed in kWh/t. The full SAGDesign test procedure also includes the Bond Work Index test after the SAG mill, determining the WBWI. These tests were used to determine whether a reduction in WSDT or WBWI would occur for treated ore. A summary of the comminution work is described in Table 10. A standard BWI was done on the -3.35+1.7 mm ore from the

RB, BB and BP tests, and the full SAGDesign test was done on the -63.5+25.4 mm ore from the RP and

CP tests. The comminution assessment the two ores is given in Section 5.3.2.

Table 10: Summary of comminution test work completed on each as-received and microwave treated ore.

Treatment Microwave System Particle Comminution Test Used (Bench/Pilot) Size (mm) (SAGDesign/Standard BWI) Reference Bench (RB) - -3.35+1.7 Standard BWI Batch Bench (BB) Bench -3.35+1.7 Standard BWI Batch Pilot (BP) Pilot -3.35+1.7 Standard BWI Reference Pilot (RP) - -63.5+25.4 SAGDesign Continuous Pilot (CP) Pilot -63.5+25.4 SAGDesign

A closing screen size of 106 µm was used after each grind cycle. Additional grind cycles were completed at the end of the test to generate more -106 µm material for further downstream work. All characterization and downstream work was done using ground ore from each of the five respective subsamples.

4.6 Surface Area Analysis Surface area was measured to assess the effect of fracture of value and gangue minerals of the microwave treated samples. As surfaces are normally created by grinding the ore, it was decided that assessing the surface area of the ground -106 µm samples from the work index tests was ideal. Surface area was measured using the BET method, a non-destructive technique that can determine the specific surface area of a fine sample by the physical adsorption of a gas, usually nitrogen, onto the surface of

46 mineral particles. The resulting amount of adsorbate gas on the surface of a single layer of molecules can then be used to calculate the surface area. The adsorption was carried out at the boiling point of the liquid nitrogen. The BET adsorption isotherm is described in Equation 10 (Naderi, 2015).

1 퐶 − 1 푃 1 [10] = ∗ + 푃 푉 ( 0 − 1) 푉푚퐶 푃0 푉푚퐶 [ 푎 푃 ]

P and P0 are the partial vapour pressure, in Pascals, of adsorbate gas in equilibrium with the surface and the saturated vapour pressure of adsorbate gas. Vm is the volume of gas adsorbed at standard temperature and pressure to produce a monolayer on the sample surface; Va is the volume of gas adsorbed in milliliters and C is a dimensionless constant. A value of Va was measured at three separate values of

P0/P, then the left of Equation 10 was plotted against P/P0. The slope was then used to calculate a value for

Vm.. The specific surface area was calculated by Equation 11, where m is the sample mass of and Na is

Avogadro’s constant (Naderi, 2015).

푉 푁 [11] 푆푆퐴 = 푚 푎 푚 ∗ 22400

A small amount (1-2 g) of -106 µm ground ore was used for each test sample. The sample had to be outgassed at 50°C under a vacuum 24 hours prior to nitrogen adsorption. Each test was done in the

Queen’s University Chemical Engineering Department, Kingston, ON. The results of the specific surface area measurements are given in Section 5.3.1.

4.7 Downstream Test Work

4.7.1 Cyanide Leaching Cyanide leaching was done to determine the effect of gold recovery between reference and microwave treated samples. The -106 µm gold ore product generated from the five grinding tests, RB, BB,

BP, RP and CP, were leached with cyanide using the bottle roll technique. The bottle roll setup used is shown in Figure 11. Two 300 g subsamples of each ground product were leached for 24 hours, one at 1.0 g/L NaCN and the other at 0.5 g/L NaCN. The tests were done in 2.5 L bottles with a target pH of 10.5-11 and 40% solids density. Solution samples were taken at intervals of 1, 3, 6 and 24 hours for gold analysis

47 by atomic absorption spectroscopy, completed by MSAnalytical in Langley, B.C. All leaching tests were completed at Sepro Laboratories. The results of the leaching tests are given in Section 5.3.3.

Figure 11: Bottle roll setup used at Sepro Laboratories for cyanide leaching.

Lime (CaO) was added intermittently to stabilize the pH above 10.5, and dissolved oxygen was measured throughout each test. Vacuum filtration was used to separate 10 mL of the aqueous phase of the slurry from the suspended solids at each time interval. The aqueous phase was then titrated, and NaOH was added with a few drops of 5-[4-(Dimethylamino)-benzylidene] rhodamine indicator into an Erlenmeyer flask and 0.01 M AgNO3 was used as the titrant.

4.7.2 Sulphide Oxidative Roasting Oxidative roasting was done on the sulphide ore to assess the impact of reference and treated material. Two 50 g samples from each of the five mill products were roasted using a laboratory scale fluidized bed roaster. First, the sulphide samples were heated under a nitrogen atmosphere in the reactor to keep the powder from packing. Then the ore was subjected to a 15-minute roast in air once the target temperature was achieved. Two roast temperatures of 1050°C and 1100°C were used for each, resulting in

10 roasts in total. During the roast, the temperature was monitored throughout using a thermocouple. After

15 minutes, the sample temperature was reduced under nitrogen to room temperature. The sulphur contents of both the initial sample and the roasted calcine were determined using an ELTRA elemental analyzer.

All roasting work was completed alongside Kingston Process Metallurgy, (Kingston, ON). These results can be seen in Section 5.3.4.

Chapter 5

Results and Discussion In this chapter, the findings of this thesis are presented. The results and discussion was split into three subsections: Section 5.1 discussed the response characterization of the ores to microwaves with permittivity data (ε’ and ε’’) and TGA; in Section 5.2, the impact of the microwave treatments was studied; in Section 5.3, the impact of the microwaves on selected downstream unit operations was discussed.

5.1 TGA and Permittivity

In order to characterize the microwave responses and heating behaviours of the two ores, their permittivities were measured as a function of temperature and frequency. Figure 12 shows the real (A) and the imaginary (B) permittivities for both ores at 912 MHz and 2466 MHz as a function of temperature. The starting values at room temperature of ε’ were 3.81 and 4.12 and for ε’’ they were 0.024 and 0.036, for the gold and sulphide ore respectively. Starting at room temperature, the permittivities increased slowly up to about 400°C. Here, most of the free and crystalline water was removed, resulting in a much more rapid increase above 400°C until around 700°C. At 700°C the gold ore exhibited a slight decrease in permittivity until about 850°C. The permittivity here increased very slowly and then more rapidly up to 1000°C. In a similar manner, the permittivities for the sulphide ore increased slowly from room temperature and then more rapidly above 400°C, indicating the removal of free and crystalline water. Once more at about 750°C, the permittivities levelled off before increasing very rapidly around 900°C. The highest values of ε’ were

5.23 and 5.64 and for ε’’ they were 1.10 and 1.02, for the gold and the sulphide ore respectively. The dielectric data showed that the microwave responses of these ores was quite similar, although the sulphide ore values were higher than the gold ore in comparison. This was due to the compositional similarities of the ores (chlorites, micas and sulphides). Worth noting was the lower frequency of 912 MHz which reported higher permittivities than 2466 MHz, indicating that the pilot treatments in this study (915 MHz vs. 2.45 GHz for bench) will show more rapid heating.

Figure 12: Real and imaginary permittivities as a function of temperature for the frequencies 912 MHz and 2466 MHz with a bulk density of 2.06 g/cm3.

Thermogravimetric analysis (TGA) was also done to determine if any of the changes in permittivity could be related to mass loss of the sample. Figure 13 compares the TGA and real permittivity for both the gold ore (Figure 13 A) and the sulphide ore (Figure 13 B) at a heating rate of 10°C/min and a frequency of

912 MHz. The main observations from this data are that two inflection points occur at approximately 580°C and 700°C, indicating dissociation reactions of one or more of the high wt. % minerals in the ore. Also, there is a rapid increase in mass loss around 400°C, likely due to the removal of crystalline water, resulting in a decomposition product with higher permittivity. To predict the reactions taking place, the differential thermogravimetric analysis (DTGA) was plotted with the permittivities.

Figure 13: TGA at a heating rate of 10°C/min and real permittivity (912 MHz) at a powder bulk density of 2.06 g/cm3 for the gold ore (left) and sulphide ore (right).

Figure 14 shows the DTGA data with the real and the imaginary permittivities for the gold ore

(Figure 14 A) and the sulphide ore (Figure 14 B) as a function of temperature. The gold ore shows two clear peaks where the rate of mass loss was highest; about 580°C and 700°C. The sulphide ore exhibited a smaller peak at the same temperature of 580°C, however no significant change at 700°C. These peaks are directly aligned with changes in permittivity and the inflection points noted in the TGA. Additionally, the sulphide ore showed a large rate of mass loss initially, indicating a large amount of free water removed.

The peaks for both ores in Figure 14 at 580°C are the dehydroxylation reaction of the chlorite, clinochlore, which was confirmed by qualitative XRD. The much greater rate of mass loss for the gold ore is explained by the higher wt. % of clinochlore present; 18.20%, in comparison to the sulphide ore which has only

6.94%. The literature confirms that clinochlore dehydroxylation occurs at roughly 550°C (Zhan &

Guggenheim, 1995), and is given by Equation 12 (Cho & Fawcett, 1986).

5푀푔5퐴푙2푆푖3푂10(푂퐻)8 → 10푀푔2푆푖푂4 + 푀푔2푆푖5퐴푙4푂18 ∙ 푛퐻2푂 + 3푀푔퐴푙2푂4 + (20 − 푛)퐻2푂 [12]

Figure 14: DTGA and the real and imaginary permittivity (912 MHz) from 25°C to 1000°C at a heating rate of 10°C/min and powder bulk density of 2.06 g/cm3 for the gold ore (left) and the sulphide ore (right).

The second significant rate of mass loss around 700°C is almost certainly the micas, specifically muscovite. Muscovite dehydroxylation occurs around 700-800°C, at which the rate of bound water loss increases drastically (Gaines & Vedder, 1964). While both ores reported mica content, 35.59% and 10.55% for the gold and the sulphide ore, respectively, XRD showed that the micas in the sulphide ore are annites.

As this second peak was not observed for the sulphide ore, it was concluded that the presence of muscovite induces a significant dehydroxylation reaction with microwaves, whereas annite does not. Also noteworthy

51 is the permittivities of the gold ore level off at 700°C, but the sulphide ore permittivities continue to increase. This can be explained by the muscovite dehydroxylation; once the reaction was over around

800°C the permittivities began to rapidly increase. Gaines and Vedder (1964) also observed increased BET surface area after the dehydroxylation of muscovite, and therefore it would be expected that dehydroxylation leads to increased surface area of this gold ore if heated to about 700°C. This was also seen by Bobicki et al. (2018), where serpentine dehydroxylation in an ultramafic nickel ore caused extensive cracking thereby increasing the surface area. If heated to these temperatures, muscovite and clinochlore would likely aid in the fracturing of the ore and therefore it would be advantageous to have a high wt. % of these minerals. Worth noting though is the temperatures to reach dehydroxylation are not economically feasible by treatment in a commercial process. However, micas and chlorites have been suggested to exhibit medium loss characteristics in microwave heating unlike other gangue minerals such as quartz (Zheng, et al., 2020). Therefore there could still be some heating effect for ores with these minerals, potentially leading to fracturing and subsequently impacting the ore competency.

Figure 15 shows the penetration depth for the two ores. At room temperature, the ores clearly have a high penetration depth that would be considered unsuitable for uniform heating due to the difference between the measured values and sample dimensions, as defined by Sun et al. (2016). Large penetration depths result in non-uniform surface heating rather than full penetration of the material (Thostenson &

Chou, 1999), and therefore this would be expected for each ore during treatment. At higher temperatures however, the penetration depth decreased. Worth noting is that the samples measured were in powder form and certainly increased the penetration depth, aligned with Lin et al. (2021) who compared the penetration depth between powder and coarser bulk samples. Therefore, it would be expected that the actual penetration depth at coarser particle sizes would be much lower than what was reported, although it cannot be concluded whether the penetration depth of these ores is suitable for the particle size range selected in the treatment tests.

Figure 15: Penetration depth versus temperature for the gold ore (left) and the sulphide ore (right) in an argon atmosphere at a heating rate of 10°C/min.

Above all, microwave systems that would be designed for ore treatment would require dielectric data for proper optimization (Meredith, 1998). Therefore, complementing the subsequent findings in this thesis with the permittivities of the ores was critical if a commercialization path using similar ores is continued by future researchers.

5.2 Influence of Microwave Treatment on Selected Ores

This section discusses the impact of microwaves on the two selected ores. The absorption of microwave radiation by the ores and its effect on their physical properties are discussed. Research was performed with both low-power (bench-scale) and high-power (pilot-scale) systems. Since the pilot-scale work is most relevant to a commercial process, the results are discussed below, while the results for the batch tests (both low and high power) are given in Appendix F.

5.2.1 Ore Response to Microwave Treatment

There are some important response signatures, such as temperature, that indicate the effect of microwave treatment on ores. For the continuous pilot tests, the gold ore was irradiated at a microwave power of 75 kW and the sulphide ore at 150 kW1. Microwave residence time was set by changing the belt speed, with 3 seconds for the gold ore and 1.5 seconds for the sulphide ore (each ore received the same

1 The higher power input was selected for the sulphide ore due to the larger presence of microwave absorbing minerals. Therefore there was more absorbing material to withstand the stronger microwave field.

53 energy dosage: 2 kWh/t for each). With a typical 75% electrical to microwave conversion efficiency

(Meredith, 1998), the approximate actual microwave energy dose to the ore was 1.5 kWh/t.

Table 11: Test parameters for the microwave treatment of the gold and sulphide ores for the continuous pilot (CP) tests.

Set Belt Tray Fill Mass Electrical Ore Sample Particle Residence Power Speed Ore Mass Throughput Energy Dose (Test ID) Size (mm) Time (s) (kW) (m/s) (kg) (t/hr) (kWh/t)

Gold (CP) -63.5+25.4 75 0.330 43.12 3.03 50 2

Sulphide -63.5+25.4 150 0.637 44.61 1.55 100 2 (CP)

In addition to the standard losses of energy in the conversion to microwaves, the actual absorbed power will be ore dependent. However, it is the actual electrical energy input that is the cost driver.

5.2.1.1 Temperature Response

Figure 16 shows the infrared view of the sample tray from the continuous pilot test (CP) for the gold ore (A) and the sulphide ore (B). The gold ore reached a maximum surface temperature of 48°C and the highest reading for the sulphide was approximately 100°C.

A B

Figure 16: Infrared surface images of the A) gold ore and B) sulphide ore immediately after microwave treatment.

Clearly, there is differential heating between various rocks, however it is unknown whether there was differential heating between minerals. It is worth noting that the bulk did not heat more than 5°C - this

54 would also be expected in a commercial unit given similar process parameters to the CP tests. As the irradiation time was only a few seconds, a heat signature comparable to that of the batch treatments would not be expected. To generate fracture, it has been suggested in the literature that structural failure can occur along mineral grain boundaries with temperature gradients as little as 5°C (Ali & Bradshaw, 2009). Thus the short high-power heating should be sufficient to create grain boundary fractures by this definition if there is differential heating between mineral grains. As rapid heating is more desirable to maximize temperature gradient, the average heating rates for the microwave treatments were calculated for comparison using bulk or surface temperature measurements over irradiation time, given in Table 12.

Table 12: Average heating rates for the microwave treatments, calculated from the average of temperatures observed over the irradiation time.

Input Energy Irradiation Average Temperature Sample Ore Test ID Power Input Time (s) Increase (°C/s) (kW) (kWh/t) Batch Bench (BB) 3.2 2133 240 0.7 Batch Pilot (BP) 75 94 45 4.4 Gold Ore Continuous Pilot 75 2 3.03 8.3 (CP) Batch Bench (BB) 3.2 2133 240 1.7 Sulphide Batch Pilot (BP) 75 63 30 8.8 Ore Continuous Pilot 150 2 1.55 19.4 (CP)

Clearly, the sulphide ore was the better heater, aligned with the permittivity measurements discussed previously. This is due to the higher presence of microwave absorbing minerals such as the sulphides and iron oxides (~3% for the gold ore in comparison to ~5% for the sulphide ore). With a higher percentage of these minerals, the ore would heat more rapidly, confirmed by the data in Table 12.

The average temperature rate during treatment increased with decreasing irradiation time and energy input. Within 30-45 seconds, the rate of temperature increase for the BP test, treated at 75 kW, was six times greater in comparison to the BB treatments. This was expected, as the bench-scale literature

55 concluded increased power input results in increased heating rates (McGill, et al., 1988). The CP tests showed the highest heating rates at 8.3°C/s and 19.4°C/s for the gold and sulphide ore respectively. These are merely estimates taken from an average of surface temperatures on the coarser rocks in the tray, while the BB and BP heating rates were based on bulk temperature. Generally, the higher the heating rate, the higher the differential temperature between gangue and value minerals and thus the greater the thermal stress (Batchelor, et al., 2015). Therefore, these heating rate estimates suggest that the continuous pilot treatment would be most effective at generating fractures.

5.2.1.2 Arcing Phenomena

Arcing is an undesirable response in microwave heating and can cause damage to the microwave power supply, as well as provide an avenue for power dissipation, further reducing the energy efficiency.

Each CP test experienced arcing when moving through the applicator; the gold ore produced ten arcs over the 3.03 s residence time and the sulphide ore had three arcs over 1.55 s. The gold ore likely had more arcs due to the longer treatment time, thus allowing more time for arcs to form. The arcs measured by the arc detector were short-lived and not detrimental to the equipment or test. However, in a commercial process this would be unsustainable as it may reduce the efficiency of the system by transferring microwave power into the arc rather than the ore (Buttress, et al., 2017). Moreover, neither the BB nor BP tests exhibited any arcs, so it is certainly the change in process variables in CP (i.e., increasing particle size, change to a more transient method of processing, more random sample geometry and particle size distribution) that induced this response – aligned with the literature (Buttress, et al., 2017; Holmes, et al., 2020). It is unknown how much microwave energy was lost in these arcs, however a further investigation of this arcing phenomena and methods to address this problem would be necessary for commercialization.

5.2.1.3 Absorbed & Reflected Power

An important aspect in microwave processing is defining both absorbed and reflected powers for any given material. To use microwave heating efficiently, the reflected power must be minimized as it can be considered wasted energy. Figure 17 shows the microwave power curve for the gold ore CP test. A

56 transparent rectangle is superimposed over the data to show when the sample trays moved through the applicator. Here, the power trend was chaotic, likely due to the high quantity of arcs that formed and this changed both the reflected power and the resulting absorbed power. This remains a challenge for further scale-up; however, a more optimized applicator designed specifically for ores would help remove this issue.

The ore sample tray averaged approximately 40 kW of absorbed power, about half the total 75 kW of forward power, and similar to the BP test (absorbed power trends from the BP tests in the pilot system are given in Appendix F).

Figure 17: Power trend data for the CP treatment of the gold ore showing forward, reflected and absorbed power, shown with the sample treatment window.

Figure 18 shows the microwave power curve for the sulphide ore CP test. The absorbed power is poor, averaging around 45 kW over the sample window. Power input was doubled to 150 kW for this test, so the poor absorption indicated that there may not have been enough of an absorbing load to make use of the additional power and thus, a greater portion of power was reflected. While the sulphide ore absorbed less of the total forward power than the gold ore, the power trend was much less chaotic and more consistent throughout the test. This is due to the smaller quantity of arcs. As mentioned previously, the CP gold ore test reported ten arcs while the CP sulphide ore test only reported three arcs. More arcs will result in increased randomness, as more power dissipates into them causing frequent fluctuations between absorbed

57 and reflected power. This is clear for the gold ore as the sample window shows this phenomena throughout, whereas the sulphide remains relatively consistent. It would be expected these arcs also increased the measured absorbed power, and it is likely that the improved absorbed power for the gold ore CP test is just microwave power used in the formation of the arcs.

Figure 18: Power trend data for the CP treatment of the sulphide ore showing forward, reflected and absorbed power, shown with the sample treatment window.

Table 13 shows the percent absorbed power of the total forward power, and of the total electrical line power supplied to the microwave (75% efficiency), for the batch and continuous high-power tests. The

CP tests show that the gold ore was a better absorbing load than the sulphide ore, with 41.5% and 25.9% power efficiencies respectively.

Table 13: Microwave power absorption efficiency in the ore samples, calculated based on both the total forward and total electrical power for the continuous tests.

Microwave Test % Absorbed/Forward Power % Absorbed/Electrical Power Gold Ore (BP) 47.9 35.3 Sulphide Ore (BP) 49.5 39.3 Gold Ore (CP) 54.7 41.5 Sulphide Ore (CP) 31.3 25.9

This power efficiency trend for the CP tests are the exact opposite of that measured in the BP tests, as the absorption was actually higher for the sulphide ore. While the sulphide ore showed better heating behaviour and dielectric properties, and resulting power absorption in the batch tests, the changes in particle size and thus increased void spacing in the bulk CP tests are likely responsible for this reverse trend in power absorption. This is an agreement with Lin et al. (2021), who suggested that the conductivity is greatly increased for coarser particles in comparison to finer particles, leading to more reflected power.

Therefore, high reflection may be inevitable for treating particle sizes similar to that of primary crushed ore. Future optimization work should be done to determine the most effective way to deliver more microwave energy to the sample.

5.2.1.4 Qualitative Observation of Treatments

Given that a main objective of the microwave treatment was to produce a more fractured ore, it was productive to qualitatively examine the physical changes. After microwave treatment, concentrated areas of high heating and visible fracturing were noted. Figure 19 shows a rock (about 5 cm in length) from the gold ore CP test, and a macrofracture across the rock can be observed. The circle denotes an area of crack initiation due to heating and the arrow shows where the crack propagated.

Figure 19: Visible macrofracture of a gold ore particle (about 5 cm in length). Circled portion indicates high localized heating, initiating a crack shown by the arrow.

Figure 20 shows a microscopic image taken of a different rock from the same treatment with microfractures. There appear to be both intergranular and transgranular microfractures; the arrow on the

59 left of Figure 20 indicates a fracture along the boundary of a shiny phase, likely a sulphide mineral, and the arrow on the right shows a microfracture through the surrounding gangue phases.

Figure 20: Observed microfracture in a gold ore particle of -63.5+25.4 mm. Arrows point to visible intergranular and transgranular breakage.

After treatment, each rock was investigated for evidence of fracture, and there were 7 rocks in which macrofracture was observed for the gold ore sample and zero for the sulphide ore. Each rock was studied for macrofractures, and the macrofractured rocks all had some degree of microfractures. It is important to note that while the gold ore samples exhibited considerable microfracturing in them, the sulphide ores had minimal microfracturing that was apparent. Both ores emitted a SO2 smell after treatment, indicating some degree of oxidation of the sulphide phases. Each ore also had some visible burn marks after heating.

5.2.2 Effect of Process Parameters on Microwave Response

5.2.2.1 Particle Size

Even if the dielectric loss is high for a sample, the heating efficiency for a coarse sample can be low (Sun, et al., 2016). With this, it is expected that with increasing particle size, the heat signature of the rock becomes less uniform due to the shallower penetration depth (cannot adequately penetrate the entire rock mass) and more localized is heating concentrated around the surface. This would explain why the heating was more effective for the bulk of the BB and BP treatments at -3.35+1.7 mm (approximately 20

60 times finer than the CP). For the CP tests, it was clear there was an issue with reflected power. Again, this is a negative for the application of microwave treatment for primary crushed ore. Also, sulphide minerals reflect more microwaves at coarser particle sizes, as high-loss mineral phases absorb microwaves in bulk at finer particle sizes, but reflect at larger particle sizes (Standish, et al., 1991). While elevated temperatures from microwave absorption were achieved for both ores with the finer particle size -3.35+1.7 mm, the coarser -63.5+25.4 mm ore showed concentrated surface heating around certain areas of each rock and sample tray. It would be expected that the microwaves did not adequately penetrate the rocks, but actually reflected from the surface minerals. This is aligned with the high penetration depth measurements discussed in Section 5.1, and due to these being much larger than the sample dimensions, non-uniform surface heating occurred (Thostenson & Chou, 1999).

Additionally, with longer processing times, the conductive heat loss from the absorbing minerals to the gangue decreases the temperature gradient between the two phases. This is supported by Wang and

Forssberg (2000), who found that conductive heat transfer has an increasingly influential role as the particle size is reduced, resulting in a lowering of the temperature gradients necessary to generate thermal stresses.

While this is less scalable, this type of heating leads to less reflective power and ensures heating of the entire sample. As mentioned, the CP tests reported concentrated areas of heating in the sample tray, likely where the arcs formed, however other areas clearly did not heat well. This would have implications to rocks coming from a primary crusher. One implication is that the microwaves would only heat small amounts of the coarse rock and not adequately generate heating throughout the same way the finer sizes do in the literature. This is an issue that needs to be addressed before microwave treatment finds commercial application. A microwave optimized with the permittivities given earlier would certainly improve this.

5.2.2.2 Power Density & Energy Inputs

As discussed in Chapter 3, high power densities with low energy inputs per tonne of ore are the most desirable parameters for creating an economic industrial microwave process (Jones, et al., 2005; Jones, et al., 2007). Achieving a power density greater than 1 x 1010 W/m3 with energy inputs less than 5 kWh/t

61 has been defined as ideal parameters to initiate breakage between mineral grains, determined by modelling and techno-economic assessment studies (Jones, et al., 2005; Bradshaw, et al., 2007; Batchelor, et al., 2015).

Table 14 shows a summary of the microwave treatment tests for each ore, with the energy inputs in kWh/t and estimated power densities, calculated using the measured dielectric properties in Section 5.1.

Table 14: Summary of microwave treatment tests completed on each ore, with energy inputs and estimated power densities.

Input Irradiation Energy Input Estimated Power Sample Ore Test ID Power Time (s) (kWh/t) Density (W/m3) (kW) Lab Batch (BB) 3.2 240 2133 8.58e+07 Batch Pilot (BP) 75 45 94 2.01e+07 Gold Ore Continuous Pilot 75 3.03 2 4.77e+06 (CP) Lab Batch (BB) 3.2 240 2133 8.58e+07 Sulphide Batch Pilot (BP) 75 30 63 2.01e+07 Ore Continuous Pilot 150 1.55 2 9.48e+06 (CP)

Clearly, a power density greater than 1 x 1010 W/m3 was not achievable based on the estimations from the current tests. The power density actually decreased with increasing scale due to the increased mass and resulting volume treated. In industrial mining processes with throughputs spanning over a few hundred to a few thousand tonnes per hour, reduced tonnage is not desirable. Therefore, to irradiate material at an industry scale while meeting the power density threshold as defined by Jones et al. (2005), a microwave capable of running higher than 150 kW would be necessary. Even with a lower power density than the bench-scale tests however, the improved electric field intensity of the BP and CP tests resulted in increased heating rates.

5.2.2.3 Sample Morphology and its Impact on Arcing

The effect of void space in the bulk ore was difficult to assess, but it is important. When commissioning the pilot plant for ore treatments, it became apparent that sample geometry and placement

62 on the sample tray had direct implications with regards to the interaction of the ore with the microwave field. As discussed, arcing was a common occurrence during the continuous treatment tests, while the BB and BP tests did not experience any. Buttress et al. (2017) reported that for an ore size range of -50.8+25.4 mm treated in their pilot-scale microwave unit, arcing was frequent, and the necessity of using filler material to close out the voids and improve material flow at high throughput was required. The main difference between the BB and BP, and the CP tests was the particle size, and the CP tests had a similar size range of

-63.5+25.4 mm to that of the ore treated by Buttress et al. (2017). With the batch treatments, much finer particles allowed for a higher bulk density, whereas with coarser particles it was more difficult to achieve this. Holmes et al. (2020) suggested that more angular and random geometry induced the arcing response

– a characteristic of coarser rocks. Therefore, minimizing the void spacing in the continuous pilot tests in order to match the BB and BP was not possible. This is a further difficulty with implementation of microwave treatment on primary crushed ore.

The arcing arising from the interaction between the microwaves and the coarser particles with both sharp edges and unpredictable morphologies was a vital part of the commissioning phase of the project.

Initial experiments showed that a -63.5+25.4 mm sample moving through the applicator without the addition of <12.7 mm ore provoked intense arcing. When the filler material was added to minimize the spacing between coarse particles during the commissioning phase, the observed arc intensity and quantities was greatly reduced and in some cases, completely eliminated. While adding fines was an adequate remedy for the work, it remains a problematic area for commercialization.

5.3 Effects on Select Unit Operations

The main scope of this thesis was to analyze the effects of microwave treatment on various downstream processes. While the initial aim of this technology in the literature was around reducing comminution energy, it could well be that the true potential of microwave treatment of ore lies within the benefits related to downstream. Perhaps the economic incentive to processes such as leaching and roasting could well be in the same magnitude as comminution energy reduction, if not more.

As discussed in Chapter 3, the key parameter that has the most significant impact in the downstream area is the increase in surface area obtained by microfracture. Wherein the previous sections discussed results related to the microwave treatments and observed properties of heated ore, Section 5.3 includes results related to the physical and chemical changes to the ore from microwaves and the subsequent effects on selected downstream processes.

5.3.1 Surface Area Analysis

The specific surface area (SSA) of each ore was measured by BET to relate the extent of fracture to downstream processing. It was felt impractical to assess the surface area of run of mine ore after treatment at coarser sizes. Therefore, SSA was determined on ground ore to determine any improvements on surface area, as the SSA at this particle size is better tied to downstream processes than pre-milled ore.

As discussed in the experimental methods given in Chapter 4, grinding of untreated and treated ore was done at 106 µm and thus the material measured by BET was -106 µm.

5.3.1.1 Gold Ore

Table 15 shows the relevant results gathered from the BET analysis for the gold ore -106 µm product. The BP treatment showed about 21% gain in surface area while the CP treatment showed about

3%. This is most likely due to the relative heating intensity of the batch versus the continuous (longer treatment time and higher temperature achieved).

Table 15: BET analysis results for the gold ore showing specific surface area, pore volume, and pore diameter. Treatments are italicized.

Specific Pore Volume Pore Diameter Test ID Surface Area (cm3/g) (nm) (m2/g) RB/RP 1.16 0.0033 12.31-12.67 BB 1.22 0.0033 12.50 BP 1.40 0.0037 11.34 CP 1.19 0.0036 11.56

Because of the relative low power absorption of the CP test, the changes in surface area were found to be within the uncertainty limits. Additionally, the longer treatment time of the BB, despite the lower power input and electric field intensity, had an increase of 5%. Pore volume appeared to have marginal increases and pore diameter shrunk in the pilot tests. This same trend was realized by Huang and Liu

(2021), and the authors attributed this to enhanced crack propagation by microwaves. Ultimately, the significant results for the BB and BP tests provide indication that there would be an improvement in downstream reaction rates. The reported uncertainty for SSA was around ±0.02 m2/g.

5.3.1.2 Sulphide Ore

Table 16 shows the BET results for the -106 µm sulphide ore. The one sample worth mentioning was the CP ground ore, showing an increase in SSA of 7% over the reference ore, well in excess of the reported uncertainty of ±0.014. The BB and BP tests showed marginal increases in SSA, although these were within the uncertainty and therefore not significant.

Table 16: BET analysis results for the sulphide ore showing specific surface area, pore volume, and pore diameter. Treatments are italicized.

Specific Surface Pore Volume Pore Diameter Test ID Area (m2/g) (cm3/g) (nm) RB/RP 1.50 0.0037 11.78-13.22 BB 1.52 0.0035 13.38 BP 1.51 0.0036 12.83 CP 1.61 0.0078 22.71

Perhaps the biggest change in this data was the pore volume and pore diameter for the CP test which nearly doubled for both. This is likely due to a large shift in particle size distribution after the microwave treatment, and indicates that the particle size of treated ore products after crushing becomes smaller – which is in agreement with Huang and Liu (2021) in their study. Worth noting is the CP test actually outperformed the BB and BP treatments in terms of SSA improvement, contrary to what was seen for the gold ore. This result was unexpected as the conditions used for the batch tests resulted in much

higher temperatures. Regardless, this result suggests that a continuous microwave treatment at high-power

can impact downstream processes by improved surface area.

5.3.1.3 Discussion

In general, the BET results indicate there was some significant improvements in surface area after

grinding. The gold ore showed significant surface area improvements for the batch treatments and the

sulphide ore only showed a significant increase for the continuous pilot test. The relevant microwave

treatment variables for each test with resulting surface area are shown in Table 17.

Table 17: Summary of microwave treatment variables power input, energy input and particle size, with measured SSA and percentage increases. Treatments are italicized.

Microwave Energy Particle Size at Specific Sample ID Test ID Power Input Input Treatment Surface Area % Increase (kW) (kWh/t) (mm) (m2/g) RB/RP - - - 1.16 - BB 3.2 2133 -3.35+1.7 1.22 5.2 Gold Ore BP 75 94 -3.35+1.7 1.40 20.7 CP 75 2 -63.5+25.4 1.19 2.6 RB/RP - - 1.50 - Sulphide BB 3.2 2133 -3.35+1.7 1.52 1.3 Ore BP 75 63 -3.35+1.7 1.51 0.7 CP 150 2 -63.5+25.4 1.61 7.3

Clearly, the use of the high-power microwave system was more effective at improving surface area

for both ores (BP for gold ore and CP for sulphide ore). An interesting trend for the gold ore between the

batch treatments (BB and BP) occurred; a lower energy input at higher microwave power was better at

improving surface area. This was certainly due to smaller conductive heat transfer and larger thermal

gradients – opposite of low-power treatments. Therefore, greater thermal stresses were reported as a result

of the higher electric field intensity – directly aligned with modelling studies in the literature for high-power

ore treatment (Jones, et al., 2005; Ali & Bradshaw, 2009). This is an important direction for this work;

reducing the kWh/t and observing better impacts is necessary for economically viable commercialization.

For the CP gold ore test, an even lower energy input was used at the same power, however this showed a smaller improvement in SSA. It is expected that this was the case due to the change in process variables discussed in Section 5.2 such as particle size, sample morphology, and transient processing. High amounts of reflected power also limited the amount of energy absorbed and thus affected the heating. While this likely limited the CP result for the gold ore, the exact opposite occurred for the sulphide ore. Even though the power absorption was limited for this sample, clearly the higher electric field intensity from the higher power input of 150 kW induced enough fracture in the ore to produce significant changes in SSA. This is aligned with the existing literature (Jones, et al., 2007). Therefore, perhaps a higher power input for the CP gold ore test would further improve the surface area of the sample.

The characteristics of the grinding products indicate that the microwave treatment had an advantageous effect on surface area by the generation of fractures. While minor, it was hoped that these changes in surface area and porosity would have beneficial impacts in downstream processes. The subsequent sections will discuss the downstream tests, starting with the comminution results.

5.3.2 Comminution Results

As discussed, the suggested effects of microwave treatment on comminution are: the ability to reduce structural ore competency by fracture, and grain boundary fracture resulting in improved liberation.

To determine the effects of these microwave treatments on the two ores in comminution, grinding energy and liberation was assessed. Grinding energy was determined by experimentally calculating the work index in kWh/t, and liberation was determined by QEMSCAN on the ground -106 µm product.

5.3.2.1 Grinding Energy

Grinding work index was measured to assess the impacts of thermally induced stresses as a result of the microwave treatment. Table 18 shows the grinding data for the RB, BB and BP tests. The cumulative particle size distributions and F80 and P80 values for each ground product is given in Appendix G, and showed no change between treated and untreated ore. Clearly, there was no significant change in work

67 index between reference and treated material for either ore, given the margin of error for the Bond Work

Index test of ±9%.

Table 18: Comminution results of the batch microwave tests at -3.35+1.7 mm, showing microwave power and energy inputs with resulting Bond Work Indices.

Sample Bond Work Test Microwave Power Microwave Energy Index, W ID Input (kW) Input (kWh/t) BWI (kWh/t) Gold Ore RB - - 8.43 BB 3.2 2133 8.55 BP 75 94 8.65 Sulphide Ore RB - - 18.78 BB 3.2 2133 18.80 BP 75 63 18.80

The lack of change in work index was unexpected in light of literature comparison that treated similar ores. Amankwah et al. (2005) reported an 18.5% work index decrease with a magnetite/hematite

(good microwave absorbers) rich gold ore in silicate gangue. A bench-scale microwave system was used with approximately 2500 kWh/t of microwave input energy, comparable to the energy input of the BB test.

The authors did not report the wt. % of the minerals, however the gold ore used in this study had a much lower thermal response, with only 0.65% iron oxides and around 2% sulphides. Therefore, there was likely not a sufficient amount of these minerals to make a measurable difference in grinding energy, and thus no change in work index.

Henda et al. (2005) did work on a similar, but very high grade sulphide ore, reporting a decrease in work index as high as 77% using a similar bench-scale system. This would indicate that microwave treatment will only have a measurable impact on work index with very high grade ores (~40% sulphides in the Henda work), since the ore treated in this study was much lower grade (~4% sulphides) and showed no decrease in required grinding energy. Although, based on the discussion with the effect of particle size, it is likely that very high grade ores would be too reflective to process at coarser sizes. Therefore the results seen for this type of ore may only be possible at the bench-scale. It is worth remembering that these batch treatments, with long processing times, would not have been economical anyway as the kWh/t of microwave energy input is much greater than the energy per tonne required to comminute the material.

Table 19 summarizes the grinding data acquired for the RP and CP tests. In addition to BWI values, the SAG Work Index was also obtained. The only difference outside of the margin of error was the 19% decrease for the sulphide ore, in WSDT. This large reduction in energy was due in part to the intermittent crushing stage required to meet the set F80 for the test, prior to starting the SAGDesign work. The microwave treated ore generated much more fine material during this stage than the untreated ore, and therefore less grinding was required to reach the target P80 of 1.7 mm. Clearly, this indicates that the ore was weakened, observed by the improved crushability. Worth noting is the 2.2% decrease in WSDT for the gold ore, although it falls within the ±5% margin of error for the SAGDesign test. All results for the BWI were within the ±9% error and therefore it was concluded that there was no energy reduction. It was also observed that the WBWI was lower in SAGDesign in comparison to the standard BWI. This was due to the smaller transfer size (T80 of 1.7 mm) in comparison to the F80 (~2.9 mm) from the tests in the previous table.

Table 19: Comminution results of the continuous pilot tests at -63.5+25.4 mm, showing microwave energy inputs with resulting SAG and Bond work indices.

Microwave Energy SAG Work Index, Bond Work Index, Sample Test ID Input (kWh/t) WSDT (kWh/t) WBWI (kWh/t) RP - 6.97 6.79 Gold Ore CP 2.00 6.82 6.99 Sulphide RP - 16.69 16.48 Ore CP 2.00 13.49 16.71

The one work index test that had a reduction in kWh/t greater than the kWh/t of microwave energy applied was the CP test for the sulphide ore. Literature values of bench-scale work index reductions have been reported in the range 10-20% with some as high as 83% (Walkiewicz, et al., 1995; Sahyoun, et al.,

2004; Marion, et al., 2016), however these were not reproducible with the bench experiments and samples used in this work. For the gold ore, the potential for further weakening of an already soft and easily breakable ore may have also contributed to the lack of energy reduction. The results from the sulphide ore are puzzling; while there was a significant reduction in grinding energy for the CP test, no such reduction was seen in any of the BWI tests. Compared to the SSA data, the CP sulphide ore test showed that the 19% reduction in WSDT is directly aligned with the 7% improvement in surface area, discussed in the previous

69 section. Interestingly, SSA was not aligned with any grinding energy reduction for the gold ore, even though the BP test reported a 21% increase. While grinding energy was mostly unaffected by the treatments, liberation assessment will provide a better understanding of the benefits to recovery.

5.3.2.2 Liberation

It may well be that the work indices after microwave treatment do not significantly change, however an effect on liberation can improve the grade and recovery in processes further downstream. It was decided that a sample of the ground -106 µm product would be analyzed for each ore, as this would be representative of an industry process. As mentioned in Chapter 4, the liberation results were put into categories, describing the percent of surface area of value sulphides exposed after grinding. In this case, the categories were:

<30% locked, 30-50% middling, 50-80% middling, 80-95% middling, >95% liberated, and 100% free, and this is shown in the following figures. As the gold associated minerals are the sulphides (mostly pyrrhotite), pyrrhotite liberation and combined sulphide liberation is shown in Figure 21.

Pyrrhotite Liberation Total Sulphide Liberation 100 100 90 90 80 80 70 70 60 60 50 50 40 40

30 30

Mass Mass % mineral Mass Mass % mineral 20 20 10 10 0 0 RB BB BP RP CP RB BB BP RP CP Locked (<30%) Middling (30-50%) Locked (<30%) Middling (30-50%) Middling (50-80%) Middling (80-95%) Middling (50-80%) Middling (80-95%) Liberated (>95%) Free (100%) Liberated (>95%) Free (100%)

Figure 21: Pyrrhotite and total sulphide liberation via QEMSCAN for the ground -106 µm gold ore product of the various reference (RB, RP) and microwave treated (BB, BP and CP) samples.

The data shows that microwave treatment somewhat increased liberation for both the batch and pilot tests. For the BB test, there was an increase in >95% liberation of pyrrhotite and total sulphides.

Interestingly however, the BP test only reported a small increase in >95% total sulphide liberation; this was

70 unexpected given the large increase in SSA. For the CP test, the main difference was a much cleaner liberation degree, as both pyrrhotite and the total sulphides reported a large increase in freely liberated particles. Additionally, there is a clear improvement in total sulphide >95% liberated particles for the CP test. This indicates that the microwave treatment had an effect on the liberation degree of this ore at the pilot-scale. The reasoning for the improvement in liberation is the grain boundary fracture due to the differential heating between value and gangue minerals, which is supported by the literature. As the sulphides are the main microwave absorbers in this ore, it would be expected that the stresses developed among these phase boundaries, thus improving the liberation after grinding. If true, this should have significant implications to gold recovery. It is worth noting that the grain size of pyrrhotite was 131 µm, therefore a high liberation degree was expected due to a 106 µm closing screen size in the grinding tests.

The liberation results of the ground -106 µm sulphide ore are shown in Figure 22. As the pay metals in this ore are copper and nickel, both chalcopyrite and pentlandite were investigated.

Chalcopyrite Liberation Pentlandite Liberation 100 100 90 90 80 80 70 70 60 60 50 50 40 40

30 30

Figure 22: Chalcopyrite and pentlandite liberation via QEMSCAN for the ground -106 µm sulphide ore product of the various reference (RB, RP) and microwave treated (BB, BP, and CP) samples.

The liberation results of the sulphide ore indicate that the microwave treatment did have some effect. For the batch tests, there were marginal changes to chalcopyrite liberation, although there were slight improvements in >95% liberated pentlandite particles. As it was concluded that SSA did not change

71 for these batch tests, it was expected that minimal changes to liberation would occur. On the other hand, the CP test showed significant increases of 23.5% and 7.5% in >95% liberated particles for chalcopyrite and pentlandite respectively. This result is consistent with the energy reduction and surface area improvements discussed in the previous sections. Clearly, this indicates that the liberation degree at the pilot-scale can be improved using continuous high-power microwave treatment.

With the exception of the CP test on the sulphide ore, the implications of the treatments did not necessarily have the expected change in energy requirements for grinding as seen in literature, but did however enhance the liberation to some degree. Due to the relationship of microwave absorption between these value minerals and the surrounding gangue, it is expected that thermal stresses that were generated around the mineral grain boundaries thus improving liberation, consistent with the literature (Orumwense, et al., 2004; Bobicki, et al., 2018). These changes to mineral liberation realized, along with surface area improvements, suggest that there will likely be implications further downstream in extractive processes.

The effects of this on cyanide leaching and oxidative roasting will be discussed in the subsequent sections.

5.3.3 Cyanide Leaching (Gold Ore)

As demonstrated in the literature, microwave treatment can successfully improve the mass transfer rate of lixiviants and improve pay metal recovery. As part of a review on the impact of microwaves on downstream processing, the gold ore was cyanided by the bottle roll technique, described in Section 4.7.1.

5.3.3.1 Kinetics and Recovery

Figure 23 displays the gold recovery curves for the -106 µm product of the batch microwave tests at a cyanide concentration of 1.0 g/L and 0.5 g/L after 6 hours. Clearly there are differences in kinetics between the various samples. For 1.0 g/L NaCN, the BB and BP material reported an 18% and 26% increase in gold recovery respectively after 6 hours when compared to the reference RB. On the other hand, the 0.5 g/L tests showed much greater improvements in gold recovery; this is likely a function of the fact that lower concentrations of lixiviant could be more mass transport controlled in leaching. Therefore the improved fracturing and higher surface area shows a much more pronounced rate effect in the more dilute solution.

The fact that the microwave treated samples seem to react similarly between the two cyanide concentrations supports this. The tests were done for 24 hours; at this point all samples reached approximately 98% gold recovery. Lab access limited sampling between 6 and 24 hours, therefore only the kinetics of the first 6 hours are shown. This data shows that the treatment had a significant impact on the kinetics of the reaction, and could have the effect of greatly reducing capital and operating costs.

Figure 23: Effect of gold recovery versus time for 300g, -106 µm samples, at 40% solids density from the RB, BB and BP tests. Kinetics of the first 6 hours are shown, at a NaCN concentration A) 1.0 g/L, and B) 0.5 g/L.

It appears once again that the pilot tests which involve a higher electric field intensity at shorter heating times, with lower overall energy inputs seem to have a smaller impact. Figure 24 displays the gold recovery curves for the CP test, at a cyanide concentration of 1.0 g/L and 0.5 g/L. While there is a 16% improvement in gold recovery for the 1.0 g/L test after 6 hours, there was no change for the 0.5 g/L test.

Figure 24: Effect of gold recovery versus time for 300g, -106 µm samples, at 40% solids density from the RP and CP tests. Kinetics of the first 6 hours are shown, at a NaCN concentration A) 1.0 g/L, and B) 0.5 g/L.

5.3.3.2 Comparison with Surface Area

A comparison of the increase in surface area and gold recovery after 6 hours for the microwave

treated samples with respect to the reference tests is given in Table 20. The most significant improvement

in gold recovery was the BP test, aligned with the largest improvement in SSA. Conversely, the lowest

improvement in SSA, the CP test, also reported the smallest change improvement in gold recovery.

Table 20: Microwave treatment parameters shown with reported increases in specific surface area (from BET) and gold recovery after 6 hours at 1.0 g/L NaCN and 40% solids density for comparison.

Rate of Increase in Increase in Energy Particle Size Power Treatment Treatment Temp. Specific Gold Test ID Input (mm) (kW) Time (min) Mass (kg) Increase Surface Recovery @ (kWh/t) (°C) Area (%) 6 hrs (%)

BB -3.35+1.7 3.2 4 0.1 2133 0.7 5.2 17.9

BP -3.35+1.7 75 0.75 10 94 4.4 20.7 26.2

CP -63.5+25.4 75 0.05 43 2 8.3 2.6 15.9

While there are only three data points, clearly the trend indicates that the greater the surface area

improvements from thermal stress fracturing, the larger the potential for improved gold recovery rates,

shown by Figure 25. This is in accordance with the literature, which described that microwave induced

fractures improve the mass transfer rate of lixiviants.

BB CP

Figure 25: Percent gold recovery increase as a function of percent SSA increase for the microwave treated samples with respect to reference tests. SSA was measured by BET, and gold recovery improvements are from the 1.0 g/L NaCN tests after 6 hours at 40% solids density.

Based on the linear trend, despite only having three data points, it appears there would be the possibility to improve gold recovery without a surface area increase for microwave treated ore. An explanation for this could be a further change in surface morphology irrespective of measurable surface area. Perhaps the muscovite gangue present as sheets with near perfect cleavage was responsible, which a large source of the fracture was observed and discussed in Section 5.2.1.4, allowing for easier mass transport of the cyanide. Regardless of the reasons, the data suggests that there is something else irrespective of microfracture from the treatment that can improve cyanidation.

Gold recovery and surface area appears to be the only linear trend from the data in Table 20. No significant trend was found for energy and power input, or rate of temperature increase between the three tests. However, there are some observations that can be made when comparing the two batch tests. The higher electric field intensity from the 75 kW power input for BP was much more effective at improving surface area and gold recovery. This certainly is attributed to the shorter heating time at higher power input, thus providing much more rapid differential heating with larger temperature gradients. Compared to the

CP test, most of the coarse sample did not heat as effectively due to the reflected power and arcing, discussed previously, and the surface area and gold recovery results confirm this.

It is clear that the high-power test was the most effective at improving surface area, but at the bench- scale particle size. However, this is obviously not scalable to industry. Still, the continuous test was sufficient at producing enticing improvements in both surface area and gold recovery, and confirms the potential benefits for high-power continuous microwave processing for the mining industry above and beyond reductions in comminution energy. For a more optimized system designed for ore treatment, it would be expected that energy delivery for the continuous tests would be much more efficient (an improvement from the poor absorbed power) and therefore the extent of fracturing would be expected to increase. This would have further implications to capital and operating costs, and higher throughputs.

It is also useful to compare these results with the literature. The work of Amankwah and Ofori-

Sarpong (2011) and Seflek and Bayat (2018) are the most relevant studies that assess gold recovery after microwave treatment, shown in Table 21.

Table 21: Comparison between the gold recovery after 6 hours at 1.0 g/L NaCN and 40% solids density for the gold ore, and the relevant literature.

MW Test ID or Literature Power Energy Input Cyanidation Increase in Gold Treatment Reference (kW) (kWh/t) Time (hours) Recovery (%) Time (min) (Amankwah & Ofori- 0.8 5 5833 8 26 Sarpong, 2011) (Seflek & Bayat, 0.6 30 1200 8 35 2018) Batch Bench (BB) 3.2 4 2133 6 18 Batch Pilot (BP) 75 0.75 94 6 26 Continuous Pilot (CP) 75 0.05 2 6 16

Both of these literature studies used very low powers and long treatment times, resulting in incredibly high energy inputs. The two literature tests reported a higher gold recovery increase than the comparable BB test, and due to the similarities in methods used (low-power, small sample mass & long treatment times) it would be expected that the difference is due to the mineralogy of the sample. The ore in the Seflek and Amankwah studies had high percentages (~30 wt. %) of iron oxides. On the other hand, this gold ore has only ~3% good microwave absorbing minerals – a huge difference. Therefore it would be expected that the gold recovery was lower with the current gold ore used due to fewer thermal stresses.

Perhaps the most interesting takeaway from the data in Table 21 is the comparison of the pilot results to the literature studies. The improvement in gold recovery for BP was the same as the one reported by Amankwah and Ofori-Sarpong (2011), although the energy input was over 60 times lower. This clearly demonstrates the significance of using high-power microwaves. Currently, there has not been any literature published on high-power microwave treatment and subsequent leaching. Therefore, this work provides a baseline for future research by including high-power batch and continuous treatments, which demonstrate the importance for investigating downstream processes in addition to comminution. If these results are at all representative of what a commercial process with this technology could look like, capital costs would go down as mentioned earlier as well as the potential to reduce the operating costs.

5.3.3.3 Cyanide Consumption

A large operating cost for gold mines is the consumption of cyanide. To align with the surface area changes observed, cyanide consumption for each leaching test was calculated to determine whether the improvement to mass transfer affected reagent use. Table 22 shows the calculated reagent consumption in kilograms of cyanide per tonne of solids processed after 24 hours for the leach tests.

Table 22: Cyanide consumption from the various leach tests at 1.0 g/L and 0.5 g/L NaCN concentrations for 300 g samples at a 40% solids density. Consumption was calculated after 24 hours of cyanidation.

Cyanide Consumption (kg/t Cyanide Consumption Test ID Surface Area (m2/g) solids), 1.0 g/L Tests (kg/t solids), 0.5 g/L Tests RB 1.04 0.94 1.16 BB 1.24 0.90 1.22 BP 1.07 0.85 1.40 RP 1.15 0.88 1.16 CP 1.34 0.85 1.19

Looking at the data, microwave treated samples appear to have consumed more cyanide during the

1.0 g/L tests; the tests at 0.5 g/L show that the treated samples consumed less cyanide. Seflek and Bayat

(2018) reported a 13.6% decrease in total cyanide consumption from an untreated to microwave treated sample at bench-scale. This is similar to the largest reduction of 9.6% in cyanide consumption for the 0.5 g/L BP test. Figure 26 shows the cyanide consumption as a function of surface area.

Figure 26: Effect of surface area on cyanide consumption after 24 hours of leaching with 300 g of -106 µm gold ore, 40% solids density, at a cyanide concentration of both 0.5 g/L and 1.0 g/L.

While there is large uncertainty, the cyanide concentration shows a trend of lower cyanide use with improved surface area. Obviously, the consumption as a function of surface area is more convincing for the 0.5 g/L tests. If significant, the lower consumption with increasing surface area would be in agreement with the literature; Moravvej et al. (2016) reported decreased sulphuric acid consumption in copper leaching after microwave treatment and concluded it was a result of improved surface area. However, it must be noted that there were certainly cyanide consuming gangue minerals in the gold ore. Pyrrhotite, pyrite and small amounts of chalcopyrite and arsenopyrite are common cyanicides (Adams, 2016) due to the preferential leaching of the copper and iron ions, and were present in this gold ore. As mentioned in Chapter

3, these minerals are all good microwave heaters and it would be expected that the fracturing and improved surface area would be concentrated around these minerals. Therefore, even though the data may weakly suggest that surface area can reduce cyanide consumption, it cannot be concluded as such, as it is unknown how much cyanide was consumed by the gold and gangue minerals for each test.

5.3.4 Roasting Tests (Sulphide Ore)

Another downstream process that was studied was roasting, and the sulphide ore was used.

Normally a concentrate is used, but it was not possible to produce this from the small ore samples.

Therefore just the ground ore product was roasted. While the leaching of microwave treated ore has been investigated in the bench-scale literature, the effects on roasting is unknown.

Table 23 summarizes the roasting tests completed on the sulphide ore, comparing the reference samples, RB and RP, to the microwave treated samples, BB, BP, and CP, using a fluidized bed roaster.

Two tests for each sample were done. The sulphur content before and after, and the total sulphur loss is shown in Table 23.

Table 23: Summary of roasting tests comparing sulphur loss of reference and microwave treated sulphide ore with surface area. Samples of 50 g were used, roasted for 15 minutes at both 1050°C and 1100°C, and sulphur content was measured by ELTRA.

Surface Area Sample ID % Sulphur Before % Sulphur After % Sulphur Loss (m2/g) RB – 1 0.97 0.14 85.6 1.50 RB – 2 0.89 <0.01 95.5 BB – 1 0.85 <0.01 98.8 1.52 BB – 2 0.86 <0.01 98.8 BP – 1 0.80 0.44 45.0 1.51 BP – 2 0.83 0.65 21.7 RP – 1 0.87 0.03 96.6 1.50 RP – 2 0.87 <0.01 98.9 CP – 1 1.31 <0.01 99.2 1.61 CP – 2 1.26 0.77 38.9 *Bold denotes tests which appear to be an outlier or show unexpected results.

Of the 10 roasting tests completed, 6 were effectively completely roasted, indicating that roasting time was too long to get comparative kinetic data. Also, there are some samples which appear to be outliers.

Particularly for the BP tests, very low desulphurization was reported and these samples also had the lowest measured sulphur content prior to entering the roaster.

In both the BB and BP tests, the sulphur content entering the roaster was lower relative to the reference material, shown in Figure 27 as a function of microwave heating rate. As seen, a higher heating rate during microwave treatment, a characteristic of higher microwave power inputs, has a greater impact on the oxidation of sulphides. The low desulphurization of the BP roasting tests is likely due to the initial degree of oxidation from the microwave treatment. As discussed in Chapter 2 of the literature review, oxidative roasting involves an initially formed oxide layer, which continues to remain porous as the inner sulphide portion of the particles reacts. It may be that the oxidation that took place before roasting formed these oxide layers and inhibited the gas-solid contact of the inner sulphides. Regardless, the two BP roasts averaged only a 33% sulphur loss, and it was concluded that the change in chemical properties from this microwave treatment adversely affected the roasting.

Figure 27: Initial sulphur content before roasting as a function of temperature change in microwave tests, measured by ELTRA elemental analyzer.

It is also of note that the CP test reported a much higher sulphur content than any of the other samples. This is likely due to the higher sulphide percentage in the fine material used to help process the coarse ore through the microwave system during treatment. While the particle size distribution of the sample remained the same, clearly the sulphur content was affected. This makes the results from the CP test potentially not line up with the other tests. As well, the second roast of CP sample reported poor desulphurization similar to BP tests, but the first CP test was completely roasted. This adds to the uncertainty in the response of this feed to roasting and so this data set was not included in the analysis.

Apart from phase change, surface area was another parameter discussed previously that was the main factor when improving downstream processes. Due to the potential error on the roasting tests and the marginal changes to the sulphide ore surface area, it was concluded that ore surface properties related to fracture showed no effect on the roasting potential of this ore. As mentioned in Chapter 3, roasting processes would likely be more affected by chemical changes (changes in phase) rather than physical changes such as improved surface area, due to their rapid and exothermic nature. Even if there was any improvements to roasting due to surface area increases, this was not something that could be accurately determined. Therefore, clearly the changes in phase from microwave treatment was the more important variable, unlike the cyanide leaching which was more surface area dependent.

The oxidation of microwave treated ore is an important consideration for commercial processing.

This can generate significant SO2 during the process, and could have both flotation and downstream processing implications. With a consistent initial sulphur content for each of the roasts, the percent of sulphur loss would not change significantly (note that a sulphide ore is being roasted and not a sulphide concentrate, which might show a more noticeable impact). While the availability of treated ore prevented concentrate roasting, if possible, this should be followed through. Perhaps the most important conclusion that can be made from this work is the adverse effects shown in the BP test due to oxidation in the microwave treatment, also observed by other researchers (Barbu, et al., 2020). For a reduced sulphur content in the feed, it was speculated that the required time to achieve the same oxidation degree could be reduced, resulting in throughput improvements and the potential to roast ores with a lower sulphur content.

These results show that this is likely not possible, as the lower sulphur content in the feed actually resulted in a worse roasting performance. This would not be as much of a concern when discussing scale-up due to the shorter residence times that would be used and the smaller temperatures reached. However, as the approximate surface temperature of the sulphide ore after the CP test was 100°C (observed by individual surface particles, not the entire sample), it is still likely that some oxidation would take place.

Unfortunately, based on the CP data, this was not something that was able to be concluded. Future work should include flotation into the test plan to assess changes to recovery, followed by roasting. This would certainly conclude whether the degree of oxidation is significant enough to impact the upgrading of this sulphide ore, as well as roasting. Additionally, the roasting of the concentrate would be much more representative of an industry process.

Chapter 6

Conclusions and Recommendations Microwave treatment of two ores was performed at bench and pilot scale to assess the impact on selected unit operations. Additional characterization work of the two ores on their microwave response was also investigated. This thesis mainly focused on the creation of fractures and improving surface area by microwaves, with the goal to reduce comminution energy consumption and improve downstream performance, namely in leaching and roasting. In both cases, this work used ores instead of concentrates because there was no flotation step that could be performed due to small sample sizes, however it still gives an indication of the potential for further benefits of high-power microwave treatment previously unknown in the literature. There are three key parts of this thesis, and each has their own findings.

6.1 Impact of Microwaves on Ores and Processing When the various microwave treatments were conducted, temperature response was a key observation that could be quantified. The sulphide ore reported greater temperatures after each treatment, and the mineralogy indicated that this was aligned with the literature due to the greater wt. % of microwave absorbing minerals (5.5% for sulphide vs. 3.1% for gold). Permittivity measurements confirmed this, showing higher real and imaginary permittivity for the sulphide ore.

High microwave energy inputs were used for the batch tests, and lower energy inputs for the continuous tests; of each, only the CP test would be economical. For the continuous pilot tests though, arcing was a frequent occurrence and likely affected microwave power absorption to the ore. It was concluded that a coarser particle size, more random sample geometry and the switch to transient processing induced the arcing response, however it was reduced by increasing the packing density with -12.7 mm ore.

A critical part of this thesis was to investigate the effect of fracturing of the ores. Interestingly, although the sulphide ore proved to be the better microwave heater of the two, it did not show any qualitative evidence of this. The gold ore however, had several coarse (-63.5+25.4 mm) rocks that had considerable macrofracture, and upon microscopic observation, evidence of microfracture. The specific surface area

82 measurements on the ground sample confirmed this, showing an increase in SSA for each gold ore treatment, with the highest improvement of 21%. The sulphide ore however only statistically improved for the continuous pilot test. Therefore, even though the sulphide appeared to be better suited based on mineralogy and dielectric properties, the gold ore was obviously more amenable to the treatment from a fracture standpoint.

While the differential heating of minerals showed some effect by fracture and SSA improvement, the energy consumption in the comminution testing was less convincing. The only relevant result was the sulphide ore CP test, which showed a 19% decrease in the SAG Work Index, WSDT; this is directly aligned with the improved surface area of the CP test. The rest of the work indices when compared to the reference showed no significant changes for either ore. Although unexpected, the conclusion was that the impact of microfracture did not weaken the ore enough to sufficiently change the grinding energy consumption, but still promoted grinding which enhanced available surface. Liberation results confirmed that the improvement of surface area was likely correlated with the value minerals – the value sulphides in each ore exhibited some degree of increase in either free particles (100%), liberated particles (>95%), or both.

Therefore it was concluded that intergranular breakage was promoted from the microwave heating.

6.2 Impact of Microwave Treatment on Leaching

Each of the ground products were subjected to downstream tests – cyanidation was done for the gold ore. It was concluded from the test results that the microwave treatment certainly influenced the leach kinetics; the highest was a 26% increase in gold recovery for the BP test. Furthermore, improvements to gold recovery after 6 hours were proportional to the surface area increases reported, indicating that higher fracture degrees would result in a higher gold recovery. The linear trend also showed that even without any surface area improvement, leaching recovery would still be increased from the heating – likely a change in the surface morphology irrespective of fracture.

Comparing the two batch tests, the higher electric field intensity of the BP test at a shorter heating time was much more effective at improving gold recovery. It was concluded that the more rapid differential

83 heating caused this. In comparison to the CP test, the sample did not heat as effectively due to reflected power and arcing limitations, and the surface area and cyanidation results confirmed this. The effectiveness of the continuous pilot treatment however still cannot be understated. The results clearly showed that this treatment was still sufficient at improving cyanidation kinetics. Cyanide consumption showed no significant trend with surface area increases. Cyanidation of a concentrate would allow for a proper conclusion given the removal of cyanide consuming gangue.

The importance of these results revolved around the absence of this high-power work in the literature. Past research showed that improvements to cyanidation kinetics was possible using low-power and high irradiation times. The work in this thesis clearly demonstrated that improved cyanidation is possible when using economical energy inputs, providing a baseline for future investigations. For a more optimized system designed for ore treatment, it would be expected that energy delivery to the ore for the continuous tests would be much more efficient and therefore the extent of fracturing would be expected to increase.

6.3 Impact of Microwave Treatment on Roasting

Oxidative roasting was done for the downstream assessment of the ground sulphide ore. It was shown that both the BB and BP tests showed a lower sulphur content after treatment, prior to entering the roaster. It was concluded that the microwave treatments caused this by oxidizing some of the sulphide phases and releasing SO2. While the results of the BB test showed no impact on desulphurization, the BP results confirmed that the oxidation adversely affected the roasting with an average of only 33% sulphur removal. This degree of oxidation from the treatment was proportional to higher heating rates, indicating that the more rapid oxidation reduced the gas-solid contact with the inner sulphide portion of the ore. It was reported that the CP test showed a much higher sulphur grade before roasting, so it was difficult to properly assess this roasting behaviour.

Interestingly, phase change appeared to be the main factor that affected roasting rather than SSA, previously concluded as the important parameter in the cyanidation tests. Due to the marginal changes in

84 surface area and the roasting data, it was concluded that surface area showed no effect on the roasting potential of this ore.

With the conclusion that roasting was adversely affected for the BP test, there would likely be implications to flotation with the reduced sulphide content. As the temperatures in the continuous process would be much lower, oxidation would likely have a smaller impact; however, unfortunately this was unable to be concluded based on the CP data. A concentrate might show a more noticeable change, as well as provide insight to how flotation recovery would be affected. While the small amounts of treated ore prevented concentrate roasting, if possible, this should be followed through. Additionally, as SSA showed no impact, perhaps microwaving sulphide ores or concentrates for heat should be the focus rather than fracture in a future application – a proposed alternative to roasting, instead of treatment prior to comminution.

6.4 Recommendations The following are recommendations for progressing this area of research:

1. Additional microwave treatment tests using a pilot microwave system to determine the optimum

process parameters by varying particle size, power input and degree of packing, as well as repeats

to confirm the reproducibility of the tests.

2. Design of a microwave system optimized for ore treatment with the provided permittivity data so

that the most effective microwave field and wave pattern is used for energy delivery for the two

ores. The microwave used in this work was manufactured for the food industry, and therefore it is

certain that power absorption could be greatly improved if designed for ores.

3. Use of other ores containing different pay metals to expand the knowledge of downstream effects,

including other gold and sulphide ores. The treatment of refractory gold ores could selectively heat

the refractory phases which would have effects on recovery, potentially eliminating the necessity

for additional autoclave or roasting steps.

4. More sampling of the pregnant leach solution between 6 and 24 hours for the cyanidation tests.

This would provide a better indication of when each sample reaches its final overall gold recovery

and give more understanding to the full kinetic change of the treated gold ore.

5. Roasting of a concentrate produced by floating microwave treated ore compared to baseline tests –

more representative of an industry process.

6. Use the pilot microwave system to heat concentrates rather than primary crushed ore. This was out

of the scope of the current work, however the smaller volumes and higher sulphide content

associated with concentrates may prove to be more beneficial to metal recovery rather than treating

primary crushed ore. Refractory phases are also excellent microwave absorbers, and selectively

oxidizing these refractory phases would be attractive for this particular application.

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Appendix A

Microwave Fundamentals

Electromagnetic Spectrum Microwaves are a form of non-ionizing electromagnetic radiation, shown in Figure 28, consisting of wavelengths between 1 mm (300 MHz) and 1 m (300 GHz). This range of frequencies includes three bands: ultra-high frequency (300 MHz to 3 GHz), super-high frequency (3 GHz to 30 GHz), and extremely high frequency (30 GHz to 300 GHz) (Haque, 1999). Microwaves have longer wave lengths and lower available energy than other forms of electromagnetic energy such as visible, ultraviolet or infrared light

(Meredith, 1998). Of the allotted frequencies, industrial microwave applications tend to operate at the 915

MHz band, whereas commercial applications such as household microwave ovens operate at the 2.45 GHz frequency band.

Figure 28: An electromagnetic wave with both an electric and magnetic field.

Electromagnetic waves are defined as the transfer of energy without the need of a material medium of propagation (Adewuyi, et al., 2020). All electromagnetic waves propagate at the speed of light (c), and are characterized by frequency (f) and wavelength (λ), which are inversely proportional (Equation 13).

푐 = 푓 ∗ 휆 [13]

Among the electromagnetic spectrum, microwaves, and less notably, radio waves (radio frequency), can be used in heating applications of materials susceptible to electromagnetic energy, also known as dielectrics.

Dielectric Heating Mechanisms Microwave energy can be used to heat certain materials, however it is not a type of thermal energy and does not heat materials in the same manner as conventional heating. While conventional heating is a surface phenomenon relying on heat transfer by conduction, convection and radiation, the heating of dielectrics in an electromagnetic field involves mechanisms which occur at a molecular level, where charged atoms attempt to realign themselves in an electromagnetic field (Robinson, et al., 2010). Both microwave and radio frequency heating are referred to as dielectric heating because their heating mechanism relies on the dielectric properties of the material. Materials with higher dielectric properties generate more heat than materials with lower dielectric properties.

According to Meredith (1997), there are two primary mechanisms in which dielectrics convert microwave energy into heat: dipole rotation and ionic conduction. Dipole rotation (Figure 29) involves the short-range displacement of a charge through the formation and rotation of electric and magnetic dipoles, whereas ionic conduction (Figure 30) requires the long-range migration of a charge (Clark & Sutton, 1996).

Water is an excellent example of a material that heats well when subjected to microwaves because the oxygen atom is negatively charged, while the hydrogen atoms are positively charged, generating frictional heat when rotated due to the alternating field (Meredith, 1998). The magnitude of the effects of these two mechanisms is dependent on the frequency of the electromagnetic field; ionic conduction is the dominant mechanism at lower frequencies, and dipole rotation is the dominant mechanism at higher frequencies

(Clark & Sutton, 1996; El Khaled, et al., 2018).

Figure 29: Dipole rotation mechanism showing material with charged dipoles attempting to realign under an increasing electric field.

Figure 30: Ionic conduction mechanism showing ions moving through an electric field.

As this transfer of energy does not rely on the diffusion of heat from the surface, the possibility of achieving rapid and uniform heating of materials becomes more attainable (Thostenson & Chou, 1999).

Haque (1999) discussed the main advantages of heating dielectric material by microwaves over conventional heating, stating that microwaves can provide: non-contact heating, rapid heating, material selective heating, quick start-up and stopping, initial heating from the interior of material, ease of energy transportation, and a high degree of safety and automation. The ability to selectively heat phases rapidly within a material is of high importance in microwave processing research, and provides a more attractive alternative to conventional heating where heating is less efficient.

Dielectric Properties The affinity for materials to absorb microwave energy to produce heat is dependent on their dielectric properties. Dielectric properties can be useful in determining whether or not materials will heat

98 when exposed to a microwave field, and progress a further understanding of how exactly the material will respond to microwave radiation. This section will review dielectric properties and their relevant relationships to the heating of materials.

Permittivity & Permeability The main factors in the quantification of material response to a microwave field are defined as permittivity and permeability. The permittivity of free space is the relationship of mechanical forces between two electrostatic charges in a vacuum; similarly, the permeability of free space is the relationship of mechanical forces between two current loops in a vacuum (Meredith, 1998). If this space contains an electric or magnetic medium, the forces will be altered by the relative permittivity or relative permeability respectively.

휀 휀∗ = [14] 휀0

µ µ∗ = [15] µ0

Equation 14 denotes the relationship of relative permittivity, where ε is the complex permittivity

-12 (F/m), and ε0 is the permittivity of free space, 8.86x10 F/m (Meredith, 1998). Likewise, Equation 15 describes relative permeability where µ is the complex permeability (H/m), and µ0 is the permeability of free space, 4πx10-7 H/m.

The dielectric and magnetic properties of any material medium are defined as complex permittivity and permeability respectively. Most materials of interest are not magnetic, and consequently the interaction is mainly determined by the permittivity (Pickles & Marzoughi, 2018). Generally, the degree to which any material will absorb and distribute microwave energy is determined by the complex permittivity described in Equation 16 (Meredith, 1998). All dielectric phenomena can be attributed directly to the polarization of a material under the influence of an applied electric field (Tinga & Nelson, 1973).

ε = ε0 · (ε′ − jε′′) [16]

The complex permittivity of a homogenous material can be described by a couple of factors. The relative dielectric constant (real permittivity), ε’, corresponds to a material’s ability to be polarized in an electric field and retain microwave energy as it passes through. The imaginary permittivity, ε’’, is a measure of a material’s ability to convert microwave energy to heat (Meredith, 1998). The imaginary permittivity is sometimes defined as the ‘dissipation factor’ or ‘loss factor’, as the measurement represents the amount of input energy lost in the material as frictional heat loss (Haque, 1999). ‘Lossy’ materials are those with high dielectric properties, acting as a producer of heat when subjected to a microwave field. The amount of heat dissipated per unit volume of material increases with increasing imaginary permittivity, with higher loss factors resulting in greater heat generation (Henda, et al., 2005). While both permittivity variables contribute to the distribution of the electric field in a specific material, the imaginary permittivity is the only factor that affects the heating rate (Bobicki, et al., 2018).

Loss tangent, tan δ, is defined as the ratio of energy dissipated as frictional losses to energy stored within a medium under an electric field. The loss tangent is defined in Equation 17.

휀′′ [17] 푡푎푛 훿 = 휀′

The variable δ, represents the loss angle, with a larger angle resulting in greater losses; a high loss material will possess a much higher loss tangent than that of a low loss material (Meredith, 1998). Materials that have a small loss tangent are considered essentially transparent to microwaves, and do not undergo significant heating, whereas materials with high loss tangents will heat rapidly (Robinson, et al., 2010).

Influence of Parameters on Dielectric Properties Dielectric properties of materials vary based on a number of factors; factors that change a material’s susceptibility to microwave heating by the variation of these dielectric properties. Notably, these variables include frequency, temperature and moisture content, and other variables such as material density, structure of material, physical state and composition of the material (Nelson & Karszewski, 1990; Meredith, 1998;

Komarov, et al., 2005).

100

The frequency can vary the heating behaviour of a material within a microwave field. As the frequency of the microwaves change, the relative contributions of dielectric heating mechanisms also change (Ure, 2017). As mentioned, the dominant heating mechanism at lower frequencies is ionic conduction, whereas dipole rotation is the preferred heating mechanism at higher frequencies. If the frequency is further increased, a point will exist where the dipoles cannot rotate as fast as the electric field changes, denoted as the relaxation point (Tinga & Nelson, 1973). Tinga & Nelson (1973) described the increase of frequency likely results in a decrease in polarization due to relaxation, and therefore the dielectric constant (ε’) decreases with increasing frequency. Once the relaxation point has occurred, some energy is lost to the material and therefore as ε’ decreases, ε’’ should increase.

Temperature has a direct effect on the microwave heating ability of materials. In most materials, dielectric properties tend to increase with increasing temperature, however, it is dependent on the dominant mechanism of heating (Komarov, et al., 2005). Increasing temperature allows for more rapid rotation of charged dipoles. Meredith (1998) stated that low loss materials show large increases in loss factor with increasing temperature, thereby creating the potential for further rapid temperature changes. The temperature at which this drastic change in loss factor occurs is known as the critical temperature, and can develop localized hot spots as well as non-uniform heating in heterogeneous mediums with varying dielectric properties (Clark & Sutton, 1996; Haque, 1999).

The moisture content of a material can have a profound effect on dielectric properties. In general, a higher moisture content results in a larger dielectric constant and loss factor (Komarov, et al., 2005). This is explained in part by free water and chemically bound water within a material structure. Komarov et al.

(2005) explained the difference in these dielectric properties: free water molecules exhibit dielectric properties similar to liquid water, whereas bound water exhibits dielectric properties similar to that of ice, much lower than liquid water. Therefore the presence of free water would have a more pronounced effect.

Power Density The average power density, often referred to as average power dissipation, by definition is the volumetric absorption of energy of a medium under an electromagnetic field, Pav (Meredith, 1998).

101

ퟐ 2 Pav = ퟐ훑퐟훆ퟎ훆′′퐄 + 2πfµ0µ′′H [18]

Equation 18 yields the expression for average power density in W/m3, described by two parts: the heating as a result of both electric (bold) and magnetic losses. E and H are defined as the electric and magnetic field intensities respectively (V/m), and f, is the frequency (Hz) (Meredith, 1998). In similar

-7 fashion to dielectric constants, µ0 is the permeability of free space, 2π x 10 H/m, and µ’’ is the relative magnetic loss factor. For most microwave heating applications in the absence of magnetic materials,

Equation 18 is simplified to Equation 19.

2 Pav = 2πfε0ε′′E [19]

Equation 19 constitutes the standard dielectric heating equation. Any increase in these values in

Equation 19 will result in an increase in power dissipation density in a material, and therefore subsequent increases in heating rates (Ure, 2017). As frequency is limited to the system and cannot be changed, and dielectric properties are material dependent, electric field intensity is the most likely variable that can be manipulated to change the power density.

Heating Rate Temperature rise with-respect-to time as a result of power dissipation in a material is expressed by

Equation 20.

dT 2πfε ε′′E2 푃 [20] = 0 r = 푎푣 dt (Cpρ) 퐶푝𝜌

The heating rate of a dielectric can be defined as a direct function of the average power density, and in addition, the materials specific heat capacity (J/kg°C) and density (kg/m3). Clearly, an increase in power density can result in higher heating rates. Therefore, high-power microwave systems are ideal for maximizing heating rates.

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Penetration Depth Microwave heating is mostly dependent on the real and imaginary permittivities, although there are several parameters affected by the dielectric properties; one key parameter is the penetration depth.

Penetration depth, Dp, provides an indication of the heating uniformity within a material by the decay strength of microwave power, where λ0 is the microwave wavelength in free space, given as Equation 21

(Thostenson & Chou, 1999). Penetration depth is defined as the distance from the surface into the material at which the propagating wave power drops to e-1 from its value at the surface.

1 − 2 ′′ 2 λ0 εr [21] Dp = {√[1 + ( ) ] − 1} ′ ′ 2π√2εr εr

The penetration depth at which internal heating takes place is dependent on the material itself and its subsequent dielectric properties. A smaller penetration depth will result in better microwave absorption compared to higher depths (Meredith, 1998). As highlighted in Equation 21, higher values of dielectric properties will result in larger penetration depths resulting in non-uniform surface heating; lower values of dielectric properties will result in volumetric heating of the material (Thostenson & Chou, 1999).

Voltage Breakdown & Arcing When the electric field component of propagating microwave energy exceeds the breakdown voltage of the atmosphere containing said microwave field, the formation of an arc will occur. Arcs are highly localized forms of power dissipation resulting in the formation of plasma, and are characterized by intense light and potential burning of material or equipment (Ali & Coffey, 1980). The breakdown and subsequent arc formation interrupt normal power transmission and causes large standing waves to occur between the microwave source and the arc. These large standing waves can be destructive to the components used in microwave power generation and transmission, and can cause high amounts of reflected power (Yen, 1978).

Arcing phenomena is difficult to understand and highly random. Microwave heating of a material generally occurs by wave propagation through air before reaching a material of interest. Arcing may occur

103 during the propagation path if the electric field exceeds the specific breakdown voltage required to produce an arc (Ali & Coffey, 1980). The breakdown voltage of air in particular, is approximately 30 kV/cm, but varies based on temperature and pressure (Meredith, 1998). Yen (1978) described arcing as a function of power level, with the time for complete arc formation of the order of a few microseconds. Further, the energy required to sustain an arc is much smaller than that of initiating one; this can cause difficulty in continuous microwave heating/processing until the arc is extinguished.

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Appendix B

Microwave Heating Behaviour of Minerals

Table 24: Heating rates of selected minerals at 1.6 kW of microwave power at a frequency of 2.45 GHz. (B) signifies a violent reaction - MnO was unable to be recorded (Ford & Pei, 1967).

Mineral Colour Time (min) Max Temperature (°C) Heating Rate (°C/min)

Al2O3 White 24 1,900 79.2 C (charcoal) Black 0.2 1,000 5,000 CaO White 40 200 5

Co2O3 Black 3 (B) 900 300 CuO Black 4 800 200 CuS Dark blue 5 600 120

Fe2O3 Red 6 1,000 166.7

Fe3O4 Black 0.5 500 1,000 FeS Black 6 800 133.3 MgO White 40 1,300 32.5

MnO2 Black (B) - -

MoO3 Pale green 46 750 16.3

MoS2 Black 0.1 900 9,000

Ni2S3 Black 3 (B) 1,300 433.3 PbO Yellow 13 900 69.2

TiO2 White - - -

UO2 Dark Green 0.1 1,100 11,000 ZnO White 4 1,100 275

Table 25: Interpretation of mineral groupings based on microwave heating response (Chen, et al., 1984).

Group 1: Poor Microwave Heater Group 2: Good Microwave Heater Arsenopyrite, bornite, Carbonates Aragonite, calcite, Sulfides chalcopyrite, covellite, dolomite, siderite galena, pyrite, pyrrhotite, Almandine, allanite, anorthite, gadolinite, Silicates muscovite, potassium Oxides Cassiterite, hematite, feldspar, quartz, titanite, magnetite zircon Nickeline, tetrahedrite, Sulfates Barrite, gypsum Others pitchblende Fergusonite, monazite, Others sphalerite (low-Fe), stibnite

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Table 26: Maximum bulk temperature recorded after microwave irradiation at 1 kW of microwave power and a frequency of 2.45 GHz (Walkiewicz, et al., 1988).

Mineral Chemical Composition Temperature (°C) Time (min)

Albite NaAlSi3O8 69 7

Chalcocite Cu2S 745 7

Chalcopyrite CuFeS2 920 1

Chromite FeCr2O4 155 7 Galena PbS 956 7

Hematite Fe2O3 182 7

Magnetite Fe3O4 1258 2.75

Marble CaCO3 74 4.25

Molybdenite MoS2 192 7

Orthoclase KAlSi3O8 67 7

Pyrite FeS2 1019 6.75

Pyrrhotite Fe1-xS 886 1.75

Quartz SiO2 79 7 Sphalerite ZnS 88 7

Tetrahedrite Cu12Sb4S13 151 7

Zircon ZrSiO4 52 7

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Appendix C

Microwave Systems

Microwave Power Generation & Transmission Magnetrons The microwave generator unit is the heart of any microwave heating system. The most common microwave generator is known as a magnetron. The attractiveness of magnetrons in microwave heating is attributed to their high- power conversion efficiencies of 70% at 2.45 GHz and 90% at 915 MHz (Meredith,

1998; Ong & Akbarnezhad, 2015). A typical 2.45 GHz magnetron is shown in Figure 31 (a), and the internal structure of the device is shown in Figure 31 (b) (Ong & Akbarnezhad, 2015).

(a) (b)

Figure 31: (a) A standard commercially available magnetron; (b) the internal structure of a magnetron, from Ong & Akbarnezhad (2015).

The magnetron creates microwaves by producing electrons using a heated filament as the cathode, placed at the center of a circular chamber. The outer part of the magnetron is coated with a copper anode block, creating an accelerated transfer of electrons, while a permanent magnet imposes a magnetic field parallel to the filament. This magnetic field forces the electrons, attracted to the outer copper anode, to spiral directly outward in a circular path rather than moving toward the anode. As electrons move past the cylindrical resonating cavities placed around the rim of the chamber, they generate an electromagnetic field.

107

Waveguides In commercial microwave processes at higher than 1kW of power, waveguides are commonly used to transmit the microwave energy. The microwaves emitted from the magnetron are delivered to a material through the waveguides, usually consisting of hollow components made of material that reflects microwaves (aluminum, copper or brass), with either a rectangular or circular cross section (Ong &

Akbarnezhad, 2015). The dimensions of the waveguide are system-specific, and depend on the microwave frequency. Lower frequencies require increasingly larger waveguides. Figure 32 shows an example of a waveguide cross-section used in high power microwave heating, and a similar waveguide attached to an applicator for microwave transmission.

Figure 32: Straight waveguide cross-section, photo from Ong & Akbarnezhad (2015).

Cooling Unit Cooling systems are often required for high-power microwave systems, and are used to absorb the reflected power (the microwaves that do not get absorbed by the material). This is necessary in order to minimize potential damage to system components associated with high amounts of reflected power. These cooling systems are designed for external cooling water to continuously circulate through the magnetron, where the reflected power can safely dissipate into the water as heat.

Microwave Applicators Once the microwave energy transmission occurs to the applicator, the energy must then be transferred into the material. Microwave applicators are structures designed to both contain and deliver microwave energy into the material, also referred to as microwave cavities. Applicators are designed to maximize the efficiency of coupling between the material being heated and the electromagnetic field. The

108 distinction between applicator types is determined by the design characteristics rather than by the processes that can be performed inside the cavity (Henda, et al., 2005). The two most notable applicators used in microwave heating of materials are the multimode and the single mode resonant cavities.

Single Mode Resonant Cavities A single mode resonant cavity, also known as a single mode or a monomode cavity, is a metallic box made of microwave reflective material with its dimensions comparable to the wavelength of the microwaves (Bradshaw, 1999; Haque, 1999). The wave pattern exists where the intensity of the microwave field is high, with a relatively small volume. It is this feature in particular that allows for the generation of a high electric field in the material in this localized section of the cavity; however, it limits the amount of material that can be treated. More importantly, there is no practical way of scaling up the process to a commercial setting, and the sample size that can be heated is limited (Bradshaw, 1999). Single mode microwave ovens were introduced commercially in 2000 and became popular in the scientific community for small scale applications, with an estimated 85% of all published microwave research using single mode microwaves (Robinson, et al., 2010).

Multimode Cavities Multimode cavities are the more versatile of the two microwave applicators; it is typically a large box with dimensions greater than the wavelength of the microwave radiation (Bradshaw, 1999; Henda, et al., 2005). In comparison to a single mode cavity, the electric field in a multimode cavity is much more complex, providing a more homogenous electric field distribution allowing for the processing of a wide variety of materials with different geometries and dielectric properties over the entire cavity volume

(Batchelor, et al., 2017). Though absorbing material can be heated anywhere inside the applicator, the microwave field lacks the electric field intensity of a single mode system. These characteristics of multimode systems make them more suitable for treatment of bulk materials in either batch or continuous processes that would be required for commercial applications.

109

Industry-Scale Microwave Processing Systems Industrial microwave systems differ from kitchen and laboratory microwaves based on their achievable power limits; the smaller scale ovens generally have a maximum power input of 1-3 kW, whereas commercial microwave units can be >75 kW. Commercial microwave systems contain all of the same key components that smaller microwaves have; however, they are commonly designed to operate in a continuous fashion and the systems are much larger. These industry-scale units generally operate at 915

MHz rather than at 2450 MHz (Meredith, 1998). High-power microwaves are also usually equipped with more than one magnetron in order to achieve desired power input levels, require multiple waveguides and therefore necessitate a large amount of cooling water to dissipate any reflected power from the operation.

To allow for continuous flow of material, a conveyor belt made from a microwave transparent material is typically used. Chokes are implemented for entry and exit of the material load through the applicator, with dimensions such that the microwaves cannot escape. Haque (1999) illustrated an industrial microwave processing system, given in Figure 33, showing multiple waveguides used to transmit microwaves from generator units to the applicator, and a conveyor belt used to move material through the chokes at the entry and exit points.

Figure 33: Commercial microwave heating unit as shown by Haque (1999).

The critical parameters in applicator performance are power efficiency, uniform power distribution, and consistent performance over a range of expected load conditions. Due to the nature of large loads and continuous processing for commercial applications, these applicators are designed to be multimode, to ensure a homogenous field distribution over the entire cavity.

110

Appendix D

Assays and XRD Plots CA36-BB-XRD

60000

50000

40000

30000

Lin (Counts) Lin

20000

10000

5 10 20 30 40

60000

50000

40000

30000

Lin (Counts) Lin

20000

10000

41 50 60 70 2-Theta - Scale CA36-BB-XRD - File: 21000.raw - Type: 2Th/Th locked - Start: 5.000 ° - End: 75.027 ° - Step: 0.020 ° - Step time: 185. s - Temp.: 25 °C (Room) - Operations: Fourier 18.000 x 1 | Background 0.977,1.000 | Import 01-070-3755 (*) - Quartz - SiO2 - Y: 88.66 % - d x by: 1. - WL: 1.78897 - Hexagonal - a 4.91580 - b 4.91580 - c 5.40910 - alpha 90.000 - beta 90. 00-029-0701 (I) - Clinochlore-1MIIb, ferroan - (Mg,Fe)6(Si,Al)4O10(OH)8 - Y: 20.98 % - d x by: 1. - WL: 1.78897 - Monoclinic - a 5.36000 - b 9.28 01-082-2450 (N) - Muscovite 2M1 - (Na0.07K0.90Ba0.01)(Al1.84Ti0.04Fe0.07Mg0.04)(Si3.02Al0.98)O10( - Y: 13.83 % - d x by: 1. - WL: 1.78897 01-083-1531 (*) - Ankerite - CaMg0.32Fe0.68(CO3)2 - Y: 5.89 % - d x by: 1. - WL: 1.78897 - Rhombo.H.axes - a 4.83000 - b 4.83000 - c 16.1670

Figure 34: Qualitative XRD analysis results for the gold ore, with the main mineral phases given at the bottom, and their calculated wt. %.

111

CA41-BBP-XRD

20000

Lin (Counts) Lin 10000

5 10 20 30 40

20000

Lin (Counts) Lin 10000

41 50 60 70 2-Theta - Scale CA41-BBP-XRD - File: 21003.RAW - Type: 2Th/Th locked - Start: 5.0 00-022-0675 (D) - Microcline, intermediate - KAlSi3O8 - Y: 12.38 % - Operations: Background 0.977,1.000 | Fourier 18.000 x 1 | Import 01-070-3755 (*) - Quartz - SiO2 - Y: 87.46 % - d x by: 1. - WL: 1.788 00-009-0466 (*) - Albite, ordered - NaAlSi3O8 - Y: 69.83 % - d x by: 1 01-073-0250 (N) - Annite - KFe3AlSi3O10(OH,F)2 - Y: 32.22 % - d x 01-089-5378 (*) - Actinolite - (Fe3.112Mn0.088Mg1.954Ca1.814Na0. 01-089-1460 (A) - Anorthite - Ca(Al2Si2O8) - Y: 18.44 % - d x by: 1. - 00-029-0701 (I) - Clinochlore-1MIIb, ferroan - (Mg,Fe)6(Si,Al)4O10(O

Figure 35: Qualitative XRD results for the sulphide ore, with the main mineral phases given at the bottom, and their calculated wt. %.

112

Table 27: Elemental analysis of the gold ore using ICP.

Element Assay (wt. %) Al 9.63 As 0.25 Ca 0.88 Fe 5.86 Mg 1.50 S 0.62 Si 26.40

Table 28: Common mineralogical phases identified in the gold ore using qualitative XRD.

Mineral Phase/Name Chemical Formula Quartz SiO2 Clinochlore (Mg,Fe)6(Si,Al)4O10(OH)8 Muscovite KAl2(AlSi3O10)(F,OH)2 Ankerite CaMg0.32Fe0.68(CO3)2

Table 29: Tabulated XRF results reported for the gold ore.

Element Assay (wt.%) Al 11.4 Ca 0.9 As 0.04 Fe 6.1 Mg 1.5 S 0.2 Si 28.38

Table 30: Elemental analysis of the sulfide ore using ICP.

Element Assay (wt. %) Al 7.79 Ca 2.92 Cu 1.03 Fe 7.53 Mg 1.87 Ni 0.32 S 1.20 Si 25.54

113

Table 31: Common mineralogical phases identified in the sulphide ore using qualitative XRD.

Mineral Phase/Name Chemical Formula Quartz SiO2 Albite NaAlSi3O8 Annite KFe3AlSi3O10(OH,F)2 2+ Actinolite Ca2(Mg2.5-4.5Fe 0.5-2.5)Si8O22(OH)2 Anorthite Ca(Al2Si2O8) Clinochlore (Mg,Fe)6(Si,Al)4O10(OH)8

Table 32: Tabulated XRF results reported for the sulphide ore.

Element Assay (wt.%) Al 8.71 Ca 3.84 Cu 0.7 Fe 7.5 Mg 1.5 Ni 0.2 S 0.7 Si 28.3

114

Appendix E

Experimental Microwave Equipment Setup

Batch Low-Power Microwave System Specifications The bench-scale microwave unit used in this work is known as a BP-211, manufactured by

Microwave Research and Applications, Illinois, USA. The microwave unit works similar to a kitchen microwave, equipped with a simple applicator design and a magnetron for microwave generation. A sample can be placed inside the cavity and a heating time was selected. The applicator was designed to have a multimode wave pattern, providing an even distribution of microwave energy throughout the cavity. The system is rated for a maximum power input of 3.2 kW and operates at a frequency of 2.45 GHz (Figure 36).

Figure 36: Bench-scale microwave unit by Microwave Research and Applications, IL, USA. 3.2 kW power input at 2.45 GHz. Pilot-Scale Microwave System Specifications The pilot-scale microwave system was designed and manufactured by Thermowave (Danvers, MA,

USA). It consists of two 75 kW microwave generators for a maximum system power input of 150 kW.

Each is comprised of a magnetron tube which produces microwaves at a frequency of 915 MHz (Figure

37A). The microwaves travel through waveguides into the applicator, with any reflected power transported back through the waveguides and where the energy dissipates into cooling water as heat, thereby preventing any damage to the magnetron. The pilot system is controlled by an HMI control panel equipped with an

Allen Bradley PLC (Figure 37B).

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Figure 37: Internal components of one microwave generator power supply (A); and HMI control panel (B).

The applicator is 1 m long, with a microwave transparent polypropylene bridge to facilitate sample movement. The applicator also includes a built-in arc sensor which shuts the system down if arcs are detected. The conveyor belt is made of microwave transparent silicone/polyester material and can reach up to 5 m/s. Microwave transparent polypropylene trays were designed to carry the ore via conveyor belt through the microwave system, and can hold up to 70 kg. Each tray is approximately 1 m in length, roughly the length of the applicator. Figure 38 A shows sample trays packed with ore material ready to move through the applicator via the conveyor, while Figure 38B shows the inside of the applicator with an empty, stationary tray for reference.

Figure 38: Conveyor belt with packed ore sample trays (A); and inside of the microwave applicator (B).

For each test, belt speed, particle packing and input power were the independently controlled variables used to manipulate dependent variables that included residence time (s), energy dosage (kWh/t), and throughput (t/hr).

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There is a ramp up period for the power after the initiation of power generation within the pilot system. During ramp-up, the microwave field strength slowly builds within the applicator and eventually reaches an equilibrium or steady-state. An example of this power trend is shown in Figure 39.

Figure 39: Example plot for power ramp-up, showing microwave power and conveyor belt speed. Once the power reaches a certain threshold, the belt switches on for ore treatment.

The microwave belt was programmed to start once a minimum forward power for each generator was achieved. After 11-12 seconds, the conveyor moved the sample through the applicator. Trays referred to as ‘dummy loads’, comprised of waste ore, were used to absorb the initial energy delivered to the system prior to the belt initiating and the sample tray moving through. Most tests were comprised of one sample tray, with two dummy load trays placed in front of the sample tray and one behind. Immediately upon test completion, the sample trays were imaged for their heat signature using a FLIR A8300sc infrared camera.

The camera was located above the belt, recording an infrared video, shown in Figure 40.

Figure 40: Infrared camera setup at exit of microwave applicator, with infrared depiction, for obtaining ore heat signature.

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Appendix F

Batch Microwave Treatment Results

Low-Power Bench-Scale Microwave Tests

Microwave Heating Behaviour The maximum temperatures measured at different heating times were used to generate microwave heating behaviour curves in the bench-scale system. As outlined in the literature, absorption of microwave energy on ores can be grouped into four separate categories, defined by their heating rates (Wong, 1975):

1. Hyperactive/excellent heater – 20°C to 200°C per second,

2. Active/good heater - 120°C to 400°C per minute,

3. Difficult to heat/poor heater – 15°C to 80°C per minute,

4. Inactive/non-heater – <5°C per minute.

Figure 41 displays microwave heating behaviour versus treatment time for the sulphide (triangular points) and gold ore (circular points) samples of 50 g and 100 g at 3.2 kW of microwave power.

Figure 41: Maximum bulk temperatures recorded for 50g and 100g gold and sulphide ore samples at a particle size range of -3.35+1.7 mm. A microwave power of 3.2 kW and frequency of 2.45 GHz was used.

Based on the heating relationships of the two ores, clearly the sulphide ore was a better heater than the gold ore. A relationship of more rapid heating for 100 g samples in comparison to 50 g samples for

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each ore was observed, explained by the additional surface area for conductive heat transfer. The maximum

temperature recorded was the sulphide, reaching 661°C after 6 minutes of heating, in contrast to the gold

ore which only reached 329°C after 12 minutes. Additionally, the highest instantaneous rate of temperature

change was 51°C/min and 122°C/min for the gold and sulphide ore respectively. Based on these heating

rates, they may be grouped into the categories defined by Wong (1975): the gold ore appears to be a

‘difficult to heat/poor heater’, and the sulphide ore an ‘active/good heater’.

The explanation for the difference in heating behaviour of the two ores is the mineralogy. Based

on the assays, the gold and sulphide ore have approximately 3% and 5% of strong microwave absorbing

minerals respectively. These minerals include: chalcopyrite; pyrite; pyrrhotite; iron oxides, specifically

hematite and magnetite; and for the sulphide ore, pentlandite. With a higher percentage of these minerals,

it would be expected that an ore would heat more rapidly, which is clearly seen in the heating behaviour

data. Greater temperature gradients between minerals induce higher thermal stress and subsequent fracture

(Jones et al., 2005; Ali & Bradshaw, 2009). Therefore it would be expected that the sulphide ore is more

suitable for the treatment.

Low-Power Batch Microwave Treatment For the bench-scale treatments (BB), four minutes was selected for both ores to balance expected

bulk temperature with process time based on the heating behaviour data in the previous section. A summary

of these treatments is detailed in Table 33.

Table 33: Summary of lab-scale treatment results for both ores, with resultant average temperature and mass loss.

Particle Average Bulk Average Input Energy Ore Sample Irradiation Power Sample Size Temperature Mass Dose (Test ID) Time (min) Input (kW) Mass (g) (mm) (°C) Loss (g) (kWh/t) Gold (BB) 4 3.2 -3.35+1.7 100 172 0.06 2133 Sulphide 2133 4 3.2 -3.35+1.7 100 399 0.17 (BB)

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After treatment, the average bulk temperature was 172°C and 399°C for the gold and sulphide ore

respectively, aligned with the microwave heating behaviour curves discussed in the previous section.

Additionally, there appears to be a larger mass loss after irradiating the sulphide ore. This is due to the

higher sulphur content, resulting in more oxidation of the sulphide phases. Worth noting is the incredibly

high microwave energy dose per tonne of ore of 2133 kWh/t. This is common in bench-scale literature due

to the limitations of a material capacity and power density.

High-Power Batch Microwave Treatment

Table 34 shows the input power and treatment time for the BP tests on the two ores, with bulk

temperature and calculated energy dose. The BP tests had a much lower input energy per tonne compared

to the bench-scale treatments. The difference in energy is significant; however, the kWh/t is still much

higher than what has been defined as the economic range in literature by Bradshaw et al. (2007) of <5

kWh/t.

Table 34: Input parameters for treatment of the gold and sulphide ores for the batch pilot (BP) tests.

Ore Sample Treatment Power Particle Sample Bulk Temperature Input Energy (Test ID) Time (s) Input (kW) Size (mm) Mass (kg) (°C) Dose (kWh/t)

Gold (BP) 45 75 -3.35+1.7 10.00 197.8 94

Sulphide (BP) 30 75 -3.35+1.7 10.00 263.0 63

Similarly to the BB treatments, the sulphide ore heated more rapidly than the gold ore. Figure 42

shows the surface temperature readings after each of the BP tests. Figure 42 shows a lack of even heat

distribution throughout the sample, indicating there is lack of microwave field uniformity in specific areas

of the applicator. This would impact the ability to maximize differential heating throughout the entire

sample. Lack of movement of these samples in particular during treatment also likely contributed to

concentrated areas of heating (Ong & Akbarnezhad, 2015).

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A B

Figure 42: Surface infrared image of 10 kg batch pilot (BP) microwave treatments for A) gold ore and B) sulphide ore.

Figure 43 shows the microwave power curves for both the gold (left) and sulphide (right) ore BP tests respectively. The total forward microwave power was plotted with the resulting reflected power and calculated absorbed power.

Figure 43: Power trend for the 10 kg BP treatment for the gold (left) and sulphide ore (right).

Based on these BP treatments, it appears that the absorbed power trend gradually increases until reaching steady state. Clearly, these batch tests at the particle size -3.35+1.7 mm have a consistent power absorption throughout the treatment. When accounting for the total timeframe of the test from start to finish, the absorption efficiency with respect to both the forward microwave power and electrical power can be calculated, given in Table 35.

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Table 35: Percentage of power absorbed in ore sample of the total forward and electrical power for the batch pilot tests.

Microwave Test % Absorbed/Forward Power % Absorbed/Electrical Power Gold Ore (BP) 47.9 35.3 Sulphide Ore (BP) 49.5 39.3

It can be seen in Table 35 that of the two batch treatments in the pilot system, the sulphide ore was more efficient at absorbing microwave energy at 39.3% electrical efficiency, compared to 35.3% efficiency for the gold ore.

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Appendix G

Particle Size Distributions

Table 36: F80 and P80 values for all as-received and microwave treated ground ore for the Bond ball mill.

Sample Test Microwave Energy F (µm) P (µm) ID Input (kWh/t) 80 80 Gold Ore RB - 2941.9 53.8 BB 2133 2933.7 51.8 BP 94 2918.0 53.4 RP - 1254.5 52.5 CP 2 1220.7 50.3 Sulphide Ore RB - 2880.8 74.1 BB 2133 2879.9 75.8 BP 63 2886.2 74.6 RP - 1203.0 75.6 CP 2 1219.1 76.1

100 90 80 70 RB - Sulfide 60 BB - Sulfide 50 BP - Sulfide 40 30 RB - Gold

Cumulative Cumulative Passing % 20 BB - Gold 10 BP - Gold 0 10 100 Particle Size (µm)

Figure 44: Particle size distributions of the ground -106 µm material for the gold ore and sulphide ore batch tests, BB and BP. Reference, RB, is also shown.

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100 90 80 70 60 RP - Sulfide 50 CP - Sulfide 40 RP - Gold 30

Cumulative Cumulative Passing % CP - Gold 20 10 0 10 100 Particle Size (µm)

Figure 45: Particle size distributions of the ground -106 µm material for the gold ore and sulphide ore continuous tests. Reference, RP, is also shown.

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Appendix H

Selected Sample Photos

Figure 46: Polypropylene tray filled with gold ore on the microwave transparent conveyor belt prior to microwave treatment. Particle size of -63.5+25.4 mm with -12.7 mm material packed to fill void spacing.

Figure 47: Polypropylene tray filled with sulphide ore on the microwave transparent conveyor belt prior to microwave treatment. Particle size of -63.5+25.4 mm with -12.7 mm material packed to fill void spacing.

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Figure 48: Gold ore particle after microwave treatment showing an area of high localized heating with subsequent burning.

Figure 49: Rocks from the bulk gold ore sample during material handling and sample preparation stages.

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Figure 50: Rocks from the bulk sulphide ore sample during material handling and sample preparation stages.

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