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How to cite this thesis

Surname, Initial(s). (2012). Title of the thesis or dissertation (Doctoral Thesis / Master’s Dissertation). Johannesburg: University of Johannesburg. Available from: http://hdl.handle.net/102000/0002 (Accessed: 22 August 2017). THE OCCURRENCE OF CHROMIUM AND OTHER TRACE ELEMENTS IN SELECTED SOUTH AFRICAN COALS

MARIA LOUISA MOKWENA

Dissertation submitted in fulfilment of the requirement for the degree of

MASTER OF SCIENCE in GEOLOGY

in the FACULTY OF SCIENCE

at the

UNIVERSITY OF JOHANNESBURG

Supervisor Prof. NICOLA WAGNER Co-supervisor Dr. ALLAN KOLKER

DECLARATION

I declare that this dissertation is my own, unaided work, except where acknowledged. It is being submitted to the degree of Master of Science in the Department of Geology, University of Johannesburg. It has not been submitted before for any degree or examination to any other University.

Signature Date

DEDICATION

I would like to dedicate this research project to my mother, Cathrine Mokwena and my brothers, Simon Mokwena and Lucas Mokwena. I really appreciate their love, support, and encouragement throughout my studies.

ACKNOWLEDGMENTS

Firstly, I would like to thank the Almighty God, who has carried me throughout this journey, despite all the challenges I encountered.

I would like to extend my sincere gratitude to my supervisor Prof. Nicola Wagner for giving me an opportunity to work on this project, for supporting and encouraging me, and for also conducting petrographic analyses, I am very grateful. I would like to thank Maseda Mphaphuli for introducing me to coal geology and for her support during this project. My special thanks to DST-NRF CIMERA for the financial support during this research project.

I would like to express my sincere appreciation to my co-supervisor Dr. Allan Kolker from the United States Geological Survey (USGS), for accepting the proposal to assist with this research project and the feedback provided. I would also like to thank Dr. Frank Dulong from the USGS for conducting X-ray diffraction (XRD) analyses. I would like to thank Philip Pieterse and Dr. Christian Reinke from the University of Johannesburg (UJ) Spectrum, for assisting during inductively coupled plasma mass spectrometry (ICP-MS) and X-ray fluorescence (XRF) analyses.

From the UJ Department of Chemistry, I would like to thank Christopher Kgatshe and Thomas Mabasa for allowing me to use the equipment and providing assistance during the selective leaching procedure. I would also like to express my sincere appreciation to Janet Smith for allowing me to use the Retsch mill, and assisting with proximate analyses at University of Witwatersrand (Wits), School of Chemical Engineering.

I would like to acknowledge the staff and fellow post-graduate students, especially in the Coal Group at UJ. I would like to thank my friends, Nomonde Hlengwa, Palesa Letswalo, Katlego Mukwevho, Nomsa Ndlovu and Mothusi Boihang for their support.

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CONTRIBUTION TO SCIENCE

The preliminary results for this study were presented at the following conferences:

 Mokwena, M.L., Wagner, N.J., Kolker, A., 2018. The occurrence of chromium and other selected trace elements in South African coals. Geocongress 2018 18-20 July 2018, University of Johannesburg (Abstract and Poster).

 Mokwena, M.L., Wagner, N.J., Kolker, A., 2018. Chromium in coal: Organic or inorganic? CIMERA Colloquium, 13 November 2018, University of Witwatersrand (Presentation).

ABSTRACT

Chromium (Cr), manganese (Mn), cobalt (Co), and molybdenum (Mo) have been reported to occur in higher concentrations than global averages in South African coals. The trace element data pool for South African coals is fairly limited, and research is required to confirm the origin and mode of occurrence of trace elements. The main aim of this study was to investigate the mode of occurrence specifically of Cr, as well as Mn, Co, and Mo in select South African coals and to determine the possible source of Cr.

Coals from the Waterberg, Soutpansberg, Witbank, Highveld, and the Nongoma Coalfields were considered. A total of 21 samples were obtained and assessed using coal petrography and chemical analyses for coal characterization, XRD and XRF for mineralogy, ICP-MS for trace elements, and the scanning electron microscope (SEM) equipped with the Energy Dispersive X-ray Spectroscopy (EDX), density fractionation, and sequential leaching to determine modes of occurrence.

The Waterberg, Soutpansberg, Witbank, and Highveld Coalfield coal samples have higher Cr concentrations compared to the Nongoma Coalfield samples and global averages for hard coals. Chromium in these coals could have originated from the Bushveld Complex, but the Cr concentrations are far much lower than those of coals formed near an ultramafic source. Chromium from primary sources could have been dissolved and been redistributed during coal formation.

Chromium is associated with both the organic and inorganic matter in the selected coal samples. High Cr concentrations were observed in the lighter density fractions, indicating an association of Cr with the organic matter, which is in agreement with literature. A positive correlation coefficient between Cr and kaolinite was determined, which implies Cr could be hosted in kaolinite. From selective leaching, most of the Cr was leached using hydrofluoric acid (HF), indicating an association of Cr with silicates. The Cr that was not leached and remained in the residue could be associated with the organic matter. These apparently conflicting results are in agreement with the literature and imply multiple modes of occurrence. It would have been of interest to determine Cr speciation in these coal samples in order to understand if high Cr contents are of health and environmental concern.

TABLE OF CONTENTS

DECLARATION ...... i

DEDICATION ...... ii

ACKNOWLEDGMENTS ...... iii

CONTRIBUTION TO SCIENCE ...... iv

ABSTRACT ...... v

TABLE OF CONTENTS ...... vi

LIST OF FIGURES ...... x

LIST OF TABLES ...... xiv

ABBREVIATIONS ...... xvi

CHAPTER 1: INTRODUCTION ...... 1

1.1. Introduction ...... 1 1.2 Background information ...... 4 1.3 Problem statement ...... 6 1.4 Study Area ...... 6 1.5 Aim and objectives ...... 7 1.6 Research questions ...... 8 1.7 Structure of the thesis ...... 9

CHAPTER 2: LITERATURE REVIEW ...... 10 2.1 Overview of coal formation ...... 10 2.1.1 Coal type ...... 11 2.1.2 Coal grade ...... 13 2.1.3 Coal rank ...... 16 2.2 Overview of coal in South Africa ...... 16 2.2.1 Coal geology of South Africa ...... 17 2.2.2 Coalfields in South Africa...... 19 2.2.2.1 The Waterberg Coalfield ...... 19 2.2.2.2 Soutpansberg Coalfield ...... 22 2.2.2.3 Witbank Coalfield ...... 23 2.2.1.1 Highveld Coalfield ...... 25 2.2.1.2 Nongoma Coalfield ...... 26

2.3 Trace elements in coal ...... 28 2.3.1 The release of trace elements ...... 29 2.3.2 Hazardous air pollutants ...... 31 2.3.3 Mode of occurrence ...... 32 2.3.4 Trace elements of interest to the current study ...... 34 2.3.4.1 Chromium ...... 34 2.3.4.2 Manganese ...... 38 2.3.4.3 Cobalt ...... 38 2.3.4.4 Molybdenum ...... 39 2.4 Analytical techniques ...... 40 2.4.1 Coal characterization ...... 40 2.4.1.1 Coal Petrography ...... 40 2.4.1.2 Proximate analysis ...... 41 2.4.1.3 Ultimate analysis ...... 43 2.4.2 Mineralogy ...... 43 2.4.2.1 XRD ...... 43 2.4.2.2 XRF ...... 44 2.4.3 ICP techniques ...... 45 2.4.3.1 ICP-MS ...... 45 2.4.3.2 Sample preparation ...... 47 2.4.4 XAFS ...... 49 2.4.5 Determining the mode of occurrence ...... 50 2.4.5.1 SEM ...... 50 2.4.5.2 Statistical methods ...... 51 2.4.5.3 Density fractionation ...... 52 2.4.5.4 Selective leaching ...... 52 2.5 Chapter summary ...... 54

CHAPTER 3: SAMPLES AND METHODOLOGY ...... 55 3.1 Sample collection ...... 55 3.2 Sample preparation ...... 57 3.3 Analyses ...... 59 3.3.1 Coal characterization ...... 60 3.3.1.1 Petrographic analysis ...... 60 3.3.1.2 Proximate analysis ...... 61 3.3.1.3 Ultimate analysis ...... 63 3.3.2 XRD ...... 63 3.3.3 XRF ...... 64 3.3.4 ICP-MS ...... 64 3.3.5 Mode of occurrence determination ...... 65 3.3.5.1 Density fractionation ...... 65 3.3.5.2 Sequential leaching ...... 65

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3.3.5.3 SEM-EDX ...... 66 3.3.6 XAFS ...... 66

CHAPTER 4: RESULTS ...... 67 4.1 Petrography...... 67 4.2 Proximate analysis ...... 71 4.3 Ultimate analysis results ...... 73 4.4 Mineralogy...... 74 4.5 Major elements ...... 77 4.6 Trace elements ...... 80 4.6.1 Impact of mill contamination on results ...... 80 4.6.2 Trace elements concentrations in samples prepared using mortar and pestle 80 4.7 Mode of occurrence ...... 82 4.7.1 Density fractionation ...... 82 4.7.1.1 Petrography ...... 82 4.7.1.2 Proximate analysis ...... 83 4.7.1.3 Trace elements ...... 84 4.7.1.4 Mineralogy ...... 86 4.7.2 Selective leaching ...... 86 4.7.3 SEM-EDX ...... 87

CHAPTER 5: DISCUSSION ...... 89 5.1 Coal characterization ...... 89 5.2 Mineralogy...... 90 5.3 Major elements ...... 91 5.4 Trace elements ...... 92 5.5 Mode of occurrence ...... 95 5.5.1 Correlation coefficients ...... 95 5.5.2 Density fractionation ...... 96 5.5.3 Selective leaching ...... 98 5.5.4 SEM-EDX ...... 100 5.6 Why do South African coals contain high Cr? Possible source ...... 101

CHAPTER 6: CONCLUSION ...... 103

6.1 Summary and conclusion ...... 103 6.2 Recommendations ...... 105

REFERENCES ...... 106

viii

APPENDIX A ...... 126

APPENDIX B ...... 137

APPENDIX C ...... 138

APPENDIX D ...... 149

APPENDIX E ...... 151

APPENDIX F ...... 153

LIST OF FIGURES

Figure 2.1: Regional cross-section of the MKB Karoo Supergroup (Falcon, 1986) 19

Figure 2.2: A stratigraphic column of the geology of the Waterberg Coalfield (Faure et al., 1996) 21

Figure 2.3: The generalized stratigraphy within the Witbank Coalfield (Cairncross and Cadle, 1988) 24

Figure 2.4: A generalized stratigraphy of the Nongoma Coalfield (Hatherly and Sexton, 2013) 28

Figure 2.5: Categorisation of trace elements based on volatility behaviour (Clarke, 1993) 30

Figure 2.6: The source of Cr from the different industries into the environment, the transportation mechanism of Cr and the reduction of Cr6+ into Cr3+ within water, soil and plants, with some of the effects (Shahid et al., 2017) 36

Figure 2.7: Simplified geological map of the Bushveld Complex (Kinnaird and McDonald, 2005) 37

Figure 2.8: Major components of ICP-MS (Tyler and Jobin, 2003) 46

Figure 2.9: Sequential steps followed in selective leaching of coal (Kolker et al., 2000) 53

Figure 3.1: Sampling the No. 4 Seam, Witbank Coalfield 57

Figure 3.2: Sample preparation and analyses flowchart 58

Figure 3.3: Retsch ZM200 with stainless steel components 59

Figure 3.4: Titanium-niobium coated cassette 59

Figure 3.5: The different analyses conducted 60

Figure 3.6: Zeiss Axio Imager M2 polarised reflected light microscope, housed at the University of Johannesburg, in the geology department 61

Figure 3.7: The Perkin Elmer TGA housed at Wits 62

Figure 3.8: A graph showing the proximate analysis data determined by TGA 62

Figure 4.1: The distribution of the maceral groups and mineral matter (vol %) 69

Figure 4.2: Proximate analysis for the selected coal samples (wt. %) 72

Figure 4.3: Mineralogy of the coal samples 76

Figure 4.4: The distribution of the major element oxides within the selected coal samples (wt. %) 79

Figure 4.5: BSE image showing macerals and minerals within sample S2M-F1.4. Figure 4.5B-D: Spectrum peaks for minerals (B) macerals (C and D) 87

Figure 4.6: BSE image showing macerals and minerals within sample S2M-F1.4. Figure 4.6B-D: Spectrum peaks for selected particles 88

Figure 5.1: Petrographic composition of the selected coalfields, calculated from the samples arithmetic mean 89

Figure 5.2: Mineralogy of the selected coalfields, calculated from the samples arithmetic mean 90

Figure 5.3: Figure 5.3: Select trace element concentrations normalized with the Clarke coal values. Error bars are included to depict the range of values determined for each element determined per coalfield. The orange dotted line indicates the global average and in comparison to the specific coalfields 93

Figure 5.4: Correlation of Cr in parent coal samples. A: fixed carbon (FC) and Cr; B: Ash and Cr; C: illite and Cr; D: kaolinite and Cr concentrations 96

Figure 5.5: Correlation between FC, ash, and Cr concentrations in density fractionated samples. A: FC and Cr; B: ash content and Cr concentrations 97

Figure 5.6: Correlation between kaolinite and Cr concentrations in density fractionated samples 97

Figure 5.7: Chromium concentration in different density separated coal samples 98

Figure 5.8: Figure 5.8: Chromium leached by different solvents and Cr remaining in the residue after sequential leaching (%) 99

Figure 5.9: An example of BSE images and elemental mapping of Cr in sample S2M- F1.4. Figure 5.9B shows the elemental map of Cr for Figure 5.9A and Figure 5.9D shows the elemental map of Cr for Figure 5.9C 100

Figure 5.10: A geological map showing the location of the Bushveld Complex and the selected coalfields in this study, indicated with numbers 102

Figure A-1: Proximate results for sample WZ9 126

Figure A-2: Proximate results for sample WZ7 126

Figure A-3: Proximate results for sample WZ5 127

Figure A-4: Proximate results for sample FBU 127

Figure A-5: Proximate results for sample FMU 128

Figure A-6: Proximate results for sample SBU 128

Figure A-7: Proximate results for sample SMU 129

Figure A-8: Proximate results for sample S2T 129

Figure A-9: Proximate results for sample S2M 130

Figure A-10: Proximate results for sample S2B 130

Figure A-11: Proximate results for sample S4T 131

Figure A-12: Proximate results for sample S4M 131

Figure A-13: Proximate results for sample S4B 132

Figure A-14: Proximate results for sample H1A 132

Figure A-15: Proximate results for sample H2B 133

Figure A-16: Proximate results for sample N1A 133

Figure A-17: Proximate results for sample N1B 134

Figure A-18: Proximate results for sample N1C 134

Figure A-19: Proximate results for sample N2A 135

Figure A-20: Proximate results for sample N2B 135

Figure A-21: Proximate results for sample N2C 136

Figure B-1: XRD diffractograph pattern for sample WZ9 138

Figure B-2: XRD diffractograph pattern for sample WZ7 138

Figure B-3: XRD diffractograph pattern for sample WZ5 139

Figure B-4: XRD diffractograph pattern for sample FBU 139

Figure B-5: XRD diffractograph pattern for sample FMU 140

Figure B-6: XRD diffractograph pattern for sample SBU 140

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Figure B-7: XRD diffractograph pattern for sample SMU 141

Figure B-8: XRD diffractograph pattern for sample S2T 141

Figure B-9: XRD diffractograph pattern for sample S2M 142

Figure B-10: XRD diffractograph pattern for sample S2B 142

Figure B-11: XRD diffractograph pattern for sample S4T 143

Figure B-12: XRD diffractograph pattern for sample S4M 143

Figure B-13: XRD diffractograph pattern for sample S4B 144

Figure B-14: XRD diffractograph pattern for sample H1A 144

Figure B-15: XRD diffractograph pattern for sample H2B 145

Figure B-16: XRD diffractograph pattern for sample N1A 145

Figure B-17: XRD diffractograph pattern for sample N1B 146

Figure B-18: XRD diffractograph pattern for sample N1C 146

Figure B-19: XRD diffractograph pattern for sample N2B 147

Figure B-20: XRD diffractograph pattern for sample N2C 147

Figure B-21: XRD diffractograph pattern for samples WZ7-F1.4, SBU-F1.4, S2M- F1.4, S4M-F1.4, S4B-F1.4, H1A-F1.4 148

Figure D-1: Elemental composition of macerals and clay minerals 151 Figure D-2: Elemental mapping across macerals and clay minerals 152

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LIST OF TABLES

Table 1.1: The concentrations of trace elements in various South African coals compared to global averages for coal (ppm) 3

Table 2.1: The different macerals present in coal (ICCP, 1998, 2001; Pickel et al., 2017) 12

Table 2.2: The most common minerals in South African coals (Bühmann, 2001) 14

Table 2.3: Typical coal qualities of the Witbank coal seams (Smith and Whittaker, 1986) 25

Table 2.4: Typical coal qualities of the Highveld Coalfield (Jordaan, 1986) 26

Table 2.5: Average Cr, Mn, Co, and Mo concentrations in South African coals compared to global averages for coals (ppm) 34

Table 3.1: List of samples obtained from the different localities (Fig. 1.1) 56

Table 4.1: Maceral groups, mineral matter (vol %) and vitrinite reflectance (%RoVmr) results 68

Table 4.2: Coal rank classification using vitrinite reflectance (%RoVmr) results of the selected coals (modified after Wagner et al., 2018) 70

Table 4.3: Proximate analysis for the selected coal samples (wt. %) 71

Table 4.4: Ultimate analysis for the selected coal samples reported on dry ash free basis (%) 73

Table 4.5: Mineralogy of the selected coal samples (vol %) 75

Table 4.6: The major element oxides present in the Waterberg, Soutpansberg, Witbank, Highveld, and Nongoma Coalfields (wt. %) 78

Table 4.7: Average Cr concentrations in the 21 selected coal samples prepared using a mill compared to samples prepared using mortar and pestle (ppm) 80

Table 4.8: Chromium, Mn, Co, and Mo concentrations in the selected coal samples, together with the certified values of SARM 19 and Clakes values (ppm) 81

Table 4.9: Percentage yields of the samples after density fractionation 82

Table 4.10: Maceral groups and mineral matter of the density fractionated samples with the parent samples indicated in bold (vol %) 83

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Table 4.11: Proximate analysis of the density separated samples with the parent samples indicated in bold, and reported as received and on a dry basis (wt. %) 84

Table 4.12: Chromium, Mn, Co, and Mo concentrations in the selected coal samples with the parent samples indicated in bold (ppm 85

Table 4.13: Mineralogy of the selected coal samples (%) 86

Table 4.14: Chromium leached by different solvents and the remaining residue (%) 86

Table 5.1: Average major element oxides in selected South African coals (wt. %) 91

Table 5.2: Average concentration coefficient (CC) 92

Table 5.3: Chromium, Mn, Co, and Mo concentrations in various South African coals determined during this study compared to global averages for coal (ppm) 94

Table C-1: Chromium concentrations for the selected samples 149

Table C-2: Manganese concentrations for the selected samples 149

Table C-3: Cobalt concentrations for the selected samples 150

Table C-4: Molybdenum concentrations for the selected samples 150

ABBREVIATIONS

CC Concentration coefficient

CV Calorific value

EDX Energy Dispersive X-ray Spectroscopy

ESRF European Synchrotron Radiation Facility

FC Fixed carbon

HAPs Hazardous air pollutants

ICP-OES Inductively coupled plasma optical emission spectrometry

ICP-MS Inductively coupled plasma mass spectrometry

ICCP International Committee for Coal and Organic Petrology

LTA Low-temperature ashing

MATS Mercury and Air Toxics Standards

MKB Main Karoo Basin

PGMs Platinum group minerals ppm Parts per million

SEM Scanning electron microscope

TGA Thermogravimetric analyzer

UJ University of Johannesburg

Wits University of Witwatersrand

USA United States of America

USGS United States Geological Survey

VM Volatile matter

XAFS X-ray absorption fine structure spectroscopy

XANES X-ray absorption near edge structure

XRF X-ray fluorescence

XRD X-ray diffraction

xvi

CHAPTER 1: INTRODUCTION

1.1. Introduction

Coal utilization has raised many environmental issues due to the emission of CO2 and hazardous air pollutants (HAPs) to the atmosphere, primarily from coal combustion (Huggins, 2002; U.S. EPA, 2011; Zhao et al., 2018). Trace elements in coals are of interest globally due to strict legislation on emissions from coal combustion as a result of detrimental effects to human health and the environment. Many countries such as Canada, China, and the United States of America (USA), have implemented legislation to control trace element emissions from thermal power stations (Sloss, 2012). With the increasing environmental legislation and drive to clean coal, trace element data has become a requirement for the coal export market.

South Africa relies heavily on coal for power generation since it lacks alternative energy sources. In South Africa, there is currently no legislation concerning trace elements emissions from coal combustion, despite the extensive use of coal. The current data pool for trace elements values for South African coals is limited. Thus, there is a need to study and understand trace elements and their mode occurrence in the coals of the region, in order to determine if potential mitigation measures are required.

Trace elements in coal occur in parts per million (ppm), and mass mining and consumption of coal can result in large quantities of potentially harmful trace elements being released to the environment (Gupta, 1999; Liu et al., 2005; Zheng et al., 2007). In South Africa, over 115 million tons of coal are burnt annually resulting in high volumes of trace elements (Garnham and Langerman, 2016). The release of these trace elements can cause various health problems (Finkelman, 1999). Millions of people in China have suffered from fluorosis and thousands from arsenism as a result of domestic coal combustion (Finkelman, 1999). Exposure to fly ash and flue gas released from coal-fired power plants can cause asthma, heart attacks, strokes, and affect the respiratory and nervous system (Finkelman and Tian, 2018).

The USA Clean Air Act Amendments, European Union, and the Canadian Environmental Protection Agency have listed some trace elements as hazardous and/or of environmental concern (U.S. EPA, 1990; EMEP, 2009). These include

arsenic (As), beryllium (Be), cadmium (Cd), cobalt (Co), chromium (Cr), mercury (Hg), manganese (Mn), nickel (Ni), lead (Pb), antimony (Sb), and selenium (Se), which all potentially occur in coal (Finkelman, 1994; Booth et al., 1999; Huggins, 2002). This study will focus on Cr, Mn, Co, and Mo as they have been reported to occur in high concentrations in South African coals, as shown in Table 1.1, compared to global averages compared to global averages (Cairncross et al., 1990; Faure et al., 1996; Wagner and Hlatshwayo, 2005; Bergh et al., 2011; Wagner and Tlotleng, 2012; Kolker et al., 2017).

Chromium is present (mainly) as trivalent Cr (Cr3+) or (rarely) as hexavalent Cr (Cr6+). Trivalent Cr is an essential nutrient for humans and animals, and it is insoluble in water. Hexavalent Cr is more toxic than trivalent Cr, which is soluble in water and it is considered as a carcinogen (Deonarine et al., 2015). Thus, Cr is included as a HAP, due to its carcinogenic and toxicological properties of Cr6+ (Deonarine et al., 2015). Exposure to Cr6+ can cause nose bleeding, ulcers, or even death under extreme circumstances in humans (Gupta, 1999; Geelhoed et al., 2002; Karar et al., 2006). Past studies have shown that Cr occurs as Cr3+ in both macerals and clay minerals (Huffman et al., 1994; Wang et al., 2008; Huggins et al., 2009).

Manganese is a key component widely used in the production of aluminium alloys and dry cell batteries as oxides (Corathers, 2003). It is also an essential nutrient for plants and animals, although it is toxic at high concentrations (Hurley and Keen, 1987). Manganese toxicity in humans mainly affects the respiratory tract and the brain. In coal, Mn is associated with carbonate minerals such as siderite (Swaine, 1990).

Cobalt is a shiny, grey, brittle metal that is used to create alloys and also to create an intense blue colour in glass and paints (Boland and Kropschot, 2011). Cobalt is an essential nutrient that assists with the metabolism and reproduction of red blood cells in humans, but high exposure to Co can cause respiratory problems (WHO, 2011). In coal, Co occurs within sulphides such as pyrite (Finkelman, 1994).

Table 1.1: The concentrations of trace elements in various South African coals compared to global averages for coal (ppm).

Trace South South Witbank Waterberg Highveld Witbank Waterberg Highveld South Average Global Average Coal Clarke element African African Coalfield Coalfield4 Coalfield Coalfield Coalfield7 Coalfield 4 African SA range9 global values hard (ppm) coals1 coals2 2 Seam3 4 Seam5 4 Seam6 Seam8 range1-8 values1-8 values10 coals11

As nd 0.9-8.2 4.6 13.0 3.1 4.7 8.6 2.0 0.9-13.0 5.9 0.5-80 5.0 9±0.7

Cd 0.4 nd nd nd 0.4 0.3 0.2 nd 0.2-0.4 0.3 0.1-3 0.6 0.2±0.04

Co 7.8 3.3-14 7.9 9.0 6.3 11.0 13.1 4.5 3.3-14 8.7 0.5-30 5.0 6±0.2

Cr 12-63 25.0 28.0 64.0 70.5 41.4 43.5 47.7 12.0-64.0 45.4 0.5-60 10.0 17±1

Hg 0.3 nd nd nd 0.2 0.3 1.4 0.3 0.2-1.4 0.5 0.02-10 0.02 0.1±0.01

Mn 30.0 nd nd 89.0 19.5 135 208 55.0 19.5-208 89.4 5.0-300 50.0 76±6

Mo 1-2.7 <1-2.7 nd 3.0 1.2 2.1 2.3 2.6 1.0-3.0 2.1 0.1-10 5.0 2.1±0.1

Ni 15.5 6.9-32 17.0 21.0 21.1 27.2 55.4 20.5 6.9-55.4 25.1 0.5-50 15.0 17±1

Pb 10.1 1.9-25 10.0 92.0 7.5 15.0 34.3 8.4 1.9-34.3 24.0 2.0-80 25.0 9±0.7

Sb nd nd 0.5 nd 0.3 0.5 0.8 0.5 0.3-0.8 0.5 0.05-10 3.0 1±0.09

Se 0.9 <0.4-0.9 0.9 3.0 1.1 1.2 1.2 1.1 0.4-3.0 1.2 0.2-10 3.0 1.6±0.1

nd: not determined 1Watling and Watling, 1982; 2Willis, 1983; 3Cairncross et al., 1990; 4Faure et al., 1996; 5Wagner and Hlatshwayo, 2005; 6Bergh et al., 2011; 7Wagner and Tlotleng, 2012; 8Kolker et al., 2017; 9Swaine, 1990; 10Zhang et al., 2004; 11Ketris and Yudovich, 2009.

Molybdenum is a key alloying agent that is used to increase the strength and resistance to corrosion of steels (Kropschot, 2010), and it is also an essential element in animal and plant nutrition (Ragagopalan, 1987). Molybdenum is not considered a HAP, but some studies have shown that high Mo in pastures can cause diseases in cattle, which can affect their growth (Nalbandian, 2012). The mode of occurrence of Mo is not clear in coal, since it can be associated with the organic matter, sulphide, and silicate minerals (Wang et al., 2008; Finkelman et al., 2018).

Whilst an understanding of the concentration of the above elements is important, the impact is dependent on the oxidation state and mode of occurrence. Information about the concentration and mode of occurrence of a potentially toxic element in coal will help determine if the use of coal might present a health or environmental risk.

Furthermore, understanding the distribution of an element in a coal deposit may help delineate areas to avoid within a coal deposit. Selective mining and/or coal washing can also be applied to effectively to reduce toxic trace elements prior to coal combustion and gasification, reducing their emission to the atmosphere. The mode of occurrence of Cr in South African coals will be investigated further in this study together with Mn, Co, and Mo.

1.2 Background information South Africa is known for its mineral wealth which contributes toward our economy. It is one of the top producers of platinum group minerals (PGMs), gold, diamonds, Mn, and coal globally (Chamber of Mines, 2016). South Africa is also one of the world’s leading producers of Cr, which is used to manufacture stainless steel. The major reserves of Cr and PGMs occur in the UG-2 Reef and Merensky Reef, within the Rustenburg Layered Suite of the Bushveld Complex (Beukes et al., 2012). Chromium incorporated in South African coals could have originated from the Bushveld Complex, but this aspect will be discussed further during this research.

The origin and distribution of trace elements in South African coals has not been studied extensively. Watling and Watling (1982) and Willis (1983) conducted studies on coals from selected coalfields in South Africa and concluded that they contained lower trace element concentrations than coals from the USA, Australia, with

exceptions of Cr, Mn, Co, and Mo. From a geochemical study by Cairncross et al. (1990) in the Witbank Coalfield, an average concentration of 28.0 ppm for Cr was obtained. Faure et al. (1996) obtained an average concentration of 64.0 ppm for Cr in coals from the Waterberg Coalfield. Wagner and Hlatshwayo (2005) studied coals from the No. 4 Seam of the Highveld Coalfield in order to understand the trace element concentration, and an average concentration of 70.5 ppm was obtained for Cr. Bergh et al. (2011) obtained an average concentration of 41.4 ppm for Cr for coals within the No. 4 Seam of the Witbank Coalfield.

As shown in Table 1.1, a global average of 0.5-60 ppm for Cr was reported by Swaine (1990); a Cr average of 10.0 ppm for hard coals was reported by Zhang et al. (2004); and more recently, an average of 17.0 ppm was reported by Ketris and Yudovich (2009). In this study, select coal samples will be analysed in order to understand the variation in Cr concentrations as shown by previous studies.

Correct sampling and sample preparation are crucial for accurate analyses. Trace element determination in coal is still a challenge as coal has to be in solution prior to ICP analysis. Coal is difficult to digest using conventional digestion methods, which use concentrated acids combined with high temperature and pressure conditions (Mesko et al., 2010).

A variety of analytical techniques are typically used to analyse coal and determine the trace elements present. A combination of inductively coupled plasma optical emission spectrometry (ICP-OES), ICP-MS, XRF, XRD, and SEM are typically used to gain a holistic picture of trace elements in coal and coal ash (Huggins, 2002).

The mode of occurrence of an element in coal is important since it determines the behaviour of the element during coal combustion, beneficiation, or leaching (Finkelman, 1993). It can be determined by direct and indirect techniques. X-ray absorption fine structure (XAFS) spectroscopy is a direct and non-destructive method that can be used to determine the oxidation state of an element. Indirect techniques include statistical methods, density fractionation, and selective leaching.

1.3 Problem statement There is a limited amount of publications on trace elements present in South African coals, especially when compared to China and the United States of America. Table 1.1 shows several studies that have been undertaken to analyse trace elements in South African coals (Cairncross et al., 1990; Faure et al., 1996; Wagner and Hlatshwayo, 2005; Bergh et al., 2011; Wagner and Tlotleng, 2012; Kolker et al., 2017). In these studies, it was revealed that Cr, Mn, and Co occur in high quantities compared to global averages for coals (Wagner and Hlatshwayo, 2005; Bergh et al., 2011; Kolker et al., 2017).

The values reported in the literature require confirmation as the current data pool is small. It would be of interest to determine why these elements in particular report such high values, and to understand the origin of these elements in South African coals. Thus, their determination, mode of occurrence and probable origin are of interest in order to predict the potential environmental and health impacts they may pose. The data from the previous studies on South African coals raise a question as to why do South African coals have high concentrations of these specific trace elements and where would these elements have originated?

1.4 Study Area Coal deposits in South Africa are hosted within the Vryheid and Volksrust Formations of the Ecca Group in the Karoo Supergroup. The Karoo Supergroup was deposited in different Karoo basins (Fig. 1.1), namely the Main Karoo, Ellisras, Tuli, Tshipise, Springbok Flats, and the Lembobo Basins. The Main Karoo Basin (MKB) is a foreland basin (Catuneanu et al., 1998), which contains the main economic coal reserves. The other Karoo basins are located north of the MKB, occurring as extensional rift basins, such as the Ellisras Basin containing a significant amount of coal reserves (Cairncross, 2001). In South Africa, coal is found in 19 coalfields ranging from low-rank bituminous in the Free State to anthracite in the KwaZulu- Natal Province (Cairncross, 2001).

Figure 1.1: The distribution of South African coalfields within the Karoo basins of South Africa. Coalfields for this study are indicated with numbers, 1-5 (modified after Catuneanu et al., 2005, and Hancox and Götz, 2014). The Waterberg and Soutpansberg Coalfields occur within the Ellisras and Tshipise Basins, north of the MKB. The Witbank, Highveld, and the Nongoma Coalfields occur within the MKB. These five coalfields are considered for this study and are discussed in greater detail in Chapter 2. Coal is currently mined in the Waterberg, Witbank, Highveld, and the Nongoma Coalfields, while exploration is underway in the Soutpansberg Coalfield. Coal that is produced in South Africa is used for domestic power generation, the feedstock for coal to liquids production, and for the export market.

1.5 Aim and objectives The aim of this study is to investigate the occurrence of Cr and other selected trace elements (Mn, Co, Mo) present in South African coals in order to determine their concentration, and to infer their mode of occurrence and origin.

The aim of this study will be achieved through the following objectives:

 Obtain coal samples from various localities in five different coalfields in South Africa.  Undertake detailed coal characterization using coal petrography, and routine chemical analysis such as proximate and ultimate analyses.  Determine the mineralogy of the coal samples using XRD.  Determine the major elements in the coal ash samples using XRF.  Determine the trace element concentration using ICP-MS.  Determine the mode of occurrence of Cr, Mn, Co, and Mo using SEM-EDX, density fractionation, and sequential leaching.  Consider the possible source of these trace elements in these coal samples.

In addition to the above objectives, the investigation will seek to determine Cr speciation using XAFS, but facilities to do so are not available in South Africa. The application to use XAFS at the European Synchrotron Radiation Facility (ESRF) was not approved.

The data will add to the limited data pool of trace elements in South African coals. This will provide information that will help understand the origin of trace elements in South African coals. It will also be of use to the coal industry and general public.

1.6 Research questions 1. What are the petrographic and geochemical differences in coals from the different coalfields in South Africa? 2. Which minerals are present in these selected South African coals? 3. Which sample preparation technique is able to completely digest the coal/ coal ash into solution, suitable for ICP analysis? 4. Which ICP technique is suitable to determine the Cr, Mn, Co, and Mo concentrations in coal between ICP-OES and ICP-MS? 5. Does Cr tend to concentrate on specific seams or coalfields? 6. Do Cr, Mn, Co, and Mo have organic or inorganic affinities in these samples? 7. What is the possible source of high Cr, Mn, Co, and Mo levels in these South African coals?

1.7 Structure of the thesis Chromium, Mn, Co, and Mo have been reported to be high in South African coals, and this study aims to investigate the occurrence and origin in selected South African coals. In order to conduct this study, literature was reviewed to gain understanding and find a way on how to go about this study. This study is outlined in six chapters.

Chapter 2 provides a review of the literature focusing on coal utilization, coal formation and coal geology with a specific focus on the five coalfields of interest. These include the Waterberg, Soutpansberg, Witbank, Highveld, and the Nongoma Coalfields. Trace elements are discussed in terms of concentration, origin, and mode of occurrence. Chromium, Mn, Co, and Mo are of interest for this study and they are discussed in greater detail. The different analytical techniques that will be used for this study are outlined, as well as their advantages and disadvantages. These include petrography, proximate, ultimate, XRD, ICP-MS, XRF, SEM, density fractionation, and sequential leaching.

Chapter 3 outlines the sampling procedures undertaken to obtain samples and the methodology used in this study. A total of 21 samples were obtained, prepared, and subjected to different analyses. These analyses include petrography, proximate, ultimate, XRD, XRF, ICP-MS, SEM, density fractionation, and sequential leaching.

Chapter 4 presents the results obtained from the different analyses conducted. The results include coal characterization, mineralogy, trace elements, and mode of occurrence.

Chapter 5 discusses the results, including the mode of occurrence and the possible source of Cr in South African coals.

Chapter 6 outlines the major conclusions of this study. Recommendations for future studies are also included.

CHAPTER 2: LITERATURE REVIEW

This chapter provides a review of coal in South Africa, as well as trace elements in coal and their mode of occurrence. An overview of the different analytical techniques used to analyse coal and determine trace elements in coal is provided.

2.1 Overview of coal formation In order to understand trace element occurrence in coal, it is necessary to gain an understanding of coal formation. Coal is a combustible sedimentary rock that was formed from buried plant debris deposited in a swamp. Coal formation involves two processes, namely: peatification and coalification (Ward and Suárez-Ruiz, 2008). Peatification is a process in which plant material is converted into peat by aerobic and anaerobic bacteria in a swamp. Peat is buried at depths for several million years and this leads to coalification due to enhanced temperature and pressure conditions. As a result, the chemical composition and coal rank changes and coal may be lignite, subbituminous, bituminous or anthracite, depending on its burial conditions (Ward and Suárez-Ruiz, 2008).

Different geological ages have led to the formation of coal in the northern and southern hemispheres. During the Carboniferous Period (359-299 million years ago), vegetation accumulated in warm periodic flooding conditions, which resulted in typically thick vitrinite-rich deposits in the Northern Hemisphere (Malvadkar et al., 2004). During the Permian Period (299-251 million years ago), typically inertinite-rich coals were formed in swamps under cold to cool temperature conditions associated with an ice-age across Gondwanaland in the Southern Hemisphere (Falcon, 1986). The high inertinite content could have been a result of high oxidation rates and microbial degradation during peatification (Falcon and Snyman, 1986).

South African coals have lateral and vertical regional differences, due to variable plant communities, changing climate, basement, sedimentary, and structural settings. This has led to a diverse organic composition and mineral matter constituents throughout the different coalfields and seams in the country (Falcon, 1986).

Coals in South Africa are typically inertinite-rich and have high mineral matter (ash) (Wagner et al., 2018), although deposits in the Limpopo Coalfields are enriched in vitrinite. Coal type, rank, and grade are discussed below.

2.1.1 Coal type Coal type refers to the nature of the organic matter found in coal, and the original plant material, which is makes up the organic components of the coal. Coal type is based on the relative amounts of constituent macerals (vitrinite, inertinite, and liptinite) (Snyman, 1989; Moore and Shearer, 2003).

Lithotypes are macroscopically identifiable layers of 5 mm in hand specimens, which are formed as a result of change in input and conditions in the mires (Wagner et al., 2018). They are distinguished from one another by colour, streak produced from coal, basic lustre, fracture pattern, texture, and type of stratification (Wagner et al., 2018). Lithotypes are divided into sapropelic and humic coals (O’Keefe et al., 2013).

Sapropelic coals are dull and compact, with a granular surface and conchoidal fracture, and they were formed in deeper water from non-woody material, such as algae (Wagner et al., 2018). Sapropelic coals include cannel, boghead, torbanite, and oil shales. Humic coals are banded with alternating layers of varying brightness, which were formed from wood and/or reed in peat swamp (O’Keefe et al., 2013). Humic coals include vitrain, clarain, durian, fusain, duroclarin, clarodurian (Stopes, 1935; Diessel, 1992).

Macerals are entities which evolved from different organs and tissues of plant material during primary accumulation, peatification, and the early stages of coalification (Falcon and Snyman, 1986). Macerals are grouped together by origin or mode of degradation and preservation. Macerals are distinguished by optical properties and chemical composition as discussed by Falcon and Ham (1988) and classified by the International Committee for Coal and Organic Petrology (ICCP) classification system (1998). The three main maceral groups are discussed.

 Vitrinite is derived from cell-wall material and woody tissue of plants. It is difficult to see the structure of vitrinite due to extreme gelification (Falcon and Snyman, 1986). Vitrinite is composed of polymers, cellulose, and lignin, and comparatively high oxygen (ICCP, 1998). Vitrinite is highly sensitive to heat as it becomes more vitreous, denser, and tougher when subjected to heat. As a result, vitrinite is used as an index to measure the maturity of the coal (Schweinfurth and Finkelman, 2003). Vitrinite appears to be shiny and bright in hand specimen and the lithotype is vitrain (ICCP, 1998). The vitrinite macerals are listed in Table 2.1 and the dominant vitrinite maceral in South African coals is collotellinite.

Table 2.1: The different macerals present in coal (ICCP, 1998, 2001; Pickel et al., 2017). Vitrinite Liptinite Inertinite

Cutinite Tellinite Suberinite Fusinite Collotellinite Sporinite Semifusinite Vitrodetrinite Resinite Funginite Collodetrinite Exsudatinite Secretinite Corpogelinite Chlorophyllinite Macrinite Gelinite Alginate Micrinite Liptodetrinite Inertodetrinite Bituminite

 Liptinite is derived from finely ground macerated remains of spores, pollen, leaf cuticles, plant resins and waxes (Padley, 1995). Liptinite macerals are more enriched in hydrogen than inertinite and vitrinite. They have a lower reflectance than other macerals of the same rank (Pickel et al., 2017), and fluoresce when illuminated with ultra-violet or blue light (Diessel, 1982). The liptinite macerals are listed in Table 2.1. Sporinite and cutinite are the dominant liptinite macerals in South African coals.

 Inertinite is a common maceral in most South African coals. It is derived from bark, stem, leaves, and roots that have been altered and degraded under oxidizing conditions during peatification (Suárez-Ruiz and Ward, 2008). Inertinite has higher C content and lower O and H content than vitrinite and liptinite (ICCP, 2001). Inertinite macerals are distinguished by the structure and high reflectance (Falcon and Synman, 1986), and they are less reactive

compared to the other maceral groups. Inertinite appears to be dull in hand- samples and referred to as durain (Falcon, 1986). The inertinite macerals are listed in Table 2.1, and the most dominant inertinite macerals in South African coals are semifusinite and inertodetrinite.

2.1.2 Coal grade Coal grade refers to the range of impurities in the coal, which is a result of mineral matter (Ward and Suárez-Ruiz, 2008). Coals in South Africa are low grade since they are typically rich in mineral matter. Mineral matter refers to inorganic mineral phases in coal (Gluskoter, 1975; Ward, 2002, 2016). Minerals are deposited during peat formation by dust particles blown by the wind, sediments from the river or by precipitation from groundwater. Mineral matter may also contain inorganic compounds and organo-metallic complexes found in the original plant (Wagner et al., 2018). Minerals in coal can occur as syngenetic or epigenetic. Syngenetic minerals occur as discrete mineral grains trapped in the coal matrix (Ward, 2002).

Syngenetic minerals are incorporated during the accumulation of plant debris by blown, washed or transported sediments when major compaction and solidification of coals has not taken place. As a result, syngenetic minerals are typically difficult to beneficiate. Syngenetic minerals occur as siderite nodules, aggregates of pyrite, and infillings of clays, quartz, carbonates, sulphates, and phosphate minerals.

Epigenetic mineral form after coalification due to external processes, such as alteration of minerals and precipitation from moving groundwater, and then deposited by the percolating waters into fractures, cavities, and pores within the coal seam (Ward, 2016). They are typically large and easily liberated during coal beneficiation (Wagner et al., 2018). Epigenetic minerals typically occur in coal fissures and void infillings as clays, quartz, calcite, dolomite, and pyrite (Ward, 2002).

Bühmann (2001) used XRD to identify minerals present in the No.4 Seam in the Witbank and Highveld Coalfields. These minerals include clays, quartz, carbonates, pyrite, and apatite. Pinetown et al. (2007) also used XRD to investigate the mineral matter in the Witbank and Highveld coals.

The most abundant minerals in South African coals are clays (kaolinite and illite), quartz, carbonates (calcite, dolomite, and siderite), pyrite, and apatite, presented in Table 2.2 (Gaigher, 1980; Bühmann, 2001).

Table 2.2: The most common minerals in South African coals (Bühmann, 2001).

Minerals in coal occur as aluminosilicates, carbonates, sulphides, sulphates, and phosphates, as discussed below.

 Aluminosilicates Aluminosilicates include clay minerals which are the most abundant minerals found in coals, and hence they are major ash contributors (Spears, 2000, Falcon, 2013). Clay minerals are formed as a result of diagenetic and hydrothermal alterations of rocks. Aluminosilicates consist primarily of silica, alumina, and water, and small quantities of iron (Fe), alkali, and alkali-earth elements (Nayak

and Singh, 2007). Kaolinite is the most common clay mineral in coals. Some trace elements such as Cr and vanadium are associated with clays (Kolker et al., 2000a; Speight, 2005).

 Carbonates Carbonate minerals in coal include calcite, dolomite, and ankerite (Table 2.2). They vary in composition because of the extensive solid solution of calcium, magnesium, iron, manganese that is possible within them. Calcite is the most common carbonate occurring in veins or cleat infillings as well as in cell cavities of macerals (Speight, 2005).

 Sulphides and sulphates Pyrite and marcasite are the dominant sulphide minerals found in coal, which are polymorphs. Marcasite is distinguished from pyrite in reflected light by its anisotropy, and it is difficult to distinguish from pyrite in SEM (Kolker, 2012). Pyrite occurs in a variety of morphologies such as framboids, isolated euhedral crystals (Roberts, 1988). Sulphides are easily oxidized and form hydrated sulphate minerals such as jarosite (Ward, 2002; Kolker and Huggins, 2007). Other common sulphates found in coals include barite and jarosite (Speight, 2005). These minerals are mainly of epigenetic origin, occurring as crack infillings (Vassilev and Vassileva, 1996).

 Phosphates Apatite is the most common phosphate mineral found in coals. Other phosphate minerals in coals include gorceixite and goyazite (Table 2.2). Phosphate minerals can be syngenetic or epigenetic in origin (Vassilev and Vassileva, 1996).

Major minerals such as silicates, sulphides, and carbonates, are the main carrier of trace elements in coal (Finkelman, 1995; Dai et al., 2012; Wang et al., 2016, Qin et al., 2018), and will be discussed in Section 2.3.3.

2.1.3 Coal rank Coal rank refers to the degree of metamorphism undergone by coal and it is a measure of the maturity of coal. It is assessed in terms of the fixed carbon (FC), moisture, volatile matter (VM), calorific value (CV), and vitrinite reflectance. As coal rank increases, the chemical composition of vitrinite correspondingly changes, and vitrinite macerals become increasingly reflective. Hence, they are used to determine the rank by vitrinite reflectance (Schweinfurth and Finkelman, 2003; Wagner et al., 2018). As coal rank increases, the CV and FC content increase, while the moisture and VM content decreases.

Coal rank ranges from lignite to anthracite. In South Africa, coal rank ranges from sub-bituminous to anthracites, with the majority of the coal being medium rank C bituminous (Wagner et al., 2018). Typically coal rank increases from west to east across South Africa, related to the greater geothermal gradients in the east as a result of dolerite outpourings during the break-up of Gondwana (Snyman and Barclay, 1989).

2.2 Overview of coal in South Africa Coal is the main source of primary energy in South Africa. This is because coal is relatively cheap to extract and is abundant in South Africa; it will certainly remain our most important energy resource for the future. Eskom is South Africa’s primary supplier of electricity and generates ~90% of the electricity used in the country. Eskom has 30 power stations in commission, consisting of 15 coal-fired stations, 1 nuclear station, 4 gas turbine stations, 6 hydroelectric stations, 1 wind station, and 3 pumped storage schemes (Eskom Integrated Report, 2018).

Coal was first mined in the Eastern Cape in 1870 in the Molteno Coalfield (Hancox and Götz, 2014) (Fig. 1.1). However, the majority of the coal deposits in South African are located in the Highveld and Witbank Coalfields, and in the Limpopo Province (Jeffrey, 2005; Bergh, 2009). Coal that is produced in the country is being mined in the Mpumalanga, Limpopo, KwaZulu-Natal, and the Free State provinces (Chamber of Mines, 2017).

Coal production in South Africa historically has concentrated in large mines, but currently, small-scale mining is increasing in output as enforced by the government (DMR, 2015). The main coal mining companies in South Africa include Anglo Thermal Coal, Coal of Africa, Exxaro, Glencore, Sasol, South32, and Universal Coal (Chamber of Mines, 2017). South Africa remains one of the most important coal producing countries in the world, being the world’s 7th producer, 6th consumer and 7th exporter of coal (Chamber of Mines, 2017).

South Africa has an abundant, cheap, and easy access to coal that is targeted at four main markets. These four markets include global exports, synthetic fuels, domestic energy supply, and domestic heating and industrial use (Steyn and Minnitt, 2010). The energy sector, synthetic fuels, and the steel industry account for more than 80% of domestic coal demand. In 2016, South Africa exported 68.9 Mt of its coal to Europe, South America, China, and India via the Richard’s Bay Coal Terminal (Universal Coal, 2017).

Coal will remain our major energy source in the future, due to its relative abundance and comparatively low cost of mining, and because South Africa does not have currently viable oil and gas deposits. Although the MKB is one of the potential resources of shale gas, because of its extensive size and appropriate host rocks, only small uneconomic amounts have been found in the shales (Geel et al., 2015). This is because the shales have been devolatized and the volatile-rich organic matter has been destroyed by dolerite intrusions (Rowsell and Swardt, 1976). The Witbank and Highveld Coalfields are becoming depleted and the Waterberg Coalfield is expected to produce coal in the future (Jeffrey, 2005; DMR, 2015). The Waterberg coals have a high percentage of ash (50–60%) in the raw product. Therefore, the coal has to be beneficiated to remove the mineral matter to improve the quality and decrease the ash content (Jeffrey, 2005).

2.2.1 Coal geology of South Africa Coal in South Africa is Permian in age, except in the Molteno Coalfield where it is Triassic in age (Cairncross, 2001; Johnson et al., 2006). The coal deposits in southern Africa were formed during the two periods in the Permian (early Permian and Late Permian) resulting in variable lithologies (Cairncross, 2001). The early Permian coal is commonly associated with sandstone, whereas the late Permian

deposits contain mudstone of non-marine terrestrial clastics. Coal is hosted in the Ecca Group of the Karoo Supergroup which was deposited in the Karoo basins. The Karoo basins formed during the break-up of Gondwana ~180 million years ago in southern Africa (Catuneanu et al., 2005).

The MKB is the largest Karoo basins (Fig. 1.1), underlying two-thirds of the present land surface of southern Africa (Smith, 1990). It a retro-arc foreland basin situated behind a magmatic arc and associated fold-thrust belt (Cape Fold Belt) (Johnson, 1996; Catuneanu et al., 1998). The other Karoo basins to the north of the MKB are extensional rift basins which formed as either cratonic grabens or half grabens, and they are structurally complex, for example, the Ellisras Basin (Cairncross, 2001; Hancox and Götz, 2014).

The Karoo Supergroup succession is 12 km thick in the southern part of the MKB and thins towards the north forming an asymmetrical basin (Tankard et al., 2009) (Fig. 2.1). The MKB Karoo Supergroup is made up of the Dwyka, Ecca, Beaufort, and Stormberg Groups (Fig. 2.1), which represent sediments deposited in glacial, marine, fluvio-deltaic, fluviolacustrine, dry and wet environments, which were later intruded by Drakensberg Group sills and dykes (Johnson et al., 1996; Cadle et al., 2000). In the north-eastern part of the MKB, the Ecca Group is represented by the basal Pietermaritzburg, Vryheid, and Volkrust Formations which hosts the major coal-bearing strata (Johnson, 2009). Coalfields pertaining to this study are discussed in Section 2.2.2.

Figure 2.1: Regional cross-section of the MKB Karoo Supergroup (Falcon, 1986).

2.2.2 Coalfields in South Africa In South Africa, coal is found in 19 coalfields, delineated on variations in origin, formation, quality and tectonic setting of the deposits. The coal ranges from low-rank bituminous in the Free State to anthracite in KwaZulu-Natal (Cairncross, 2001). The Waterberg and Soutpansberg Coalfields in the Limpopo Province, the Witbank and Highveld in the Mpumalanga Province and the Nongoma Coalfield (Fig. 1.1) in the KwaZulu-Natal Province considered for this study are discussed further.

2.2.2.1 The Waterberg Coalfield The Waterberg Coalfield contains 40 to 50% of South Africa’s remaining coal resources (Hancox and Götz, 2014). The coalfield currently supplies coal to the Eskom’s Matimba Power Station and the new Medupi power station. The Waterberg Coalfield is located north of the Waterberg Mountain range in the Limpopo Province, and covers an area of 360,000 hectares (Hancox and Götz, 2014).

The Waterberg Coalfield occurs within the fault-bounded Ellisras sub-basin (Catuneanu et al., 2005). The coalfield is structurally deformed, with major normal faults striking east-west and northwest-southeast. The Daarby Fault has caused displacements which have separated the coalfield into a shallow and deep

underground mining area. This has allowed shallow coal to be mined at the Grootegeluk Coal Mine owned by Exxaro (Cairncross, 2001).

The basement of the Waterberg Coalfield consists of the rocks of the Transvaal Supergroup and the Bushveld Complex (Johnson et al., 1996). The basement is unconformably overlain by the sedimentary succession of the Karoo Supergroup (Beukes et al., 1991). The coal seams are hosted within the Vryheid and Volksrust/Grootegeluk Formations of the Ecca Group (Cairncross, 2001) (Fig. 2.2). The coal-bearing sequence is divided into 11 zones, which are equivalent to coal seams (Cairncross, 2001; Hancox and Götz, 2014).

The Vryheid Formation is 55 m thick. It is made up of coarse sandstones, carbonaceous mudstones and coal seams (Fig. 2.2) that range from 1 to 6 m thick. The coal seams are divided into zones 1, 2, 3 and 4, and they are predominantly dull coals (inertinite-rich) that are mainly used for power generation.

The overlying Volksrust/Grootegeluk Formation (Fig. 2.2) is between 70 to 90 m thick and hosts the thick interbedded coal type in 7 coal zones, which range from zone 5 to 11(Faure et al., 1996). The Grootegeluk Formation consists of carbonaceous mudstones that are interbedded with thin coal seams. The coal seams are vitrinite- rich and high mineral matter and require beneficiation (Wagner et al., 2018).

Several products are produced from the Grootegeluk Formation following advanced coal beneficiation, which includes semi-soft coking coal, metallurgical coal for local and export market and power station middlings products (Wagner et al., 2018). The coal quality within the coal-mudstone sequence increases vertically. The average coal qualities are moisture (1.8%), VM (22.6%), FC (35.8%), ash (26.7%), and CV (9.0 MK/kg) (Wagner et al., 2018)

Figure 2.2: A stratigraphic column of the geology of the Waterberg Coalfield (Faure et al., 1996).

2.2.2.2 Soutpansberg Coalfield The Soutpansberg Coalfield is situated north of the Soutpansberg Mountain Range in the Limpopo Province (Brandl, 1981). The coalfield is structurally deformed with east-west and northwest-southeast trending normal faults (Cairncross, 2001) and it is preserved in a half-graben structure (Hancox and Götz, 2014). The Soutpansberg Coalfield is sub-divided into three separate smaller sub-basins. These include the Pafuri (Eastern Soutpansberg), Tshipise (Central Soutpansberg) and Mopane (Western Soutpansberg) sub-basins, which are sometimes referred to as coalfields.

The Karoo Supergroup in the Soutpansberg Coalfield overlies the rocks of the Soutpansberg Group and the Beit Bridge Complex (Malaza, 2013). Like elsewhere in South Africa, the basal part of the Karoo succession is formed by the Dwyka Group; the Dwyka Group equivalent is referred to as the Tshidzi Formation. It is 5 to 20 m thick, composed of diamictite and coarse-grained sandstone (Hancox and Götz, 2014). The overlying Madzaringwe Formation is ~220 m in thickness, an equivalent of the Ecca Group, and comprises of sandstone, siltstone, and shale containing two coal seams (McCourt and Brandl, 1980).

The lower seam is 2.5 m thick and the upper main seam is 3.5 m thick, and consists of nine coal bands which are separated by carbonaceous shale (McCourt and Brandl, 1980). The Madzaringwe Formation is overlain by the Mikambeni Formation, which is 20 to 150 m thick. It is composed of siltstone, carbonaceous mudstone, and sandstones (Hancox and Götz, 2014). Thin coal seams occur throughout, and the coal is generally bright and vitrinite-rich.

The coal rank increases towards the east of the basin (Sparrow, 2012). A total of 267 Mt of recoverable coal are estimated to be in the Soutpansberg Coalfield (Jeffrey, 2005). Exploration for both thermal and coking coal set to take place in the Soutpansberg Coalfield, with a number of new mines planned in the near future (Hancox and Götz, 2014).

2.2.2.3 Witbank Coalfield Coal was first mined in the Witbank Coalfield in 1895 at the Landau Colliery, just outside the town of Witbank (Schalenkamp, 2006). The coalfield has since been supplying both metallurgical coal and thermal coal for both local and export markets (Hancox and Götz, 2014). Many of the major coal-fired power stations in South Africa are situated within the coalfield.

The Witbank Coalfield is situated north of the MKB (Fig. 1.1), from the town of Springs in the west to Belfast in the east (Hancox and Götz, 2014). The underlying basement of the Witbank Coalfield consists of rocks in the Witwatersrand Supergroup, Transvaal Supergroup, and Bushveld Complex. As a result the basement contributed to the nature of the sedimentary fill in the lower part.

Coal is hosted within the Vryheid Formation. The coal seams are flat-lying to gently undulating, with a regional dip of 1–3º to the south. Five seams were identified (Fig. 2.3) and only, the No. 2, 4, and 5 Seams are economically important (Hancox and Götz, 2014).

The No. 1 Seam is thin, discontinuous, and interlaminated with sandstone and mudstone and consists predominantly of dull coal with high ash content. The No. 2 Seam is 6.5 m thick and accounts for ~70% of the coal resources in the Witbank Coalfield (Snyman, 1998). It was mined as a metallurgical product due to the low ash content, and exploitation has declined over the past 20 years, leading to the No. 4 Seam becoming the main source of export products.

Figure 2.3: The generalized stratigraphy within the Witbank Coalfield (Cairncross and Cadle, 1988).

The overlying No. 4 Seam varies from 2.5 m to 6.5 m in thickness, and accounts for ~25% of the coal in the Witbank Coalfield. The No. 4 Seam contains dull coal which is predominately supplied to local power stations. The coal in the No. 4 Seam has high ash and inertinite compared to the No. 2 Seam, and as a result it is beneficiated.

The No. 5 Seam has been extensively eroded, but where present it is 0.5 m to 2 m thick. The coal is generally of a high quality, with low ash (Table 2.3), high fixed carbon (FC), and low phosphorus, and it is marketed as a metallurgical product (Hancox and Götz, 2014). The coals are characterised as bituminous based on vitrinite reflectance of 0.49–0.62 %RoVmax (Cairncross, 2001).

Table 2.3: Typical coal qualities of the Witbank coal seams (Smith and Whittaker, 1986). No. 1 Seam No. 2 Seam No.4 Seam No.5 Seam CV (MJ/kg) 24.0 21.2 22.2 28.7 Moisture (%) 1.7 5.3 2.6 2.5 VM (%) 21.0 21.5 20.7 32.0 Ash (%) 25.4 23.3 27.6 13.1 FC (%) 51.9 49.9 49.1 52.4

Dolerite dykes and sills are common and render most of the coal un-mineable in the coalfield. Dykes of 0 to 1 m thick are common, and the most prominent dyke is the Ogies dyke, which is 15 m thick, 100 km long, and strikes east-west. Sills of 15 to 50 m thickness transgress seams causing tilting and displacement of seams (Jeffrey, 2005). Samples from the No. 2 Seam and No. 4 Seam were obtained for this study from the Ntshovelo Colliery near Delmas.

2.2.1.1 Highveld Coalfield The Highveld Coalfield is located in the Mpumalanga Province, south of the Witbank Coalfield (Fig. 1.1) and it is hosted within the MKB. It covers an area of 700,000 hectares, from Nigel in the west to Davel in the east (Hancox and Götz, 2014). The Sasol’s synthetic fuels industry uses coal resources from the Highveld Coalfield which have moderate ash, and low vitrinite (Wagner et al., 2018). These coals are also used for power generation and other domestic industries.

The basement is made up of the rocks of the Witwatersrand Supergroup, Transvaal Supergroup, and Bushveld Complex (Cairncross, 2001). As a result the Karoo succession varies in thickness due to the uneven nature of the basement in the coalfield (Fig. 2.1).

The Highveld coal seams occur in the Vryheid Formation of the Ecca Group. They have similar nomenclature to the Witbank Coalfield, identified from the base as the No. 1 Seam up to the No. 5 Seam (Fig. 2.3). The No. 1 Seam is well developed in the eastern part of the coalfield. The No. 2 Seam is often not well-developed, and it is generally not economic. The No. 3 Seam is thin, and varies from 0.5 m to 1 m in thickness (Hancox and Götz, 2014).

The No. 4 Seam is the major economic seam in the Highveld Coalfield, ranging from 1 to 11 m in thickness. The No. 5 Seam ranges in thickness from 0.30 to 3 m in thickness and developed at a depth of 15 to 150 m. The coal seams are generally flat-lying to gently undulating, with a regional dip to the south. Dolerite dykes and sills are common and often positioned above the coal zone. Typical coal qualities of these seams are presented in Table 2.4.

Table 2.4: Typical coal qualities of the Highveld Coalfield (Jordaan, 1986). No. 2 Seam No.4 Seam No.5 Seam CV (MJ/kg) 20.5 21.9 25.9 Moisture (%) 3.8 2.5 3.2 VM (%) 19.9 20.2 32.7 Ash (%) 29.3 27.9 17.1 FC (%) 47.0 49.4 47.0

2.2.1.2 Nongoma Coalfield The Nongoma Coalfield is situated in central KwaZulu-Natal and stretches from Gluckstadt in the west to the east where it shares a boundary with the Somkele Coalfield (Fig 1.1). The Nongoma Coalfield is the southern extension of the Kangwane Coalfield and contains both the Ecca and Beaufort Group coals (Barker, 1999). The Nongoma Coalfield has a limited mining and exploration history and thus, is among the most poorly understood coalfield in South Africa; there are several internal reports, which are proprietary (Hancox and Götz, 2014).

Rio Tinto’s Zululand Anthracite Colliery is located 48 km northeast of Ulundi, and it is the only operating mine in the Nongoma Coalfield. The Zululand Anthracite Colliery was opened in 1985 to supply low ash, high carbon anthracite to the domestic and export markets. The high-rank anthracite coals are typically used as reducing agents in the production of titanium, ferrochrome, ferromanganese, iron, and steel in the metallurgical industry (Jeffrey, 2005).

Turner and Whateley (1983) studied the structural and sedimentological controls of coal deposition in the Nongoma graben, and concluded that the sedimentation was contemporaneous with graben formation. The Nongoma graben developed in response to crustal thinning and extensional rifting, prior to the continental break-up of Gondwana (Turner and Whateley, 1983). At the Mbila area, major faults strike southwest. The strata dip at 10 to 15° towards the east, except where dolerite sills caused a localized steepening of the dip up to 25°.

The Dwyka Group lithologies occur at the base of the succession and are 10 m thick in the Mbila area. The Ecca Group is represented by the Vryheid Formation which is 100 m thick and dominated by sandstone, carbonaceous siltstones, and mudstones. The overlying Volksrust Formation comprises of siltstones and mudstones (Hatherly and Sexton, 2013).

Three coals seams (M−1, M, and M+1) are recognized in the Ecca Group in the Mbila area (Fig. 2.4), and they are generally thin (Barker, 1999). At the Mbila area, the M-1 Seam is 0.2 m thick and overlying this seam is a 1 m parting, which is overlain by the M Seam. The M Seam is 1 to 1.2 m thick and it is the only economic seam in the Mbila area. The M Seam has an ash value of 24.98%, VM of 7.5%, FC of 83%, and low phosphorus (Thirion, 1982). Six samples were obtained from the M Seam for this study in order to understand their geochemistry.

Figure 2.4: A generalized stratigraphy of the Nongoma Coalfield (Hatherly and Sexton, 2013). 2.3 Trace elements in coal Many studies have been conducted in order to determine the concentration of trace elements in coals from different coalfields globally (Finkelman, 1981, 1995; Cairncross et al., 1990; Swaine, 1990; Faure et al., 1996; Finkelman, 1999; Wagner and Hlatshwayo, 2005; Bergh et al., 2011; Dai et al., 2011, 2012, 2015; Munir et al., 2018; Qin et al., 2018; Zhao et al., 2018), amongst many others. There is a notable variation in trace element concentrations within South African coals (Watling and Watling, 1982). The concentration of trace elements can differ from coal to coal even between coals in the same coalfield as shown in Table 1.1.

Coal contains most of the elements in the periodic table (Swaine, 1990). These elements can be separated into three groups, depending on the concentrations within which they occur on a whole coal basis:

 Major elements: carbon (C), hydrogen (H), nitrogen (N), oxygen (O), sulphur (S). These occur at concentrations >1 wt. %.  Minor elements in coal mineral matter: aluminium (Al), calcium (Ca), Fe, potassium (K), magnesium (Mg), Mn, sodium (Na), silicon (Si), titanium (Ti). They are present in concentrations between 0.1–1 wt. %  Trace elements: which are the constituents with concentrations <0.1 wt. %. These elements include As, Be, Cd, Co, Cr, Hg, Mn, Ni, Pb, amongst others and they are reported in ppm.

The difference in concentration of trace elements in coals could be related to the conditions during peat accumulation, coalification and differences in the geological conditions (Zheng et al., 2008; Sun et al., 2010). Hydrothermal fluids have been reported to be responsible for the enrichment of certain trace elements, such as Pb and Zn (Dai et al., 2012).

2.3.1 The release of trace elements Trace elements are emitted during coal combustion, gasification, and leaching, and the quantities emitted are dependent on the concentration, physical and chemical properties of the element in coal (Finkelman, 1999; Nalbandian, 2012). The extent to which trace elements may be hazardous is dependent on the period of exposure, concentration, and oxidation state (Gupta, 1999).

The human body requires trace elements, although the absence or excess of these may cause malfunctioning of the body or even death in extreme cases. Essential trace elements such as Cr, Co, copper (Cu), Fe, iodine (I), Mn, Mo, Se, and zinc (Zn) control the metabolic processes of the body (WHO, 1973). Trace elements find their way into the human body either by direct absorption through the skin, by inhalation or ingestion.

Many trace elements are captured in fly and bottom ash, while others are emitted to the atmosphere in the combustion flue gas. The partitioning depends upon the degree of volatilization of the particular element and its mode of occurrence. Trace elements are separated into three groups (Fig. 2.5), and this is based on their relative volatility coupled with their partitioning behaviour between combustion byproducts and emissions (Clarke, 1993) as discussed below:

 Class 1 elements do not volatilize and concentrate in coarse residues or become equally partitioned between coarse residues and finer ash particles, typically become enriched in the bottom and fly ash (Nalbandian, 2012). The release of these elements into the atmosphere is directly related to efficient control of total particulate emissions. Examples include Be, Co, Cr, and Mn.

Figure 2.5: Categorisation of trace elements based on volatility behaviour (Clarke, 1993).

 Class 2 elements are partially volatilized in the combustor or gasifier, condense downstream and become concentrated in the finer particles in fly ash. Group 2 elements become enriched with decreasing particle size because volatile elements condense on the surface of smaller particles in the flue gases as cooling occurs. These elements include As, Cd, Pb, and Sb.

 Class 3 elements are the most volatile elements and they are not enriched in solid phases (Nalbandian, 2012). This includes bromine (Br), chlorine (Cl), Hg, and Se, which are emitted in the vapour phase. During combustion and gasification, volatile elements, such as Hg, may remain in the gas phase.

The overlap between the classes of certain elements shows volatile behaviour in some studies, but partitioning into solid residues in others, due to wide variations in the operating conditions, of boilers and gasifiers especially with regards to temperature, that controls element volatility (Clarke, 1993). To further understand the trace elements in this study, the environmental and health impacts and mode of occurrence are discussed.

2.3.2 Hazardous air pollutants Trace elements in coal have become of interest to many researchers due to increasing public concerns on their potential environmental effects as a result of coal production and utilization (Willis, 1983; Cairncross, 1990; Wagner and Hlatshwayo, 2005; Liu et al., 2007; Qi et al., 2008; Chen et al., 2011, Qin et al., 2018; Zhao et al., 2018). A number of trace elements in coal such as As, Cd, Hg, Pb, Se, are potentially toxic from a health perspective (Clarke and Sloss, 1992; Swaine, 2000; Zhao et al., 2018). Although they occur in low concentrations, continuous emission from coal combustion can increase their concentration in the environment (Liu et al., 2005; Zheng et al., 2007; Dai et al., 2012). The release of these trace elements can cause various health problems which include cancer, cardiovascular diseases, neurological illness, kidney disease, encephalopathy, and impaired cognitive function (Qi et al., 2007; Dai et al., 2014).

The 1990 Clean Air Amendment Act and the National Ambient Air Quality Standards of the USA require the use of coal in an environmentally friendly manner (Huggins, 2002). In the past, the focus has been mainly on the gaseous pollutants released from coal combustion, such as SO2, CO2, CO, and NOx. However, trace element emissions from combustion have become of interest to researchers and regulatory authorities due to their toxicities towards living organisms (Karayigit et al., 2000; Zhang et al., 2008; U.S. EPA, 2011).

The U.S. National Research Council (U.S. NRC, 1980) classified trace elements based on known adverse health effects or their abundances in coal, and determined their level of concern:

 Major concern: These elements are highly toxic to most biological systems at concentrations above critical levels, and include: As, Cd, Hg, Mo, Pb, and Se.  Moderate concern: These elements are present in coal combustion residues at elevated levels, and they are potentially toxic, and include: Cr, Cu, and Ni.  Minor concern: These elements are in residues, but they are of little environmental concern and include: Br, Cl, Co, Mn, Na, and Sb.

The US Clean Air Amendments Act of 1990 identified eleven trace elements which are commonly found in coal as potentially hazardous, and these include Cr, Mn, and Co, which are interest for this study. The identification of HAPs by the EPA in the 1990s eventually led to Mercury and Air Toxics Standards (MATS) under which potentially hazardous trace elements except Mo are regulated (U.S. EPA, 2011). Understanding the mode of occurrence of trace elements in coals can help minimize their hazardous release into the environment during coal utilization (Sun et al., 2012,

Dai et al., 2012, 2016; Qin et al., 2018). The mode of occurrence of trace elements is discussed in the following section.

2.3.3 Mode of occurrence The mode of occurrence refers to the organic or inorganic affinity of the element, and how an element is chemically bound and physically distributed in coal (Finkelman, 1994). The mode of occurrence of trace elements determines the behaviour of the element during cleaning, combustion, conversion, and leaching of the coal (Finkelman, 1980; Zhao et al., 2014; Wang et al., 2016). It can also provide useful information about coal genesis, and environmental and health impacts (Dai et al., 2012, 2016; Li et al., 2017; Zheng et al., 2017; Qin et al., 2018).

The low concentration and dispersed nature of many trace elements in coal makes it difficult to determine their modes of occurrence (Finkelman, 1999). Several techniques are used to determine the elemental modes of occurrence by assessing the textural relationships of the minerals and the chemical form of the elements.

These techniques include SEM, density fractionation, and selective leaching which are discussed in Section 2.5.8.

Coal is composed of organic and inorganic matter containing various major and trace elements that have an organic or inorganic or both affinities (Finkelman, 1999; Ren et al., 2006; Dai et al., 2012; Riley et al., 2012; Sutcu and Karayigit, 2015; Ward, 2016). The majority of trace elements are associated with the mineral matter as discrete mineral phases such as Zr in zircon; substituted elements in the major mineral phases such as As in pyrite; and/or bound up in the organic matter (Riley and Dale, 2000).

Organically bound trace elements are difficult to remove using pre-combustion cleaning processes because that necessitates removing organic matter needed for coal combustion (Bergh et al., 2011). They can only be released from the coal either by deep chemical leaching, which is expensive, or combustion which can cause pollution. Some trace elements can be associated with mineral matter, as replacement ions in minerals, or absorbed onto minerals. Trace elements associated with minerals may be liberated through coal washing (Bergh et al., 2011).

Querol et al. (1995) identified several affinities of trace elements with minerals. Chromium has an association with clay minerals such as kaolinite or montmorillonite. Arsenic, Cd, Co, Hg, Ni, Pb, Se are associated with iron sulphides, such as pyrite. Cobalt and Mn have an association with carbonates, such as calcite or dolomite.

The mode of occurrence of certain trace elements can differ from coal to coal. Clarke (1993) mentioned that elements like Be, Co, Cr, Cu, Mo, Ni, and Se can be found in different mineral associations. However, the mode of occurrence of some trace elements such as Hg, Se, As, and Cd is fairly constant. Chromium, Mn, Co, and Mo are further discussed in the following section.

2.3.4 Trace elements of interest to the current study Chromium, Mn, and Co have been reported to be high in South African coals compared to global averages for coals (Table 1.1), and it is of interest to understand why these elements are high in South African coals. The arithmetic mean values for Cr, Mn, Co, and Mo from different coalfields in Table 1.1 was calculated and used together with the Coal Clarke values for hard coals to construct Table 2.5. In this section Cr, Mn, Co, and Mo are discussed.

Table 2.5: Average Cr, Mn, Co, and Mo concentrations in South African coals compared to global averages for coals (ppm). Trace Average SA Coal Clarke element values1-8 values for hard coals9 Cr 45.4 17±1 Mn 89.4 76±6 Co 8.7 6±0.2

Mo 2.1 2.1±0.1

1Watling and Watling, 1982; 2Willis, 1983; 3Cairncross et al., 1990; 4Faure et al., 1996; 5Wagner and Hlatshwayo, 2005; 6Bergh et al., 2011; 7Wagner and Tlotleng, 2012; 8Kolker et al., 2017; 9Ketris and Yudovich, 2009)

2.3.4.1 Chromium Chromium in South Africa is a specific focus in this study due to high Cr concentrations reported in South African coals (Table 1.1 and Table 2.5). Chromium is the 21st most abundant element in the Earth’s crust at about 100 ppm (Barnhart, 1997). It is the 24th element on the periodic table, with an atomic mass of 51.996 g.mol. Chromium has six isotopes, but only four are naturally occurring (50Cr, 52Cr, 53Cr, and 54Cr). Chromium-51 is a radioactive isotope which is used in medical research. Chromium occurs in several oxidation states, and only Cr3+ and Cr6+ are the most stable species in the aquatic and terrestrial environments (Panda and Choudhury, 2005).

Trivalent Cr is an essential nutrient for humans and animals, but not for plants, and it is insoluble in water (Shrivastava et al., 2002). Trivalent Cr is dominant in coals and tends to be absorbed onto clays, sediments, and organic matter. Other researchers have determined Cr to be entirely trivalent in both macerals and illite (Huffman et al.,

1994; Goodarzi and Huggins, 2005; Wang et al., 2008; Huggins et al., 2009). Hexavalent Cr is more mobile and toxic, and exposure to it can cause nose bleeding, inflamed skin, ulcer, bronchitis, pneumonia, cancer, or even death (Gupta, 1999; Geelhoed et al., 2002; Karar et al., 2006; Deonarine et al., 2015). Chromium is listed as a HAP by the 1990 US Clean Air Amendment Act due to Cr6+ which is toxic, carcinogenic and soluble in water. Concerns have been raised about the possible effects on human health as a result of its release to the atmosphere during coal combustion (Huggins et al., 2000). In order to understand the behaviour of Cr in coal combustion, it is important to understand the mode of occurrence and oxidation state.

The mode of occurrence of Cr in coals has always been somewhat problematic. Finkelman (1994) mentioned that the mode of occurrence of Cr has low confidence, and results are frequently inconclusive. In a study by Gluskoter et al. (1977), Cr was found to have a mixed organic-inorganic affinity. Chromium can be associated with organic matter and/or minerals. Chromium often shows an organic association in bituminous coals and this could be due to the occurrence of microminerals of Cr oxides or oxyhydroxides within coal macerals (Huggins et al., 2000; Wang et al., 2003; Huggins and Huffman, 2004). Chromium was reported to be concentrated in the light fractions of the coal, indicating an organic association (Wagner and Tlotleng, 2012).

Chromium is mostly associated with clay minerals, specifically illite (Huffman et al., 1994; Kolker et al., 2000a; Xu et al., 2003; Wang et al., 2008; Riley et al., 2012). In a study by Querol and Chenery (1995), Cr was detected using LA-ICP-MS, and was found to be mainly associated with clay minerals (kaolinite and illite) in Spanish sub- bituminous coal. In a study by Finkelman et al. (2018), 60% of Cr was leached, mostly by HF indicating an association with silicates (clay). The 40% that was not leached could be associated with the organic matter. Hence, the mode of occurrence of Cr still remains inconclusive since it has a mixed association, as shown by different studies.

The interest in Cr originates from the widespread use of this metal in various industries such as the metallurgical and chemical industries as depicted in Figure 2.6 (Nriagu and Pacyna, 1988; Kotaś and Stasicka, 2000). Chromium is used to cover the surface of metals in order to protect it from corrosion and give it a bright and shiny appearance. During ferrochrome production, small quantities of Cr6+ are formed as an unintended by-product and released into the environment (Kotaś and Stasicka, 2000). Figure 2.6 shows the source of Cr from the different industries into the environment, the transportation mechanism of Cr and the reduction of Cr6+ into Cr3+ within the environment (Shahid et al., 2017).

Chromium is mined as chromite ore, and South Africa is the world’s leading producer of chromite, with 75 % of the world’s Cr resources hosted in the Bushveld Igneous Complex in South Africa (Naldrett, 2004). The Bushveld Complex is the world’s largest layered igneous intrusion, covering an area of approximately 65,000 km2. It is located in the north-eastern part of South Africa, within the Kaapvaal Craton and intruded around 2.05 billion years ago (Olsson et al., 2010). It extends 450 km east- west and 250 km north-south (Eales and Cawthorn, 1996).

The rocks of the Bushveld Complex are distributed among the five limbs (Cawthorn and Walraven, 1998), as shown in Figure 2.7. These include the mafic to an ultramafic suite and the granite and granophyre suite (von Gruenewaldt et al., 1985). The mafic suite which is known as the Rustenburg Layered Suite is subdivided into 5 zones. These include the Marginal Zone, the Lower Zone comprising pyroxenite and harzburgite, the Critical Zone composed of chromitite, norite, and pyroxenite, the Main Zone comprising of gabbronorite, and the Upper Zone composed of anorthosite, gabbro, and magnetitite (Eales and Cawthorn, 1996; Kruger, 2005; McDonald and Holwell, 2011). Interest and exploration efforts within the Bushveld Complex is due to the mafic rocks of the Rustenburg Layered Suite, which host vast amounts of PGEs, Cr, Ni, V, and Ti.

Figure 2.7: Simplified geological map of the Bushveld Complex (Kinnaird and McDonald, 2005).

South African coals have been reported to contain high Cr concentrations (Table 1.1). To explain the enrichment of Cr in Highveld coals, Wagner and Hlatshwayo (2005), thus suggest that Cr from primary sources, such as chromite, was subsequently dissolved and redistributed. According to Finkelman (1994) coals having over 500 ppm of Cr usually contain chromite. Brownfield et al. (1995) and Ruppert et al. (1996) studied coals enriched in Cr and concluded that the occurrence of high Cr in coal may be due to the deposition of detrital Cr-bearing minerals in the peat-forming environment. Coals which are enriched in Cr often have a nearby ultramafic deposit that contains abundant Cr-bearing spinels like chromite, chromium

magnetite, and Cr-bearing ferromagnesian silicates such as olivines (Huggins et al., 2000). It is likely that this finding is applicable to the South African context, except that Cr contents are lower, and no detrital Cr-minerals have previously been reported. The origin of Cr in the selected South African coals will be discussed in Chapter 5. 2.3.4.2 Manganese Manganese is a transition element in group 7 of the periodic table. The element has an atomic number of 25, an atomic mass of 54.938 g.mol, with several oxidation states (Mn2+, Mn3+, Mn4+, Mn6+, Mn7+), and one naturally occurring isotope (55Mn). Manganese is an essential nutrient in human and animal health which serves as a cofactor for various enzymes and high exposure to Mn can be toxic to humans and animals (WHO, 2011). Manganese toxicity in humans can cause nerve damage, bronchitis, and affect the respiratory tract and the brain. Manganese is classified as a HAP, however, no health or environmental problems due to Mn in coal mining and utilization have been reported (Swaine, 1990). In coals, Mn is generally present in high concentrations (76 ppm), higher than any of the HAPs (Table 1.1).

Manganese is typically associated with carbonates such as siderite and ankerite, although some Mn has been determined to be organically associated in low-rank coals (Swaine, 1990; Finkelman, 1994). The organic form of Mn as a carboxyl compound has been observed using X-ray absorption near edge structure spectroscopy (XANES) analysis (Huggins and Huffman, 1996). Float-sink and leaching experiments show an association of Mn with carbonates, though small amounts may be associated with clays, pyrites or organic components of coal. Manganese was mainly concentrated in the sink fractions indicating a mineral association in the Waterberg coals (Wagner and Tlotleng, 2012). In a study by Finkelman et al. (2018), Mn was largely leached by HCl indicating an association with carbonates. 2.3.4.3 Cobalt Cobalt is mainly used to impart a rich blue colour to glass and ceramics. It is also used in the production of rechargeable batteries, superalloys, and catalysts. Cobalt has an atomic mass of 58.933 g.mol, two oxidation states, Co2+ and Co3+, with Co2+ being the most common oxidation state, and has a single stable isotope (59Co) (Pourret and Faucon, 2016).

Cobalt is an essential element to humans in the form of vitamin B12, and helps with synthetic reactions in metabolic processes and the production of red blood cells (Facts, 2006). Excess of Co within the body might increase the action of thyroid and bone marrow resulting in overproduction of erythrocytes, fibrosis in lungs, and asthma (Lombaert et al., 2008). South African coals have been reported to have high Co concentrations compared to global averages for coals (Table 2.5).

Cobalt can occur in organic and inorganic forms in coal (Lombaert et al., 2008). According to Finkelman (1981), Co in coal is associated with clays and sulphides, such as pyrite and may also be associated with the organic matter as suggested by Swaine (1990) and Dale et al. (1992). Finkelman (1994) suggested that there is a degree of uncertainty about the mode of occurrence of Co in coal, and the leaching behaviour of Co does not resolve this uncertainty. In a study by Finkelman et al. (2018), Co was mainly leached by HCl indicating an association with monosulphides and carbonates. A significant amount of Co was not leached indicating an organic association. Thus, the mode of occurrence of Co still remains inconclusive or it may have multiple forms, similar to Cr.

2.3.4.4 Molybdenum Molybdenum has an atomic number of 42, an atomic mass of 95.960 g.mol and occurs in group 6 of the periodic table. It has five main oxidation states (Mo2+, Mo3+, 98 Mo4+, Mo5+, and Mo6+) and seven stable isotopes of which Mo is the most abundant (Cotton and Wilkinson, 1972).

Molybdenum is not classified as a HAP, but some publications cite Mo to be of environmental concern (Booth et al., 1999; Bushell and Williamson, 1996). High levels of Mo in pastures cause diseases in cattle and sheep which results in poor growth and anaemia (Nalbandian, 2012). Molybdenum has been determined to affect the lactation of cows, absorption and metabolism of Cu.

Finkelman (1981) mentioned that the mode of occurrence of Mo is still unclear from literature. Molybdenum can be associated with sulphide minerals and organic matter in coal Swaine (1990), but Wang et al. (2008) regarded pyrite as the main carrier of Mo in coals. Molybdenum is commonly found in the lighter fractions in float-sink tests

indicating an organic affinity. Sequential leaching indicated that some Mo is organically associated since it remains in the residual material minerals (Riley et al., 2012). In a study by Finkelman et al. (2018), Mo was mainly leached by HF, although a significant amount of Mo remained unleached. Most of Mo in coal may be primarily associated with the silicates (clays) and organics. Wagner and Tlotleng (2012) support the organic association of Mo, concentrated in the light fractions of the Waterberg coals.

2.4 Analytical techniques Different analytical techniques have been used by researchers through the years in order to characterize coal and determine the mineralogy, geochemistry, and mode of occurrence of trace elements in coal. These include coal petrography, proximate and ultimate analyses, XRD, XRF, ICP-MS, SEM, XAFS, density fractionation, and selective leaching; these are discussed further in this section.

2.4.1 Coal characterization Coal can be characterized in terms of organic matter and inorganic matter. Petrographic, proximate, and ultimate analyses are typically used to characterize the coal, as discussed in the next section.

2.4.1.1 Coal Petrography Coal petrography is the microscopic study of organic and inorganic components in coal (Wagner et al., 2018). Coal petrography is a fundamental technique that is used to characterize coal and mineral matter (Cloke and Lester, 1994). Petrographic techniques have gained importance in the determination of:  Coal type (maceral composition)

 Maceral and mineral matter association in coal (microlithotypes)

 Textural relationships between mineral matter and macerals

 Coal rank (vitrinite reflectance)

Petrography can reveal information that can be used to understand coal origin, formation, sedimentary basin development, depositional environment (Teichmfiller, 1989). This information can be used during beneficiation, combustion, and coke- making (Wagner et al., 2018).

Many petrographic studies on South Africa coals have been published including Steyn and Smith (1977), Falcon et al. (1984), Falcon (1986), Holland et al. (1989), Taylor et al. (1998), and Wagner et al. (2018). Holland et al. (1989) undertook a maceral group and mineral matter analyses on the coals from the Witbank Coalfield. It was revealed that these coals are inertinite-rich, with the inertinite content commonly greater than 55% on the whole coal basis (Holland et al., 1989). The vitrinite content generally varies between 0 to 10% and liptinite generally less than 10% on the whole coal basis (Holland et al., 1989).

Maceral and microlithotypes studies are based on the qualitative and quantitative determination of macerals by conducting a point count (by volume) on polished blocks under a light microscope equipped with an oil immersion objective lens. In determining the relative abundance of macerals in coals, the maceral that is observed on the crosshair under the eyepiece is classified and counted, as per SABS ISO 7404-part 4. Although petrographic studies have paved the way towards the understanding of coal geology and coal geochemistry, it is also important to highlight some of the challenges related to the use of the technique, as it is time- consuming and requires a skilled petrographer to analyse the coals (Gluskoter, 1975).

2.4.1.2 Proximate analysis Several test methods of proximate analysis have been used including ASTM D3172, ASTM D7582–15, and ISO 1171 (Speight, 2005). However, a rapid method using a Perkin Elmer thermogravimetric analyzer (TGA) has been developed (Cassel and Menard, 2012). Perkin Elmer was an early developer of the methodology for proximate analysis by TGA for coal (Cassel and Menard, 2012). The Perkin Elmer STA 8000 easily meets these analytical requirements for proximate analysis by TGA. These include the ability to accurately record the weight of a sample, as it is heated over a temperature range and held isothermally at designated temperatures, then

change the sample’s atmosphere from inert to oxidizing and software to facilitate and automate the analysis (Cassel and Menard, 2012). Results obtained from this technique are comparable with results obtained using ASTM/ISO/SANS standards (Cassel and Menard, 2012).

The four parameters that are measured in proximate analyses include moisture, VM, FC, and ash:

 Moisture is determined by the loss of weight when a sample is dried in the furnace, and represents the water molecules that are bonded in the coal. The percentage of water present depends on the coal rank. Anthracites has the least quantity of moisture, bituminous more, and lignites have the most (Speight, 2005).

 Volatile matter is derived from organic matter and mineral matter and includes

gases such as CH4, CO2, CO, and N2, hydrocarbons, and water from clay minerals. These volatiles influence the ignition temperature and combustion characteristics of coals, which also depends on the coal rank. Anthracites have less volatiles (2-12%), bituminous (15-45%), sub-bituminous (28-45%), and lignites have more (24-32%) (Speight, 2005).

 Fixed carbon is the C solid residue that remains once all volatile matter is driven off (Falcon and Ham, 1988). The FC content gives an indication of the heating value of coals. Anthracites have the highest FC (75-85%), followed by bituminous (50-70), then sub-bituminous (30-57%), and the lignites (25-30%) have the lowest.

 Ash is an inorganic residue that remains after all the C is burnt off at 900°C (Falcon and Ham, 1988). It is calculated by taking the difference from 100% of total surface moisture, VM, and FC, as shown by Equation 1:

Ash= 100 – (moisture + VM + FC) Equation 1

The proximate analyses of any coal are represented in weight percentages and can be reported on the following bases:  As Received (ar): includes moisture  Dry Basis (db): excludes moisture  Dry Ash Free (daf): excludes moisture and ash

2.4.1.3 Ultimate analysis Ultimate analyses have been used by several researchers to determine the composition of the organic matter in coal (Chelgani et al., 2010; Yi et al., 2017; Wang et al., 2018). These analyses determine C, H, N, S, and O content in coal. Carbon and H are the major combustible elements found in coals typically ranging from 65- 95% and 2–5% respectively. The nitrogen content in coal samples typically varies between 1–2%. The analyses determine the C, H, N, and S content, and O is calculated by difference as shown by Equation 2.

Oxygen = 100 – (C + H + N + S +Ash + Moisture) Equation 2

2.4.2 Mineralogy In order to understand the concentration and distribution of trace elements, it is important to study the coal mineral matter. Minerals in coals are normally found in sedimentary rocks. The most abundant minerals in coals are clays, quartz, various carbonates and sulphides, minor rutile, and apatite (Riley and Dale, 2000).

2.4.2.1 XRD X-ray diffraction is the best developed and reliable method used for distinguishing minerals in coal (Rao and Glukoster, 1973; Ward et al., 1999, 2001; Ruan and Ward, 2002; Pinetown et al., 2007; Ward, 2016). X-ray diffraction is a non-destructive technique. The diffracting X-rays from an ordered crystal provide a crystalline structure of a mineral. Each crystal has its own distinct diffraction pattern, where positions of the lines are determined by the ordered internal arrangement of atoms which allows for mineral identification (Studer, 2008).

X-ray diffraction can provide both qualitative and quantitative information (Sharma, 2012). The initial use of XRD on coal characterization was purely on a qualitative and semi-qualitative basis. Semi-qualitative analyses were obtained by measuring

the relative intensities of the main diffraction peaks, to identify individual mineral phases (Vassilev et al., 1997). A direct and quantitative determination of mineral matter in coals was developed using Rietveld methods (Hutton and Mandile, 1996; Ward and Taylor, 1996; Matjie et al., 2016). For quantitative analyses, both the peak intensity and the peak position must be accurate to extract accurate qualitative results. An internal standard is used to calibrate the instrument.

Analysing coal by XRD may be problematic since the non-crystalline organic components produce an accentuated background, which could mask the peaks of selected minerals (van Alphen, 2005). Samples should be subjected to low- temperature ashing (LTA) prior to analysis to consume the organic fraction. It is possible to perform XRD directly on coal without prior LTA, which shows XRD patterns for crystalline components and amorphous C. This results in three very broad peaks with diminishing intensity and increasing width. The undulating background is superimposed on the sharp peaks of the mineral matter and needs to be subtracted in order to quantify for the minerals (Wertz, 1990).

Quantitative analyses are dependent on good sample preparation to produce an accurate diffraction pattern. Samples are prepared using a back loading preparation method and analysed with a diffractometer with the detector and fixed slits with Ni- filtered Cu K-alpha radiation. The relative phase amounts of coal are estimated using the Rietveld method.

2.4.2.2 XRF X-ray fluorescence is a common, non-destructive technique used to analyse coal ash (Evans et al., 1997; Wiley, 2003; Gupta, 2007; Chand et al., 2009). Coal ashes are subjected to XRF analyses, for the determination of major elements (Al, Ca, Fe, K, Mg, Na, P, S, Si, and Ti) reported as oxides. The measured ash oxides are indicative of the common minerals occurring in coal.

The basic principle of XRF is to measure the wavelength and intensity of characteristic X-ray photons emitted from the sample. As a result, the sample is excited by high-energy gamma rays which result in secondary X-rays (Wiley, 2003). This allows the identification of the elements present in the sample and the determination of their concentration. It is fully automated for unattended operation of

samples, and it can measure concentrations from 1 ppm to 100 wt.% (Huggins, 2002).

Sample preparation for XRF involves pulverization and pelletization. The coal ash sample is pressed to a pellet (disc-shaped) or converted to a homogeneous glass disc (bead) by fusion with lithium tetraborate. The fused bead technique overcomes heterogeneity effects and is generally required for obtaining the highest analytical accuracies (Norrish and Hutton, 1969). The disadvantages of XRF include low sensitivity and detection limits to trace elements, and matrix corrections are required for best precision. Whole coal samples have been analysed, but coarse minerals grains caused problems and so working on low-temperature ash has more precision and enriches the concentrations (Huggins, 2002).

2.4.3 ICP techniques In the last few decades, ICP-OES and ICP-MS have been applied to analyse a wide range of samples (Zhou et al., 1996; Sandroni and Smith, 2002; Han et al., 2006; Marin et al., 2008) including coal and coal ash (Wang et al., 2004; Antes et al., 2010; Low and Zhang, 2012). The techniques are capable of simultaneous multi-elemental analysis with excellent reliability and sensitivity. Trace element determination in coal and rock samples is still a challenge since coal and rock samples have to be in solution and sample preparation procedures are complex and time-consuming (Antes et al., 2010). Coal and rock samples are digested by conventional digestion methods, using concentrated acids combined with high temperature and/or pressure conditions (Mesko et al., 2010). As a result, high volumes of concentrated acids are used, and final digests contain high concentrations, which require dilution (Mesko et al., 2010). 2.4.3.1 ICP-MS Inductively coupled plasma mass spectrometry is an established method since the 1980s for multi-trace element determination in coal (Finkelman et al., 2002; Shao et al., 2003; Wang et al., 2006; Wagner and Tlotleng, 2012; Dai et al., 2013; Li et al., 2014). This instrument has several advantages including low detection limit, high sensitivity, simultaneous determination of multi-trace elements in a run, and perform isotopic measurements (Davidson and Clarke, 1996; Palmer, 1997; Beauchemin, 2017).

Spectral interference is a common problem in ICP-MS analyses and to minimize spectral interferences resulting from the interaction of samples with the argon plasma, collision/reaction cell technology is being used for coal samples (Li et al., 2014). It is important to match the matrix of the standard to that of the sample to ensure the instrument responds in the same manner when analysing the samples (Thomas, 2004).

The sample in solution is delivered to a nebulizer using a peristaltic pump. The nebulizer converts this solution into an aerosol that is then carried by the carrier gas through a spray chamber where large droplets are drained out (Beauchemin, 2017) as shown in Figure 2.8. The resulting fine aerosol, which typically constitutes 2–5% of the sample, is injected into an argon-plasma that has a temperature of 6000–8000 K (Beauchemin, 2017). Inside the plasma torch, solution undergoes atomization and ionization. Only a small amount of the ions produced in the plasma proceed to the mass spectrometer. The mass spectrometer consists of a sampler cone and a skimmer cone, in which a small amount of the free ions generated by the plasma are transmitted (Beauchemin, 2017).

Figure 2.8: Major components of ICP-MS (Tyler and Jobin, 2003).

An electrostatic lens focuses ions onto the entry to the true mass spectrometer (Beauchemin, 2017) (Fig. 2.8). The ions pass through the ion optics, which optimizes the ion paths to eliminate the neutral species and light by a photon stop. Then ions pass through a quadrupole mass filter, which separates the ions on account of their mass-to-charge ratios before the selected ions reach the detector (Tyler and Jobin, 2003).

2.4.3.2 Sample preparation Coal is composed of organic matter and mineral matter, both of which are associated with trace elements. Thus, complete decomposition of both organic matter and mineral matter is required before the determination of trace elements can be done, using ICP. This can be done by ashing or digestion at elevated temperatures. During ashing or digestion, volatile elements such as Hg and Se are lost. As a result, Hg and Se are typically determined separately on a whole coal basis (Huggins, 2002). Sample preparation needs to be controlled to prevent the adsorption and diffusion of volatile elements through the containers. The different sample preparation techniques that can be used for ICP analyses are discussed:

 Ashing is widely used to pre-concentrate elements by a factor of 5-10 and thereby improving the sensitivity and precision of the analyses (Lachas et al., 1999; Huggins, 2002). Ashing eliminates organic matter with combustion. The resultant ash is fused with lithium meta-borate (Lau et al., 2000) and dissolved

with nitric acid (HNO3). Fusion using an alkaline flux is often considered susceptible to increase blank values due to contamination from the flux (Laitinen et al., 1996; Lachas et al., 1999). This method is not favourable for determining less abundant trace elements in coal (Wang et al., 2003). Combustion techniques have been considered suitable before digestion because of their relatively low reagent consumption and high digestion efficiency. The accuracy of the recoveries can be evaluated using certified reference materials (Antes et al., 2010).

 Microwave-assisted digestion for ICP analysis is an extensively used sample preparation technique. It is a rapid and efficient dissolution technique for coal samples (Sandroni and Smith, 2002; Marin et al., 2008) and it is possible to minimize loss of volatile elements and cross contamination (Rodushkin et al., 2000; Wang et al., 2006 Dai et al., 2014, Mketo et al., 2016).

Microwave-assisted digestion is normally carried out using concentrated

inorganic acids such as, HNO3, HF, hydrochloric acid (HCl), perochloric acid

HClO4, sulphuric acid (H2SO4), and hydrogen peroxide (H2O2) (Ojeda and Rojas, 2013; Fadda et al., 1995; Richaud et al., 2000), although restrictions

may arise from the use of HClO4 and HF. For example, HF has a negative matrix effect and also etches the glassware of the instruments, and boric acid is required to react with fluoride (Nadkani, 1980). However, boric acid is unfavourable for ICP-MS because of the matrix effect and spectrometric interferences. Perochloric residue in ductwork is explosive, and specially designed fume hoods with a wash down cycle are required to use it safely.

Microwave digestion involves high-pressure dissolution in a sealed vessel, where the elevated temperatures attained lead to faster sample breakdown and analyte dissolution. The digested solution is introduced into the ICP for analysis. The digestion efficiency of the proposed method is evaluated using coal certified reference materials, such as SARM 19. However, even using concentrated acids and high temperature and/or pressure, incomplete coal digestion has been reported (Srogi, 2007).

The selection of the reagents depends on the type of sample to be digested. Organic samples are generally decomposed with oxidizing reagents such as HNO3 or H2O2 (Kotz et al., 1972). In a study by Rodushkin et al. (2000) good recoveries were achieved by the HNO3/H2O2 digestion for almost all the examined trace elements in three standard coals. Wang et al. (2004, 2006) also reported good elemental recoveries when coal samples were digested HNO3 and H2O2 using the microwave- assisted digestion.

2.4.4 XAFS During coal combustion, trace elements are released and their toxicity depends on their oxidation state (Huggins, 2002). It is important to determine the oxidation state of trace elements in order to understand the health and environmental effects they may pose. For example, As3+ is more toxic than As5+, while Cr6+ is more toxic than Cr3+. Trivalent As and Cr6+ are classified as Group I carcinogens.

X-ray absorption fine structure spectroscopy is a direct, non-destructive technique that is capable of determining the oxidation state of trace elements in coal (Huffman et al., 1994; Huggins and Huffman, 1996; Huggins, 2002; Riley et al., 2012). A synchrotron source of intense X-ray radiation is used to perform the analysis. X-rays are fluoresced by the sample in response to the X-ray absorption edge and compared to that of standards with known speciation of that element (Stam et al., 2011). The fluxes are intense and spectral signals can be obtained from ppm levels in coal and ash.

X-rays probe the physical and chemical structure of the sample at an atomic scale and determines the valence state of an element (Huggins, 2002). X-ray absorption fine structure spectroscopy is limited to elements with atomic numbers above 20 that occur in coal in excess of 5 ppm (Najih et al., 1995). X-ray absorption fine structure spectroscopy is also capable of determining the relative percentages of the two Cr oxidation states (Najih et al., 1995; Huggins et al., 2000; Stam et al., 2011). Huggins et al. (2000) examined the mode of occurrence of Cr using XAFS and two distinct Cr occurrences were observed, Cr3+ associated with illite and the organic Cr3+ associated with macerals in Kentucky coals.

X-ray absorption fine structure spectroscopy has shown to be an effective technique for observing the chemical structure around an element (Huffman et al., 1994; Huggins and Huffman, 1996; Riley et al., 2012). The technique is direct and non- destructive and sample preparation is quick. The technique is performed at a synchrotron source which is not widely available and limited for use. XAFS is available at the National Synchrotron Light Source in the Brookhaven National Laboratory, at the Stanford Synchrotron Radiation Laboratory in Stanford University, CA, and also at the ESRF in Grenoble, France among other such facilities.

2.4.5 Determining the mode of occurrence Trace elements are present in coal in both minerals and macerals. Trace elements that are organically bound are not desired from the standpoint of coal utilization and are difficult to remove by coal cleaning processes. They may be released during combustion or by deep chemical leaching (Schweinfurth; 2009). In contrast, trace elements associated with minerals may be removed during coal cleaning processes.

Both direct and indirect methods have been employed to investigate the modes of occurrence of trace elements in coals (Finkelman, 1999; Liu et al., 2016, Qin et al., 2018). Several direct techniques have been used to investigate trace elements and include SEM and XAFS since they are non-destructive and performed directly on macerals and minerals.

Indirect methods which are commonly used include statistical analysis (correlation coefficients, factor, and cluster analyses), density separation, and sequential leaching (Qin et al., 2018). These techniques are informative, less expensive and time-consuming than direct methods. However, the accuracy of the results should be confirmed through a comparison of multiple approaches. The direct and indirect techniques are discussed further as they were used in this study.

2.4.5.1 SEM The scanning electron microscope can determine mineral mater in coal qualitatively and semi-quantitatively. It uses a focused beam of high energy electrons to generate a variety of signals at the surface of a specimen. Data is collected over a selected area of the surface of the sample and a two-dimensional image is generated. This image reveals information about the composition, texture, orientation, and the relationship between the materials (Zygarlicke and Steadman, 1990). It can be used to determine the mineral associated with trace elements since some minerals occur as isolated grains, mineral inclusions within the coal matrix, or in mineral partings. The association of trace elements with minerals can influence how they behave during coal separation and combustion (Huggins, 2002).

The EDX separates the characteristic X-rays of different elements into an energy spectrum which is used to determine the abundance of specific elements. It can also be used to determine the chemical composition of the material down to a spot size of

a few microns and create elemental maps over the selected area. Areas ranging from approximately 1 cm to 5 microns in width can be viewed (Zygarlicke and Steadman, 1990) with magnification ranging from 20X to 30,000X, and a spatial resolution of 50 to 100 nm (Reimer, 1998) and the field emission SEM (FE-SEM) can extend the area viewed in SEM to the sub-micron scale. The SEM-EDX is not powerful enough in determining trace element occurrences that are dispersed at low abundance levels in coal.

Swaine and Goodarzi (1995) suggested that the most effective method for determining the mode of occurrence of trace elements is SEM-EDX, as it also provides textural relationships of the minerals. Finkelman (1981, 1982, and 1994) showed that using SEM-EDX is another effective method of detecting and analysing trace elements in coal as small as one micron in diameter in polished blocks.

2.4.5.2 Statistical methods Attempts are made to understand the affinities of trace elements by statistical analysis, using the correlation between macerals, minerals, major and trace elements, fixed carbon, ash yield, and sulphur contents (Munir et al., 2018). Pearson’s correlation coefficient is a statistical analysis that can be performed on the data. It can be used to explain the relationship between different components in coal and reflect the linear relationship between two variables and ranges from –1 to +1 (Conaway, 2001).

Correlation coefficients are used to infer the mode of occurrence of the element indirectly, between trace element, macerals, and mineralogy (Yan et al., 2014). A negative correlation coefficient between trace elements and ash indicates an organic affinity of trace elements in coals, and a low correlation coefficient suggests a mixed organic-inorganic affinity (Dai et al., 2008; Mastalerz and Drobniak, 2012). A positive correlation between trace elements and ash indicates an association with the mineral matter (Swaine and Goodarzi, 1995).

In a study by Liu et al. (2016), 22 coal samples from nine provinces in China were studied and a positive correlation of Cr with ash yield (0.64) was determined. This indicates that the mineral matter is the primary host of Cr (Swaine and Goodarzi,

1995). This is consistent with the conclusions from other studies (Spears and Zheng, 1999; Zhou et al., 2010).

2.4.5.3 Density fractionation Density fractionation was one of the first systematic methods to obtain information relating to the mode of occurrence of trace elements in coal (Gluskoter et al., 1977). Early attempts to determine the mode of occurrence of elements in coal was based on the organic/inorganic affinity of the elements during density separation experiments (Zubovic et al., 1960). Different liquids such as bromoform, zinc chloride are used to separate coal at different densities (Callahan, 1987). Several other authors who have used density fractionation include Finkelman (1980), Feng and Hong (1999), Querol et al. (2001), Li et al. (2005); Huggins et al. (2009), Bergh et al. (2011), Wagner and Tlotleng (2012) and Kolker et al. (2017).

The organic affinities of select trace elements are estimated from the partitioning of trace elements in different density fractions at different specific gravities. If the element of interest has an organic affinity it will report to light specific gravity fraction, for example, float 1.4. Trace elements concentrated in the light coal fractions are often regarded as bound to the organic matter while elements concentrated in the heavier-specific-gravity fraction, for example, sink 1.9 are associated with the inorganic constituents (minerals) (Swaine, 1992; Wagner and Tlotleng, 2012). There is no assurance that a specific form of an element will be present or absent in a particular fraction, since it is based on the specific gravity of coal particles.

2.4.5.4 Selective leaching Selective leaching is primarily based on a leaching principle, developed by Finkelman and Simon (1984) and used by Palmer et al. (1998, 2000) and Finkelman et al. (2018). Selective leaching provides qualitative and quantitative information about the association of specific elements with individual minerals in the different fractions (Huggins et al., 2009). Each coal can respond differently to the individual solvent or the order of solvent application, as a result, a limited number of solvents is selected.

Various elements associated with different minerals are removed by specific solvents and elements which are not removed by the solvents may be present in the organic matrix or occur in insoluble phases such as zircon (Kolker et al., 2000b). For the purpose of the current study, four solvents were selected: ammonium acetate

(CH3COONH4), HCl, HF, and HNO3 and each of these solvents can dissolve more than one phase. Figure 2.9 outlines the sequential steps followed in selective leaching of coal. Extraction is carried out in stages and the residue is taken to the next leaching stage. After each stage of the leaching, a multi-element chemical analysis is performed on the leachate using ICP to determine the amounts of trace elements removed by that particular leaching solvent (Huggins, 2002).

Figure 2.9: Sequential steps followed in selective leaching of coal (Kolker et al., 2000b).

Incomplete extraction of an element at any stage, and overlapping solubility in different solvents can influence the accuracy of this technique (Wijaya and Zhang, 2011; Spears, 2013). Regardless of these limitations, this technique can provide reliable quantitative information on the modes of occurrence of elements in coal (Finkelman et al., 2018). The mode of occurrence of an element is determined by its

association with major mineral groups (e.g. sulphides, silicates, carbonates, etc.) and with the organic component. Finkelman et al. (2018) summarized the previous studies which have attempted to quantify the modes of occurrence of different trace elements in coal.

2.5 Chapter summary In this Chapter, South African coal resources were discussed. In South Africa coal is hosted in different types of basins and in 19 coalfields. The Waterberg, Soutpansberg, Witbank, Highveld, and the Nongoma Coalfields were discussed as they are applicable to the current study. Trace elements were discussed in terms of concentration, origin, mode of occurrence together with their health and environmental effects. Chromium was discussed in terms of its characteristics, occurrence, and probable origin in South African coals. A variety of analytical techniques are used to study the different components in coal were discussed. A combination of petrography, chemical analyses, ICP-MS, XRF, XRD, SEM-EDX, density fractionation, and selective leaching will be used to analyse the selected coal samples used in this study.

CHAPTER 3: SAMPLES AND METHODOLOGY In order to meet the aim and objectives of the study, a variety of samples were acquired and general coal characterization, geochemistry, and mineralogical analyses were conducted. This chapter explains how the samples were obtained from the different South African coalfields and how the samples were prepared for the different analyses. The analytical work for petrography, proximate, and ultimate analyses, XRD, XRF, and ICP-MS is outlined. All samples were assessed for the Cr, Mn, Co, and Mo content, and the select samples (those with high Cr values) were assessed using correlation coefficients, density fractionation, and sequential leaching to determine the mode of occurrence. From the density separated samples, one sample with a high Cr value was studied with SEM-EDX to further understand the mode of occurrence.

3.1 Sample collection Coals from five South African coalfields were considered for this study, namely: 1) the Waterberg, 2) Soutpansberg, 3) Witbank, 4) Highveld, and 5) Nongoma Coalfields (Fig. 1.1). Samples were obtained from different seams in some localities.

1. Waterberg Coalfield: three samples were supplied by the mine from a belt cut obtained from the three different coal zones, namely Zone 8-9, 6-7, and 5. 2. Soutpansberg Coalfield: two boreholes were selected from the Makhado Coal Project, with samples taken from the middle and bottom of each borehole, representing the No. 6 Seam of the Madzaringwe Formation. Samples from the top of the borehole were not enough to be used for this study. 3. Six samples were obtained from the Ntshovelo Colliery near Delmas (Fig. 3.1), in the Witbank Coalfield. The roof, middle, and floor of the No. 2 Seam and No. 4 Seam were sampled at the mine face by the mine personnel. 4. The Highveld Coalfield samples were supplied by the mine from two different underground mine faces mining the No. 4 Seam. 5. The Nongoma Coalfield samples were supplied by another MSc student who sampled pillars of an operating underground mine. Samples from the roof, middle and floor were selected.

Thus, a total of 21 samples were obtained for this study (Table 3.1).

Table 3.1: List of samples obtained from the different localities as shown in Figure 1.1.

Formation Coal seam Sample identification Sample Coalfield Waterberg Zone 8-9 WZ9

1. Waterberg Grootegeluk Bench 3,4,5 Waterberg Zone 6-7 WZ7

Waterberg Zone 5 WZ5 Borehole F578004 FBU bottom-upper Borehole F578004 FMU middle-upper 2. Soutpansberg Madzaringwe No. 6 Seam Borehole S65801LD SBU bottom-upper Borehole S65801LD SMU middle-upper Seam 2 top S2T

No. 2 Seam Seam 2 middle S2M

Seam 2 bottom S2B 3. Witbank Vryheid Seam 4 top S4T

No. 4 Seam Seam 4 middle S4M Ecca Group Ecca

Seam 4 bottom S4B

Highveld area 1 H1A 4. Highveld Vryheid No. 4 Seam

Highveld area 2 H2B

Nongoma area 1 roof N1A

Nongoma area 1 middle N1B

Nongoma area 1 floor N1C 5. Nongoma Vryheid Main seam Nongoma area 2 roof N2A

Nongoma area 2 middle N2B

Nongoma area 2 floor N2C

Figure 3.1: Sampling the No. 4 Seam, Witbank Coalfield.

3.2 Sample preparation The Witbank and Highveld Coalfield samples were sampled from the mine face by an excavator, which resulted in particles larger than 16 mm in size. The samples were crushed at UJ, Doornfontein Campus, using a jaw and roll crusher to achieve a 2–4 mm particle size. After every sample was prepared, the crusher was cleaned and a clean polystyrene layer was inserted into the collecting pan to prevent contamination. The samples were reduced to a representative sample (2–4 mm) using the cone and quarter method. The Waterberg, Soutpansberg, and Nongoma Coalfield samples were already crushed to -4mm when received, prepared by other students using comparable crushing techniques. Each sample was split using a manual splitter, with one half being stored and the other half being split again into half.

The quarter sample was milled to -1 mm using an ultra-centrifugal Retsch ZM200 mill housed at Wits School of Chemical and Metallurgical Engineering. This sample was mixed with epoxy resin and hardener in a 30 mm crucible and left in vacuum to solidify overnight. The blocks were then polished using a Struers Tegraforce polisher to create a relief free surface with a final polish of 0.04 µm, required for coal petrography and SEM analyses. The blocks were prepared following SABS ISO 7404 part 2 at the UJ Department of Geology sample preparation facility.

The other quarter sample was milled to -250 µm and the size fraction was used for chemical analyses such as proximate and ultimate analyses, XRD, and XRF. For ICP-MS analyses, the samples were pulverized using a mortar and pestle and sieved to pass -106 µm to enable more surface area for digestion (Fig. 3.2). The sample was not milled using the Retsch ZM200 due to possible contamination of Cr from the mill components. The mill has a stainless steel push-fit rotor, ring sieves, and cassette (Fig. 3.3). The potential contamination from the mill is discussed further in Section 4.6.

After assessing the ICP-MS data, six samples with the highest Cr concentrations were selected to determine the mode of occurrence of Cr specifically. Samples were selected from the retained portions and milled using the newly acquired Retsch ZM200 to -1mm and -250 µm. This mill was purchased with a pure titanium push-fit rotor, titanium ring sieves with the reinforced rim and titanium-niobium coated cassette (Fig. 3.4) to prevent metal contamination and thus replacing the tedious mortar and pestle and sieving steps.

Polished mount blocks for petrography & SEM (-1mm)

Density fractionation (2-4 Retsch ZM 200 mm) Proximate, ultimate, XRD & XRF (250µm)

Jaw and roll crusher (2-4mm) ICP-MS (106 µm)

Mortar and pestle

Selective leaching (250 µm)

Figure 3.2: Sample preparation and analyses flowchart.

Figure 3.3: Retsch ZM200 with stainless steel components

Figure 3.4: Titanium-niobium coated cassette.

3.3 Analyses Various analyses, namely petrography, proximate, and ultimate analyses were conducted in order to study and understand the selected coals used in this study (Fig. 3.5). The mineralogy of coal was determined by XRD and XRF. In order to understand the geochemistry of the coal samples, ICP-MS analyses were conducted. Density fractionation, sequential leaching, and SEM-EDX were used to determine the mode of occurrence of the selected trace elements (Fig. 3.5). The procedures followed are outlined in the next sub-sections.

Coal Characterization -Petrography -Proximate - Ultimate

Mineralogy Geochemistry ANALYSES -XRD -ICP-MS -XRF

Mode of occurence -Density fractionation -Sequential leaching -SEM

Figure 3.5: The different analyses conducted.

3.3.1 Coal characterization Coal can be characterized in terms of organic matter and inorganic matter. Petrographic, proximate, and ultimate analyses are typically used to characterize the coal.

3.3.1.1 Petrographic analysis Petrographic analyses (maceral and mineral group analyses and vitrinite reflectance) were used to characterize the coal samples in terms of the major organic and inorganic components. A Zeiss Axio Imager M2 polarised reflected light microscope, with a 50X oil immersion objective, coupled to Hilgers Diskus Fossil software analyses was used to conduct the analyses at the Department of Geology, UJ (Fig. 3.6). The samples were analysed using live images generated by a digital colour camera for maceral analysis using a point count technique, generally following SABS ISO 7404 part 3. The maceral groups and minerals were distinguished by their reflectance and morphology, and a total of 500 points for each sample were recorded on macerals and mineral matter reported as percentage volume (vol %).

Figure 3.6: Zeiss Axio Imager M2 polarised reflected light microscope, housed at the University of Johannesburg, in the Geology Department.

The maceral terminology and classification used in this study are based on Taylor et al. (1998) and ICCP Maceral Classification System 1994 (ICCP, 2001). Mean random vitrinite reflectance analysis was used to determine the rank of the coal samples, essential following SABS ISO 7404 part 5. The reflectance system was calibrated using a Yttrium-Aluminium-Garnet standard with a certified value of 0.900. 100 readings were obtained on vitrinite particles, and the mean vitrinite reflectance value (%RoVmr) and the standard deviation was reported.

3.3.1.2 Proximate analysis Proximate analyses were conducted to determine the moisture, volatile matter, fixed carbon, and ash content using a Perkin Elmer TGA with Pyris software (Fig. 3.7) housed at Wits. Procedures from Donahue and Rais (2009), Cassel and Menard, (2012), and Mphaphuli (2017) were followed as explained in Section 2.4.1.2. About 10 mg of coal sample was loaded in a crucible and placed in the furnace (Fig. 3.7). The furnace was heated from 30 ºC to 700 ºC under nitrogen atmosphere and then from 700 ºC to 900 ºC under an oxygen atmosphere for coal combustion. Pyris Software was used to process the data. An example of a TGA graph is presented in Figure 3.8, also see Appendix A. The moisture content was determined from 0 ºC – 110 ºC, volatile matter from 110 ºC – 700 ºC, and fixed carbon from 700 ºC – 900 ºC. The ash content was determined using Equation 1 (Section 2.4.1.2).

Figure 3.7: The Perkin Elmer TGA housed at Wits.

Figure 3.8: A graph showing the proximate analysis data determined by TGA.

3.3.1.3 Ultimate analysis Ultimate analyses are elemental analyses which determine the weight percentage of C, H, N, and S present in the coal, following the ISO 1170:2008 standard. Oxygen is calculated by difference using Equation 2 (Section 2.5.1.3). The ultimate analyses for this study were conducted at Bureau Veritas, Centurion, South Africa. A sample of 0.1 g (250 μm) was used for the analysis. The moisture content of each sample was determined concurrently with the analyses. The samples were run in duplicates including a reference material in order to monitor acceptable instrument performance and calibration for the sample matrix. The results were reported as a percentage by mass, and calculated to dry ash free basis.

3.3.2 XRD X-ray diffraction was used to identify minerals present in South African coals as explained in Section 2.4.2.1. The coal samples were dried at 100 ºC and subjected to LTA prior to XRD analysis to remove the organic matter that masks the peaks. The samples were prepared as 25 mm pressed-pellets for XRD. The Phillips PANanalytical X-ray Diffractometer at the United States Geological Survey (USGS) Reston, Virginia, was used to conduct the analyses, operating at 45 kilovolts and 40 milliamps. The fixed slits with Ni-filtered Cu K-alpha radiation was used to scan the samples from 5º to 65º over a range of 2θ counting for 90 seconds every 0.017º. The X’Pert Highscore Software was used to process the X-ray spectrum and estimate the proportions of major, minor, and trace mineral phases in the parent samples using the Rietveld method.

The XRD analyses for the density separated samples were done at XRD Analytical and Consulting in Pretoria, South Africa, due to time constraints. The coal samples were prepared using a backloading preparation method and analysed using a Malvern Panalytical Aeris diffractometer, with PIXcel detector and fixed slits with Fe filtered Co-Kα radiation. The mineral phases were identified using X’Pert Highscore plus software and the relative phase amounts were estimated using the Rietveld method.

3.3.3 XRF X-ray fluorescence was used to determine the major oxide elements in high- temperature ash samples. The coal samples were ashed following ISO standard 1171:2010 at 815 ºC for two hours to remove the organic matter and dried at 110 ºC before fusion in the oven. A sample of 0.70 g was weighed, and mixed with 0.1 g oxidant and 6.00 g lithium bromide flux in a platinum crucible. The sample was heated to 1105 ºC to create a melt that was poured into a dish, which solidified into a glass disc; the pellet was used to analyse the elements. A MagiX Pro spectroscope housed at the UJ Spectrum analytical facility was used for the analysis and calibrated for 15 elements using various certified standards.

3.3.4 ICP-MS In order to conduct ICP analysis, the sample must be in solution, and as a result, coal was digested prior to analysis. For this study, procedures from Rodushkin et al. (2000) were followed. The coal samples were digested using a 2-stage microwave digestion sequence. About 0.10 g (106µm) of the coal sample was placed in a clean vessel with a mixture of 5 ml 65% HNO3 and 2 ml H2O2 and digested using Mars 6 microwave for 1 hour at a temperature of 120 0C. After the first run was complete, the vessels were removed and vented in a fume hood. After the venting procedure, 2 ml of 65% HNO3 and 1 ml H2O2 was added to each sample, sealed and placed in the microwave for the second digestion at 140 0C.

After digestion, the solution was carefully collected and filtered to remove the insoluble matrix. The solution was transferred into 50 ml volumetric flasks and filled to the mark using MilliQ water. A 10X times dilution factor was used, and 1 ml was diluted to 10 ml using 1% HNO3. The solution was then analysed for Cr, Mn, Co and Mo concentrations using the Perkin Elmer NexION 300 ICP-MS housed at the UJ Spectrum.

Blank samples and a digested reference material (SARM 19) were included to check the accuracy of the digestion and analytical methods. The recovery rates for the selected trace elements in the reference material (SARM 19) were within the certified range. A multi-element standard solution including elements of interest was used to prepare calibration solutions for trace element concentrations. The calibration curves for the selected trace elements were linear and within the expected ranges, showing

that the analytical method for trace elements determination was accurate and consistent.

Several factors were taken into account to ensure the quality of the results. The apparatus was washed thoroughly with soap and rinsed with MilliQ water and dried between samples. All prepared standard solutions were kept at 4 ºC in the refrigerator before analysis. Each sample was digested and analysed three times. Blank reagents and reference materials for coal were used in each sample batch to verify the accuracy digestion and analysis.

3.3.5 Mode of occurrence determination This study applied several methods to investigate the modes of occurrence of Cr in the selected South African coal samples. Density fractionation and selective leaching were conducted on six samples with high Cr concentrations.

3.3.5.1 Density fractionation Several authors have used density fractionation to determine trace elements and their affinities in coal (Bergh et al., 2011; Wagner and Tlotleng, 2012; Mguni, 2015; Kolker et al., 2017) as outlined in Section 2.4.5.3. Density fractionation was used to determine Cr affinity in the coal samples. The samples were prepared at Bureau Veritas. The coal samples (-4 mm) were placed in various beakers with different density solutions, and allowed to separate into float and sink fractions (float 1.4, 1.7, 1.9, and sink 1.9). After the separation was completed, the samples were rinsed and dried. The masses and cumulative yields of the different fractions were measured. The 24 samples were subjected to petrography, proximate, XRD, and ICP-MS analyses, following the procedures discussed above.

3.3.5.2 Sequential leaching Sequential leaching was undertaken to determine the elements that were removed by different acids, to investigate their association with certain minerals. The selective leaching procedure was developed by Finkelman and Simon (1984). It was later refined by Palmer et al. (2000) and used by Finkelman et al. (2018). Procedures from Kolker et al. (2000b) outlined in Section 2.4.5.4 were used for this study.

About 5 g of coal sample was sequentially leached using four different solvents. The vacuum filtration system was used to obtain leachates and ICP-MS was used to analyse the leachates. All the leaching apparatus were washed and rinsed with MilliQ water. The sequence of solvents that was adopted follows:

 35 ml 1 M CH3COONH4 was added to remove elements associated with exchangeable cations and a portion of carbonate-hosted cations.  35 ml of 3 M HCl was used to remove cations associated with the carbonates and mono-sulphides;  35 ml of 40% concentrated HF was used to removed cations associated with silicates;

 35 ml of 2 M HNO3 was used to remove elements associated with the sulphides.

10 ml of MilliQ water was used to wash the previous solution between each step. Any unleached cations may be encapsulated in the coal matrix or associated with the insoluble mineral phases.

3.3.5.3 SEM-EDX The association of Cr with organic and inorganic components was examined using SEM, as explained in Section 2.4.5.1. The UJ Tescan Vega SEM, equipped with EDX was used. A semi-quantitative EDX analysis was conducted as well as grain morphology and appearance were determined using back-scattered electron imaging (BSE). Energy Dispersive X-ray Spectroscopy was used to acquire elemental maps and spectrum peaks of different minerals. The sample with high Cr from density fractionation was analysed. A polished coal-epoxy pellet coated with carbon was analysed using SEM.

3.3.6 XAFS X-ray absorption fine structure spectroscopy is a direct, non-destructive, synchrotron technique used to determine the oxidation of an element. This technique is not widely available and in order to obtain beam-time, the researcher needs to apply at synchrotron facilities such as the ESRF. Unfortunately for this study, the application was unsuccessful (see Appendix B).

CHAPTER 4: RESULTS

This chapter provides the results obtained from the different analyses conducted on the 21 samples collected from the five coalfields in South Africa. These include general characterization (petrography, proximate, and ultimate analyses), mineralogy (XRD and XRF), and trace elements determined by ICP-MS as well as the mode of occurrence results. The results are discussed in Chapter 5.

4.1 Petrography Maceral groups, mineral matter, and vitrinite reflectance results are presented in Table 4.1. The Waterberg samples contain high mineral matter, which is primarily constituted of clays and quartz (Table 4.1). The Waterberg coal samples have high liptinite compared to other coal samples, and variable amounts of vitrinite and inertinite. The coals are medium rank C bituminous as determined by vitrinite reflectance (Table 4.2).

Coal samples from the Soutpansberg Coalfield are dominated by vitrinite and have less liptinite, inertinite, and mineral matter compared to the other coal samples. In terms of the mineral composition, samples FBU, FMU, and SBU are dominated by silicate minerals and sample SMU is dominated by carbonates (Table 4.1). Sample FMU is medium rank B bituminous coal while FBU, SBU SMU are medium rank C bituminous coals as determined by vitrinite reflectance (Table 4.2).

The Witbank Coalfield is represented by samples from the No. 2 Seam and No. 4 Seam, with three samples taken from each seam. These samples are inertinite-rich and have a variable amount of vitrinite and liptinite (Fig. 4.1). The Witbank coal samples have less mineral matter compared to the Waterberg coal samples. These coal samples are medium rank C bituminous coals (Table 4.2).

Samples from the Highveld Coalfield have comparable amounts of inertinite, liptinite, and mineral matter. Sample H1A contains more vitrinite than sample H2B. The coal samples are characterized as medium rank C bituminous coals (Table 4.2).

Table 4.1: Maceral groups, mineral matter (vol %), and vitrinite reflectance (%RoVmr) results.

Maceral groups (vol %) Mineral matter (vol %) Vitrinite Coalfields Samples reflectance Vitrinite Liptinite Inertinite Total Clays Quartz Pyrite Carbonates Others Total %RoVmr WZ9 22.2 3.5 6.7 32.4 52.9 7.1 4.5 2.9 0.2 67.6 0.63 Waterberg WZ7 19.3 7.4 15.8 42.5 31.7 19.2 3.0 1.4 2.2 57.5 0.64 WZ5 5.0 9.5 25.5 40.0 25.1 31.3 1.0 0.6 2.0 60.0 0.68 FBU 44.2 0.3 24.1 68.6 28.6 0.0 1.4 1.4 0.0 31.4 0.95 FMU 56.6 0.4 19.1 76.1 19.1 0.0 2.6 2.2 0.0 23.9 1.26 Soutpansberg SBU 64.8 0.2 10.5 75.5 18.6 0.0 1.8 4.1 0.0 24.5 0.89 SMU 42.8 1.6 17.8 62.2 7.4 0.0 1.2 29.2 0.0 37.8 0.95 S2T 10.2 2.0 77.0 89.2 6.6 2.2 1.6 0.4 0.0 10.8 0.61 S2M 13.4 2.2 71.0 86.6 8.4 1.0 0.0 4.0 0.0 13.4 0.65 S2B 6.4 2.0 85.2 93.6 2.0 1.4 1.0 2.0 0.0 6.4 0.62 Witbank S4T 2.3 1.9 84.5 88.7 9.7 1.6 0.0 0.0 0.0 11.3 0.73 S4M 4.8 2.8 83.2 90.8 7.6 1.2 0.4 0.0 0.0 9.2 0.73 S4B 20.0 2.1 69.5 91.6 5.9 0.2 2.3 0.0 0.0 8.4 0.64 H1A 12.2 4.0 47.2 63.4 9.3 19.2 4.5 3.6 0.0 36.6 0.64 Highveld H2B 6.3 3.3 48.3 57.9 8.6 25.3 3.7 4.5 0.0 42.1 0.64 N1A 22.0 0.0 67.5 89.5 3.2 0.2 3.8 3.2 0.0 10.4 3.64 N1B 8.0 0.0 90.8 98.8 1.2 0.0 0.0 0.0 0.0 1.2 3.65 N1C 59.6 0.0 37.2 96.8 0.4 0.0 2.8 0.0 0.0 3.2 4.24 Nongoma N2A 36.4 0.2 50.8 87.4 3.7 1.0 2.3 5.6 0.0 12.6 3.61 N2B 22.2 0.0 74.6 96.8 1.4 0.0 1.2 0.6 0.0 3.2 3.61 N2C 40.0 0.0 57.2 97.2 1.6 0.0 1.2 0.0 0.0 2.8 4.16

Coalfields Waterberg Soutpansberg Witbank Highveld Nongoma 100

Mineral 50

vol % vol Matter Inertinite 40 Liptinite

30 Vitrinite

0 WZ9 WZ7 WZ5 FBU FMU SBU SMU S2T S2M S2B S4T S4M S4B H1A H2B N1A N1B N1C N2A N2B N2C Sample Medium-rank bituminous High-rank anthracites

Figure 4.1: The distribution of the maceral groups and mineral matter (vol %).

Table 4.2: Coal rank classification using vitrinite reflectance (%RoVmr) results of the selected coals (modified after Wagner et al., 2018).

Coal samples from the Nongoma Coalfield are inertinite-rich. They have variable amounts of vitrinite and do not have liptinite, due to rank (Fig. 4.1). These samples have less mineral matter compared to other coal samples. The Nonogma coal samples are characterized as high-rank anthracites (Table 4.2).

4.2 Proximate analysis To further characterize the coals, proximate analyses were used. Moisture, VM, FC, and ash content in these coal samples were determined as explained in Section 3.3.1.2. The Waterberg samples have a low FC ranging from 20.4 to 28.7 wt. % (Table 4.3) due to the high ash content (Fig. 4.2) and this can be linked to the high mineral matter, observed in the petrographic analyses as shown in Section 4.1. The Soutpansberg samples have a high VM and FC, and low ash content compared to the Waterberg coal samples (Table 4.3). The low ash content can be linked with the lower mineral matter determined by petrographic analyses.

Table 4.3: Proximate analyses for the selected coal samples (wt. %).

As received Dry basis Dry ash free Coalfield Sample Moisture VM FC Ash VM FC Ash VM FC WZ9 1.5 20.7 24.8 53.0 21.0 25.2 53.8 45.4 54.5 Waterberg WZ7 1.1 17.8 28.4 52.8 18.0 28.7 53.4 38.6 61.6 WZ5 1.0 15.3 20.2 63.6 15.5 20.4 64.2 43.2 57.1 FBU 0.5 20.8 51.3 27.4 19.8 45.9 34.3 30.1 69.9 FMU 0.5 11.5 58.5 29.5 11.6 58.8 29.6 16.4 83.6 Soutpansberg SBU 0.5 20.2 39.0 40.3 17.9 50.7 31.4 26.1 73.9 SMU 0.5 19.7 45.7 34.1 23.6 48.2 28.2 32.8 67.2 S2T 3.2 19.0 54.2 23.6 19.6 56.0 24.4 26.0 74.0 S2M 2.9 17.1 54.2 25.8 19.1 53.6 27.4 26.2 73.8 S2B 3.2 18.4 46.3 32.1 19.0 47.8 33.2 28.4 71.6 Witbank S4T 11.0 20.1 55.5 13.4 22.6 62.4 15.1 26.6 73.4 S4M 3.8 17.9 58.5 19.8 18.6 60.8 20.6 23.4 76.6 S4B 5.2 22.2 63.6 9.4 23.4 67.1 9.9 26.0 74.0 H1A 5.0 20.6 43.6 30.8 21.4 39.9 38.7 34.9 65.1 Highveld H2B 5.1 17.1 46.9 30.9 18.2 39.0 42.8 31.8 68.5 N1A 1.9 6.7 76.7 14.7 6.4 78.1 15.5 7.6 92.4 N1B 2.2 7.5 72.1 18.2 8.7 81.1 10.2 9.7 90.3 N1C 2.1 6.9 78.2 12.8 7.9 73.2 18.9 9.7 90.3 Nongoma N2A 2.3 11.7 79.3 6.7 11.4 66.8 21.8 14.6 85.4 N2B 2.2 8.3 74.9 14.6 8.6 64.5 26.9 11.7 88.3 N2C 1.6 7.9 75.6 14.9 9.7 69.8 21.4 12.3 88.9

Coalfields Waterberg Soutpansberg Witbank Highveld Nongoma 100

50 wt. % Ash

40 FC VM

0 WZ9 WZ7 WZ5 FBU FMU SBU SMU S2T S2M S2B S4T S4M S4B H1A H2B N1A N1B N1C N2A N2B N2C

Medium-rank bituminous Sample High-rank anthracites

Figure 4.2: Proximate analysis for the selected coal samples on dry basis (wt. %).

Samples from the Witbank Coalfield have FC ranging from 47.8 to 62.4 wt. %, and an ash content ranging from 9.9 to 33.2% (Table 4.3). This can be linked to the high organic matter and low mineral matter observed during petrographic analysis. Sample H1A and H2B from the Highveld Coalfield have high moisture and VM compared to other coal samples (Table 4.3). They have sub-equal amounts of FC and ash. The Nongoma coal samples have high FC and low ash contents (Fig. 4.2), which is typical for anthracites. This is in agreement with the high organic matter and low mineral matter observed in petrographic analyses

4.3 Ultimate analysis results Ultimate analyses were conducted to determine the C, H, N, O, and S content in the coal samples. Sample WZ9, WZ7, and WZ5 have a low C content compared to other coal samples, and this is in agreement with the proximate analysis. Sample WZ9 and WZ7 have a high S content (Table 4.4), and this can be linked with pyrite occurrence observed during petrographic analyses.

Table 4.4: Ultimate analysis for the selected coal samples reported on a dry ash free basis (%).

Coalfields Sample C H N S O Waterberg WZ9 38.7 3.2 0.7 2.9 54.5 WZ7 34.2 2.9 0.7 1.8 60.5

WZ5 28.6 2.4 0.6 0.6 67.8 FBU 60.2 3.9 1.2 1.4 33.4 Soutpansberg FMU 58.7 3.4 1.1 1.4 35.4 SBU 61.0 3.7 1.2 0.6 33.5 SMU 59.2 3.7 1.1 0.6 35.4 S2T 53.5 3.8 1.3 0.6 40.8 S2M 52.4 3.5 1.1 0.5 42.5 Witbank S2B 52.3 3.4 1.1 0.7 42.5 S4T 52.1 3.4 1.1 0.6 42.8 S4M 55.1 3.4 1.2 0.8 50.2 S4B 64.6 4.1 1.5 1.1 28.6 H1A 48.0 3.2 1.1 1.4 46.2 Highveld H2B 47.8 3.3 1.0 1.3 46.5 N1A 70.4 2.3 1.4 1.1 24.8 N1B 80.9 2.3 1.6 1.0 14.2 Nongoma N1C 69.7 2.0 1.5 1.9 24.9 N2A 73.3 2.5 1.6 2.6 20.1 N2B 81.4 2.5 1.7 1.2 13.3 N2C 80.5 2.4 1.6 0.9 14.6

Coal samples from the Soutpansberg Coalfield have a high C content and this is in agreement with the high FC determined from proximate analysis. Sample FBU and FMU have a high S, while sample SBU and SMU have a low S content. The S content may be due to the presence of pyrite in these coals (Table 4.4). The S content is high for S4B and low for the other samples. This is because sample S4B contains more pyrite compared to other samples. The Highveld and Nongoma samples also have a high S content

4.4 Mineralogy The different minerals phases present in coals, as determined by using XRD are summarised in Table 4.5. The XRD diffractograph patterns for these samples following LTA are included in Appendix C. The results are reported as including and excluding the organic carbon and although the mineralogy excluding organic carbon is presented in Figure 4.3.

Sample WZ9 is composed of roughly sub-equal amounts of kaolinite and quartz, followed by a minor amount of illite and trace amounts of pyrite, feldspars, carbonates, and illite/smectite mixed-layer. Sample WZ7 contains kaolinite, quartz with a minor amount of illite and trace amounts of feldspars, ankerite, and pyrite. Sample WZ5 is dominated by kaolinite followed by quartz with a minor amount of illite and trace amounts of pyrite, and illite/smectite mixed-layer (Fig. 4.3).

From the Soutpansberg Coalfield, sample FBU and FMU are dominated by quartz followed by kaolinite, illite, calcite, ankerite, and trace pyrite, and chlorite. Sample SBU is dominated by carbonated followed by kaolinite and quartz with minor amounts of illite, and trace amounts of siderite and goethite (Table 4.5). Sample SMU is mostly quartz followed by carbonates, kaolinite, with trace amounts of illite and pyrite.

Sample S2T, S2M, and S2B from the No.2 Seam are dominated by kaolinite, and quartz with minor amounts of carbonates, illite, and trace amounts of pyrite, anhydrite, sphalerite, and alunite. Samples S4T, S4M, and S4B from the No.4 Seam are dominated by kaolinite followed by quartz, with minor amounts of illite and trace amounts of K-feldspar, pyrite, illite, illite/smectite mixed-layer, and goethite.

Table 4.5: Mineralogy of the selected coal samples (vol %).

Includes organic carbon Excludes organic carbon Sample QTZ FLD CAR ILL I/S KAO CHL PY OTH OM QTZ FLD CAR ILL I/S KAO CHL PY OTH WZ9 22.7 0.6 0.6 4.0 1.1 24.9 0.0 2.3 0.6 43.4 40.0 1.0 1.0 7.0 2.0 44.0 0.0 4.0 1.0 WZ7 17.9 0.0 0.6 4.5 0.0 30.8 0.0 1.1 1.1 43.9 32.0 0.0 1.0 8.0 0.0 55.0 0.0 2.0 2.0 WZ5 14.8 0.0 0.0 4.0 0.7 45.7 0.0 0.7 1.3 32.8 22.0 0.0 0.0 6.0 1.0 68.0 0.0 1.0 2.0 FBU 15.7 0.0 2.2 4.9 0.0 7.1 0.0 0.9 0.0 69.2 51.0 0.0 7.0 16.0 0.0 23.0 0.0 3.0 0.0 FMU 13.9 0.0 2.0 11.6 0.0 3.6 0.7 1.0 0.3 66.9 42.0 0.0 6.0 35.0 0.0 11.0 2.0 3.0 1.0 SBU 6.0 0.0 10.0 3.0 0.0 14.4 0.0 0.0 0.0 66.6 18.0 0.0 30.0 9.0 0.0 43.0 0.0 0.0 0.0 SMU 14.0 0.0 2.5 3.5 0.0 10.8 0.0 1.0 0.0 68.2 44.0 0.0 8.0 11.0 0.0 34.0 0.0 3.0 0.0 S2T 6.0 0.0 0.0 1.3 0.3 24.6 0.0 0.3 0.7 66.8 18.0 0.0 0.0 4.0 1.0 74.0 0.0 1.0 2.0 S2M 5.2 0.0 2.8 0.7 0.7 23.9 0.0 0.7 0.7 65.4 15.0 0.0 8.0 2.0 2.0 69.0 0.0 2.0 2.0 S2B 4.5 0.0 1.7 0.7 1.0 25.4 0.0 0.7 0.7 65.2 13.0 0.0 5.0 2.0 3.0 73.0 0.0 2.0 2.0 S4T 6.7 0.4 0.0 0.4 1.2 29.0 0.0 0.0 1.6 60.9 17.0 1.0 0.0 1.0 3.0 74.0 0.0 0.0 4.0 S4M 10.3 0.3 0.0 1.7 0.0 15.2 0.0 0.0 1.1 71.3 36.0 1.0 0.0 6.0 0.0 53.0 0.0 0.0 4.0 S4B 4.4 0.2 0.0 0.4 0.6 11.7 0.0 0.6 0.7 81.5 24.0 1.0 0.0 2.0 3.0 63.0 0.0 3.0 4.0 H1A 7.8 0.4 1.6 0.0 0.8 26.0 0.0 1.2 1.2 61.1 20.0 1.0 4.0 0.0 2.0 67.0 0.0 3.0 3.0 H2B 6.3 0.4 2.4 2.0 0.8 26.1 0.0 0.8 0.8 60.5 16.0 1.0 6.0 5.0 2.0 66.0 0.0 2.0 2.0 N1A 0.0 0.0 0.0 13.5 0.0 0.0 0.9 4.1 0.2 81.2 0.0 0.0 0.0 72.0 0.0 0.0 5.0 22.0 1.0 N1B 0.0 0.0 1.7 4.0 0.0 0.0 12.9 2.1 0.4 78.8 0.0 0.0 8.0 19.0 0.0 0.0 61.0 10.0 2.0 N1C 0.0 0.0 0.6 5.6 0.0 0.0 1.7 0.8 0.0 91.3 0.0 0.0 7.0 64.0 0.0 0.0 20.0 9.0 0.0 N2B 0.0 0.0 0.2 6.8 0.0 0.0 3.4 0.8 0.1 88.7 0.0 0.0 2.0 60.0 0.0 0.0 30.0 7.0 1.0 N2C 0.0 0.0 0.0 6.6 0.0 0.0 1.9 1.6 0.0 89.8 0.0 0.0 0.0 65.0 0.0 0.0 19.0 16.0 0.0

QTZ= quartz, FLD= K-feldspar and plagioclase, CAR= calcite, ankerite, dolomite and siderite, ILL= illite and muscovite, I/S= illite/smectite or rectorite, KAO= kaolinite, CHL= chlorite, PY= pyrite, marcasite and sphalerite, OTH= any other unqualified phase(s) not accounted for in the previous categories (goethite, alunite, barite, hematite, anhydrite, bassanite), and OM= organic matter.

Coalfields Waterberg Soutpansberg Witbank Highveld Nongoma 100

70 OTH PY 60 CHL 50 KAO I/S 40 ILL CAR 30 FLD

20 QTZ

0 WZ9 WZ7 WZ5 FBU FMU SBU SMU S2T S2M S2B S4T S4M S4B H1A H2B N1A N1B N1C N2B N2C Sample Medium-rank bituminous High-rank anthracite

Figure 4.3: Mineralogy of the coal samples. QTZ= quartz, FLD= K-feldspar and plagioclase, CAR= calcite, ankerite, dolomite and siderite, ILL= illite and muscovite, I/S= illite/smectite or rectorite, KAO= kaolinite, CHL= chlorite, PY= pyrite, marcasite and sphalerite, OTH= any other unqualified phase(s) not accounted for in the previous categories (goethite, alunite, barite, hematite, anhydrite, bassanite), and OM= organic matter.

Samples H1A and H2B are dominated by kaolinite followed distantly by quartz and trace amounts of feldspars, calcite, ankerite, pyrite, illite/smectite mixed-layer, and sphalerite. Sample N1A is dominated by illite (Table 4.5), followed by quartz and pyrite with minor amounts of chlorite, and trace amounts of plagioclase and rutile. Sample N1B is dominated by chlorite with minor amounts of illite, siderite, and pyrite and trace amounts of ankerite and marcasite. Sample N1C is dominated by illite, followed by chlorite, with minor amounts of pyrite and calcite, and trace amounts of siderite. Samples from area 2 (N2B and N2C) are dominated by illite and chlorite with a minor amount of pyrite and trace amounts siderite and rutile (Table 4.5).

4.5 Major elements The major element oxides were determined in ash by XRF. The Waterberg,

Soutpansberg, Witbank, and Highveld coal samples have high SiO2, Al2O3, Fe2O3,

CaO, and low TiO2, MgO, Na2O, K2O, SO3, P2O5, and BaO (Table 4.6). This can be linked with the mineralogy observed in these samples. Silicates are the dominant minerals, hence the high SiO2 and Al2O3 is expected. CaO, Fe2O3, and SO3 can also be expected as a result of carbonates and pyrite minerals present. Cr2O3, NiO, and

V2O5 were below the detection limit for XRF (Table 4.6) by the fused glass disc XRF method, which is not intended for trace elements. These elements could have been determined by XRF on pressed pellets of sample powder if ICP-MS was not available.

Table 4.6: The major element oxides present in the Waterberg, Soutpansberg, Witbank, Highveld, and Nongoma Coalfields (wt. %).

Coalfield Sample SiO2 Al2O3 Fe2O3 TiO2 CaO MgO Na2O K2O MnO P2O5 SO3 BaO Cr2O3 NiO V2O5 Sum WZ9 39.6 23.8 9.5 1.2 11.1 1.5 0.5 1.9 0.0 6.9 1.0 0.1 0.0 0.0 0.0 97.0 Waterberg WZ7 57.5 28.8 3.1 1.4 3.6 1.0 0.1 0.7 0.1 0.5 1.6 0.2 0.0 0.0 0.0 98.7 WZ5 61.7 23.7 5.3 1.2 2.2 0.9 0.1 0.9 0.1 0.1 1.7 0.1 0.0 0.0 0.0 97.8 FBU 59.7 14.8 9.3 0.9 4.8 2.3 0.1 1.2 0.1 0.1 5.1 0.1 0.0 0.0 0.1 98.5 FMU 64.0 15.7 4.9 0.9 6.5 0.7 0.2 1.7 0.1 0.1 3.3 0.1 0.0 0.0 0.0 98.2 Soutpansberg SBU 45.9 39.5 1.8 2.2 1.3 0.5 0.1 2.1 0.0 1.2 2.2 0.6 0.0 0.0 0.0 97.5 SMU 64.8 21.1 7.3 0.8 1.4 0.5 0.0 1.2 0.1 0.1 0.9 0.0 0.0 0.0 0.0 98.2 S2T 52.7 30.4 3.0 1.7 6.1 1.5 0.0 0.6 0.0 0.7 1.3 0.2 0.0 0.0 0.0 98.0 S2M 53.3 13.5 18.9 1.3 4.1 2.3 0.1 1.0 0.2 0.2 4.9 0.1 0.0 0.0 0.1 100 S2B 43.1 27.7 17.2 1.3 1.6 0.4 0.8 3.5 0.0 0.3 0.7 0.1 0.0 0.0 0.0 96.7 Witbank S4T 34.1 15.5 6.5 1.2 26.8 6.3 0.0 0.6 0.2 0.8 6.1 0.2 0.1 0.1 0.1 98.4 S4M 50.9 27.3 5.9 1.2 4.8 0.7 1.0 0.9 0.1 0.1 4.3 0.1 0.0 0.0 0.0 97.0 S4B 58.0 28.8 6.3 1.9 0.7 0.2 0.0 1.0 0.0 0.7 0.8 0.3 0.0 0.0 0.0 98.8 H1A 52.4 25.6 4.9 1.6 6.2 1.7 0.2 1.0 0.1 0.6 3.8 0.3 0.0 0.0 0.0 98.4 Highveld H2B 52.1 26.0 4.9 1.8 6.3 1.8 0.2 1.0 0.1 0.5 3.9 0.3 0.0 0.0 0.0 98.9 N1A 46.3 28.0 9.1 1.1 5.5 0.4 0.6 3.4 0.0 0.1 2.8 0.1 0.0 0.0 0.0 97.4 N1B 35.9 22.2 16.4 1.6 10.7 0.7 0.3 2.1 0.1 3.8 3.2 0.1 0.0 0.0 0.0 97.0 N1C 29.5 19.9 24.0 1.0 11.9 0.7 0.2 2.0 0.2 0.1 7.0 0.0 0.0 0.0 0.0 96.5 Nongoma N2A 48.5 29.8 11.4 1.8 2.9 0.6 0.0 0.9 0.0 0.8 0.8 0.2 0.0 0.0 0.0 97.6 N2B 41.5 31.1 22.9 2.0 0.5 0.2 0.1 0.6 0.0 0.5 0.2 0.1 0.0 0.0 0.0 99.7 N2C 39.0 28.4 19.1 1.6 7.2 0.6 0.2 2.1 0.0 0.3 0.8 0.1 0.0 0.0 0.0 99.4

Coalfield Waterberg Soutpansberg Witbank Highveld Nongoma 100

V2O5 80 NiO Cr2O3 70 BaO

60 SO3 P2O5

50 MnO wt. % K2O 40 Na2O MgO 30 CaO TiO2 20 Fe2O3 Al2O3 10 SiO2

0 WZ9 WZ7 WZ5 FBU FMU SBU SMU S2T S2M S2B S4T S4M S4B H1A H2B N1A N1B N1C N2A N2B N2C Sample

Figure 4.4: The distribution of the major element oxides within the selected coal samples (wt. %).

4.6 Trace elements 4.6.1 Impact of mill contamination on results As explained in Section 3.2, the possible contamination from the milled samples was investigated by comparing the samples milled and samples pulverized by hand using a ceramic mortar and pestle. The samples that were milled have high Cr values compared to the samples pulverized using the mortar and pestle, indicating possible contamination from the mill (Table 4.7).

Table 4.7: Average Cr concentrations in the 21 selected coal samples prepared using a mill compared to samples prepared using mortar and pestle (ppm). Cr concentration Coalfields Milled Pestle Waterberg 45.1 24.7 Soutpansberg 31.5 23.9 Witbank 42.7 29.1 Highveld 42.1 35.2 Nongoma 8.5 2.0

4.6.2 Trace elements concentrations in samples prepared using mortar and pestle For the verification of the results, SARM 19 was included in the analyses and the recoveries were within the certified values. The samples were analysed in triplicate (Appendix D) and the average concentrations from these runs are included in Table 4.8.

Table 4.8 shows the concentration of Cr, Mn, Co, and Mo for the selected coal samples together with the Clarke values for hard coals, which are taken as global averages (Ketris and Yudovich, 2009). Sample WZ9 and WZ7 have high Co and Mo concentrations compared to the global averages. Sample WZ5 has a Co value that is comparable to the global average and a lower Mo concentration. Samples from the Soutpansberg Coalfield are enriched in Cr, Mn, Co, and Mo compared to the global averages except for sample FMU which has low Cr (4.7 ppm) and Mo (1.5 ppm) concentrations (Table 4.8).

The Witbank and Highveld coals are enriched in Cr compared to global hard coals, which have an average Cr concentration of 17.0 ppm (Ketris and Yudovich, 2009). Coal samples from the Witbank Coalfield have low Mn concentrations (average 41.8

ppm) compared to the Highveld coals (average 184.2 ppm) and the global hard coals (71.0 ppm). The Witbank coals have low Co while the Highveld are comparable to the Clarke values for global hard coals.

Table 4.8: Chromium, Mn, Co, and Mo concentrations in the selected coal samples, together with the SARM 19 certified values and Clakes values (ppm). Sample Cr Mn Co Mo SARM 19 Certified values 50.0 157.0 5.6 2.0 Obtained values 47.5 150.3 6.4 1.9 WZ9 22.6 207.5 9.7 2.9 Waterberg WZ7 28.3 144.8 8.0 2.1 WZ5 23.1 138.9 5.4 0.6 FBU 21.9 145.6 11.9 7.6 FMU 4.7 175.4 13.6 1.5 Soutpansberg SBU 43.0 87.9 23.1 17.0 SMU 25.9 238.9 12.0 4.2 S2T 23.7 57.2 3.8 0.7 S2M 37.3 9.5 2.6 0.9 S2B 25.0 4.1 7.1 0.2 Witbank S4T 29.2 75.7 2.4 0.3 S4M 29.2 6.3 1.6 0.2 S4B 30.4 3.5 3.7 0.4 H1A 37.1 182.2 5.1 0.9 Highveld H2B 33.3 186.1 5.3 1.0 N1A 1.8 52.7 8.8 0.8 N1B 2.4 36.8 3.8 0.3 N1C 1.2 244.8 7.8 2.4 Nongoma N2A 1.8 94.4 13.2 1.0 N2B 2.4 48.3 3.7 0.5 N2C 2.5 80.4 12.2 0.8 Global averages Clarke values 17.0 71.0 6.0 2.1 for hard coals

The Nongoma Coalfield samples have low Cr concentrations (average 1.99 ppm) compared to the other four coalfields in this study and the global average of 17.0 ppm. They have variable Mn concentrations with an average of 92.9 ppm, which is higher than the global average for coal. The Nongoma coal samples also have high Co concentrations (8.24 ppm) and low Mo concentrations (0.97 ppm). The reason for the relative enrichment and depletion in the selected coals will be discussed in Chapter 5.

4.7 Mode of occurrence From Table 4.8, samples with high Cr concentrations were selected for density fractionation and sequential leaching in order to determine the mode of occurrence. The different density fractions obtained were subjected to petrography, proximate, XRD, and ICP-MS analyses.

4.7.1 Density fractionation The six samples with the highest Cr values were subjected to density fractionation and this resulted in 24 samples, and the percentage yields per density fraction are reported in Table 4.9.

Table 4.9: Percentage yields of the samples after density fractionation.

%Yield Density WZ7 SBU S2M S4M S4B H1A F1.40 15.6 38.3 8.3 8 1.3 10.7 F1.70 19.4 26.9 59.8 52.5 70.4 50 F1.90 10.6 9.3 19.5 28.7 19.9 15.7 S1.90 54.4 25.5 12.4 10.7 8.4 23.6

4.7.1.1 Petrography The light fractions are rich in vitrinite (sample WZ7-F1.4, S2M-F1.4, S4B-F1.4, H1A- F1.4) and inertinite (sample SBU-F1.4 and S4M-F1.7) as shown in Table 4.10. The heavy fraction samples are rich in mineral matter, dominated by quartz (sample WZ7-S1.9, S4M-S1.9, and H1A-S1.9), and clays (sample SBU-S1.9, S2M-S1.9, and S4B-S1.9). Sample S2M-S1.9 and S4B-S1.9 contain more pyrite and sample SBU- S1.9 contains more carbonates compared to the other coal samples (Table 4.10).

Table 4.10: Maceral groups and mineral matter of the density fractionated samples, with the parent samples indicated in bold (vol %).

Maceral group (vol %) Mineral matter (vol %) Sample Vitrinite Liptinite Inertinite Total Clays Quartz Pyrite Carbonates Others Total WZ7-P 19.3 7.4 15.8 42.5 31.7 19.2 3.0 1.4 2.2 57.5 WZ7-F1.4 83.2 5.3 6.5 95.0 2.2 2.0 0.0 0.0 0.8 5.0 WZ7-F1.7 49.0 7.4 23.3 79.7 7.5 10.4 0.6 0.6 1.2 20.3 WZ7-F1.9 16.1 6.0 22.5 44.6 7.8 39.8 1.4 1.2 5.2 55.4 WZ7-S1.4 3.7 6.3 8.3 18.3 17.5 58.3 3.1 1.2 1.6 81.7 SBU-P 64.8 0.2 10.5 75.5 18.6 0.0 1.8 4.1 0.0 24.5 SBU- F1.4 10.2 3.6 70.2 84.0 2.8 9.8 0.0 3.4 0.0 16.0 SBU- F1.7 35.6 0.3 44.0 79.9 5.4 9.2 0.0 5.5 0.0 20.1 SBU- F1.9 10.0 0.2 39.4 49.6 15.8 26.0 0.0 8.6 0.0 50.4 SBU- S1.4 3.3 0.0 16.1 19.4 58.4 9.1 2.5 10.4 0.2 80.6 S2M- P 13.4 2.2 71.0 86.6 8.4 1.0 0.0 4.0 0.0 13.4 S2M- F1.4 73.7 2.8 21.5 98.0 1.4 0.0 0.2 0.4 0.0 2.0 S2M- F1.7 81.5 0.4 16.7 98.6 0.0 1.0 0.2 0.2 0.0 1.4 S2M- F1.9 2.4 1.2 63.3 66.9 11.7 14.5 0.6 6.3 0.0 33.1 S2M- S1.4 2.8 0.2 16.9 19.9 39.4 12.1 19.1 9.3 0.2 80.1 S4M-P 4.8 2.8 83.2 90.8 7.6 1.2 0.4 0.0 0.0 9.2 S4M- F1.4 30.9 7.6 57.3 95.8 1.2 2.0 0.2 0.8 0.0 4.2 S4M- F1.7 3.8 3.0 74.0 80.8 4.0 14.8 0.0 0.4 0.0 19.2 S4M- F1.9 2.6 1.0 65.5 69.1 7.7 22.8 0.4 0.0 0.0 30.9 S4M- S1.4 2.2 0.2 27.8 30.2 15.4 52.6 1.8 0.0 0.0 69.8 S4B-P 20.0 2.1 69.5 91.6 5.9 0.2 2.3 0.0 0.0 8.4 S4B- F1.4 71.6 7.0 19.6 98.2 1.4 0.2 0.2 0.0 0.0 1.8 S4B- F1.7 5.7 4.3 76.9 86.9 0.8 9.3 0.0 3.0 0.0 13.1 S4B- F1.9 2.0 1.0 63.8 66.8 9.8 18.5 0.0 4.9 0.0 33.2 S4B- S1.4 2.6 0.6 19.1 22.3 29.7 16.3 26.3 5.4 0.0 77.7 H1A-P 12.2 4.0 47.2 63.4 9.3 19.2 4.5 3.6 0.0 36.6 H1A- F1.4 72.0 3.6 23.4 99.0 1.0 0.0 0.0 0.0 0.0 1.0 H1A- F1.7 13.2 9.4 63.6 86.2 0.8 11.4 0.0 1.6 0.0 13.8 H1A- F1.9 4.0 2.6 46.1 52.7 3.2 37.9 3.6 2.4 0.2 47.3 H1A- S1.4 2.0 0.4 11.4 13.8 9.0 64.2 7.4 5.2 0.4 86.2

4.7.1.2 Proximate analysis For all the samples, the lighter fractions have a high VM and FC, and low ash content (Table 4.11). The heavy fractions are mineral-rich have, and have a low VM and FC, and high ash content (Table 4.11). The VM, FC, and ash contents are relatively high on dry basis.

Table 4.11: Proximate analysis of the density separated samples with the parent samples indicated in bold, and reported as received and on a dry basis (wt. %).

As received Dry basis Sample Moisture VM FC Ash VM FC Ash WZ7-P 1.1 17.8 28.4 52.8 18.0 28.7 53.4 WZ7-F1.4 3.4 31.7 48.6 16.4 32.8 50.3 17.0 WZ7-F1.7 2.6 28.5 44.3 24.6 29.3 45.5 25.3 WZ7-F1.9 2.0 20.8 13.1 64.1 21.2 13.4 65.4 WZ7-S1.4 1.6 16.0 7.4 75.0 16.3 7.5 76.2 SBU-P 0.5 19.7 45.7 34.1 19.8 45.9 34.3 SBU- F1.4 1.4 26.3 66.1 6.2 26.7 67.0 6.3 SBU- F1.7 1.4 22.2 54.0 22.4 22.5 54.8 22.7 SBU- F1.9 1.5 18.4 17.4 62.7 18.7 17.7 63.7 SBU- S1.4 1.3 13.5 8.3 77.0 13.7 8.4 78.0 S2M- P 2.9 17.1 54.2 25.8 17.6 55.8 26.6 S2M- F1.4 4.7 33.3 55.3 6.7 34.9 58.0 7.0 S2M- F1.7 4.3 20.4 54.2 21.2 21.3 56.6 22.2 S2M- F1.9 2.1 17.2 33.4 47.3 17.6 34.1 48.3 S2M- S1.4 2.1 15.5 9.9 72.5 15.8 10.1 74.1 S4M-P 3.8 17.9 58.5 19.8 18.6 60.8 20.6 S4M- F1.4 5.2 24.5 61.6 8.7 25.8 65.0 9.2 S4M- F1.7 4.5 16.7 57.0 21.9 17.5 59.7 22.9 S4M- F1.9 3.3 18.3 38.1 40.3 18.9 39.4 41.7 S4M- S1.4 3.1 17.8 35.4 43.7 18.4 36.5 45.1 S4B-P 5.2 22.2 63.2 9.4 23.4 66.7 9.9 S4B- F1.4 4.7 30.4 64.2 0.7 31.9 67.4 0.7 S4B- F1.7 4.5 18.7 58.9 17.9 19.6 61.7 18.7 S4B- F1.9 4.3 17.6 26.1 52.0 18.4 27.3 54.3 S4B- S1.4 2.7 19.7 23.0 54.6 20.2 23.6 56.1 H1A-P 5.0 20.6 43.6 30.8 21.7 45.9 32.4 H1A- F1.4 5.2 29.6 64.6 0.6 31.2 68.1 0.6 H1A- F1.7 4.2 20.8 61.1 13.9 21.7 63.8 14.5 H1A- F1.9 4.0 20.0 34.0 42.0 20.8 35.4 43.8 H1A- S1.4 2.9 17.4 10.4 69.2 17.9 10.7 71.3

4.7.1.3 Trace elements The 24 density separated samples were digested and analysed using ICP-MS for trace element concentrations. Table 4.12 provides the concentrations of the four selected trace elements in the samples along with the parent samples (identified with the suffice–P) for comparison purposes. Organic-rich samples (lighter fractions) generally contain high Cr concentrations compared to the mineral-rich (heavy fractions) and these are also higher than the parent samples (Table 4.12).

Table 4.12: Chromium, Mn, Co, and Mo concentrations in the selected coal samples with the parent samples indicated in bold (ppm).

Sample Cr Mn Co Mo Ash WZ7-P 28.3 144.8 8.0 2.1 53.4 WZ7-F1.4 45.4 59.7 14.1 6.5 17.0 WZ7-F1.7 32.5 179.8 15.6 2.9 25.3 WZ7-F1.9 26.6 179.5 8.5 1.2 65.4 WZ7-S1.4 12.6 211.7 6.9 1.2 76.2 SBU-P 43.0 87.9 23.1 17.0 31.4 SBU- F1.4 65.2 21.1 22.4 27.4 6.3 SBU- F1.7 40.3 114.1 57.2 18.4 22.7 SBU- F1.9 33.6 124.0 16.6 7.0 63.7 SBU- S1.4 25.4 238.4 12.2 5.6 78.0 S2M- P 37.3 113.9 2.6 0.9 27.4 S2M- F1.4 67.0 41.4 11.9 1.8 9.2 S2M- F1.7 36.2 105.4 2.7 0.3 22.9 S2M- F1.9 31.2 122.5 5.6 0.5 41.7 S2M- S1.4 29.0 192.3 1.5 0.3 45.1 S4M-P 29.2 6.3 1.6 0.2 20.6 S4M- F1.4 40.7 8.5 2.8 2.6 7.0 S4M- F1.7 36.5 12.6 1.8 0.1 22.2 S4M- F1.9 22.7 15.1 1.6 0.2 48.3 S4M- S1.4 14.6 23.5 5.3 2.3 74.1 S4B-P 30.4 3.5 3.7 0.4 9.9 S4B- F1.4 21.4 16.1 9.9 1.3 0.7 S4B- F1.7 29.2 64.3 1.9 0.4 18.7 S4B- F1.9 21.5 93.3 1.0 0.2 54.3 S4B- S1.4 18.7 239.3 8.3 6.2 56.1 H1A-P 37.1 182.2 5.1 0.9 38.7 H1A- F1.4 42.9 42.7 10.1 1.7 0.6 H1A- F1.7 29.5 112.1 3.2 0.3 14.5 H1A- F1.9 37.8 100.8 2.2 0.7 43.8 H1A- S1.4 27.5 165.0 3.3 1.6 71.3 SARM 19 48.6 155.5 7.4 1.9 Clarke values 17.0 71.0 6.0 2.1

Sample SBU-F1.4 and S2M-F1.4 have the highest Cr concentrations in the light fractions. Manganese is generally enriched in the heavy fractions, especially in sample S4B-S1.9, SBU-S1.9, and WZ7-S1.9. Cobalt and Mo seem to be associated with both the light and heavy fractions of the coal (Table 4.12). The mode of occurrence of these elements will be discussed further in Section 5.5.

4.7.1.4 Mineralogy Six samples with high Cr from density separated samples (Table 4.12) were selected for XRD analysis. The XRD diffractograph patterns for these samples are included in Appendix C. The dominated mineral phases in these samples are kaolinite, followed by quartz, except for sample WZ7-F1.4, which is dominated by quartz (Table 4.13). There are trace amounts of calcite, dolomite, siderite, and alunite.

Table 4.13: Mineralogy of the selected coal samples (%).

%Organic Cr Sample Kaolinite Calcite Quartz Dolomite Siderite Alunite %Ash carbon (ppm) WZ7-F1.4 7.3 0.0 9.2 0.2 0.3 0.0 17.0 83.0 45.4 SBU-F1.4 3.4 0.2 2.1 0.3 0.0 0.1 6.3 93.7 65.2 S2M-F1.4 6.2 1.3 1.3 0.3 0.0 0.1 9.2 90.8 67.0 S4M-F1.4 3.7 0.0 2.6 0.0 0.0 0.6 7.0 93.0 40.7 S4B-F1.4 0.6 0.0 0.1 0.0 0.0 0.0 0.7 99.3 21.4 H1A-F1.4 0.5 0.0 0.1 0.0 0.0 0.1 0.6 99.4 42.9

4.7.2 Selective leaching Six samples with high Cr concentrations (WZ7, SBU, S2M, S4M, S4B, and H1A) were selected for selective leaching using four different solvents, as explained in Section 3.3.5.2. The concentration of Cr was converted into percentages to see how much of it was leached by each solvent (Table 4.14).

Table 4.14: Chromium leached by different solvents and the remaining residue (%).

Sample Solvents WZ7 SBU S2M S4M S4B H1A %Cr

CH3COONH4 0.0 0.0 0.0 0.0 0.0 0.0 HCl 7.6 0.1 6.2 36.8 15.3 32.0 HF 41.8 97.4 28.3 36.9 24.2 19.2 HNO3 13.8 0.2 3.0 11.0 6.2 30.1 Residue 36.8 2.4 62.5 15.3 54.3 18.7

There was no ammonium acetate leachable Cr in these samples. Most of the Cr was

leached using HF, followed by HCl and HNO3 in sample WZ7, SBU, S4M, and H1A (Table 4.14). Chromium in sample S2M and S4B was not completely leached and it remained in the residue (Table 4.14). Each element seems to be leached by different acids and this could have implications on the mode of occurrence of Cr which is further discussed in Section 5.5.3.

4.7.3 SEM-EDX From the density separated samples, one sample with a high Cr concentration (S2M- F1.4) was selected for SEM-EDX analysis. The selected sample is organic-rich, enriched in vitrinite. From the observations, macerals contain thin layers of silicates minerals (Fig. 4.5A and Fig. 4.6A). From spot analysis, C and O are present in the macerals (Fig. 4.6C) confirming the organic particles. Aluminium and Si are present in the clay minerals, with some Ba, Ca, K, Mg, Na, P, and Ti (Fig. 4.5B and Fig. 4.5C and D). To further investigate the mode of occurrence of Cr, elemental mapping on macerals and silicate minerals is discussed in Section 5.5.4, see also Appendix E.

Figure 4.5: BSE image showing macerals and minerals within sample S2M-F1.4. Figure 4.5B-D: Spectrum peaks for minerals (B) macerals (C and D).

Figure 4.6: BSE image showing macerals and minerals within sample S2M-F1.4. Figure 4.6B-D: Spectrum peaks for selected particles.

CHAPTER 5: DISCUSSION

The discussion of the results focuses on coal characterization, mineralogy, trace elements, mode of occurrence and correlation coefficients. The possible source of Cr in the selected coal samples is also discussed.

5.1 Coal characterization Petrographically, the Waterberg coals have a high mineral matter content (Fig. 5.1) which is dominated by clays and quartz; this was also noted by Wagner and Tlotleng (2012) for Grootegeluk Formation samples from the Waterberg Coalfield. The Soutpansberg samples are rich in vitrinite (Fig. 5.1), and have potential as coking coals; this was noted by Saad and Pinhiero (2001), Mphaphuli (2017), and Wagner et al. (2018). The Witbank, Highveld, and Nongoma samples are inertinite-rich (Fig. 5.1), in agreement with Falcon (1986) and Pinhiero et al. (1999). The Nongoma coals are characterized as high-rank anthracites while the other coal samples are characterized as medium rank bituminous based on the vitrinite reflectance (ECE- UN, 1998), which is in agreement with Pinhiero et al. (1999).

100 90 80 70

60 50 vol % vol 40 30 20 10 0 Waterberg Soutpansberg Witbank Highveld Nongoma Coalfields

VITRINITE LIPTINITE INERTINITE MINERAL MATTER

Figure 5.1: Petrographic composition of the selected coalfields, calculated from the samples arithmetic mean.

In terms of the proximate data, the Nongoma coals have the highest FC, and low VM and ash contents, which are characteristic of anthracites, in agreement with Jeffrey (2005). The Waterberg ROM coal samples have high ash content, and this can be linked with the high mineral matter observed petrographically. The other three coalfields have variable FC and ash contents. The Nongoma and Waterberg coals have high S content, while the other coalfields have variable S contents.

5.2 Mineralogy The Waterberg coal samples contain high kaolinite and quartz, while the Soutpansberg samples have high quartz and less kaolinite (Fig. 5.2). The coals from the Witbank and Highveld Coalfields contain kaolinite as the dominant mineral. The Nongoma coals are different from the above-mentioned coals and contain higher amounts of illite and chlorite, with some pyrite (Fig. 5.2). Overall clays are the most abundant minerals in these coal samples, in agreement with Falcon (2013) and Wagner et al. (2018).

100

80 OTH 70 PY 60 CHL

% 50 KAO I/S 40 ILL 30 CAR 20 FLD

10 QTZ

0 Waterberg Soutpansberg Witbank Highveld Nongoma Coalfields

Key: QTZ= quartz, FLD= K-feldspar and plagioclase, CAR= calcite, ankerite, dolomite and siderite, ILL= illite and muscovite, I/S= illite/smectite or rectorite, KAO= kaolinite, CHL= chlorite, PY= pyrite, marcasite and sphalerite, OTH= any other unqualified phase(s) not accounted for in the previous categories (goethite, alunite, barite, hematite, anhydrite, bassanite)

Figure 5.2: Mineralogy of the selected coals, determined from the arithmetic means per coalfield.

The dominant clay in the bituminous coal samples is kaolinite, but in the anthracite samples, it is illite. Vassilev and Vassileva (1996) also noted that kaolinite decreases and illite increases as coal rank advances for South African coals, and Snyman and Botha (1993) found a higher concentration of illite in Kwazulu-Natal coals compared to Witbank coals. Hence, the higher illite content, and the presence of chlorite, in the KwaZulu-Natal Nongoma coals is anticipated.

5.3 Major elements The major elements present in the coal samples were determined from prepared coal ash, as oxides. The selected coals are enriched in Si, Al, Fe, Ca, P, and S, and depleted in Na, Mg, and K (Table 5.1). The enrichment of these elements can be linked with the mineralogy determined. Silicates in coals occur mostly in quartz and clay minerals, whilst Al is primarily found in the form of clay minerals (Saikai and Ninomiya, 2011). Vassilev et al. (1997) classified coals with high Al and Si content to be of detrital origin. The presence of CaO and MgO can be associated with calcite and dolomite minerals. The presence of P2O5 suggests a possibility of the occurrence of phosphate minerals such as apatite, especially in the Waterberg coals (Maledi, 2017).

Table 5.1: Average major element oxides in selected South African coals (wt. %), calculated from the arithmetic means of the selected samples.

Major oxides (wt. %) Coalfields SiO2 Al2O3 Fe2O3 TiO2 CaO MgO Na2O K2O MnO P2O5 SO3 Waterberg 52.9 25.4 6.0 1.3 5.6 1.1 0.2 1.2 0.0 2.5 1.4

Soutpansberg 58.6 22.8 5.8 1.2 3.5 1.0 0.1 1.5 0.1 0.4 2.9

Witbank 50.4 23.9 9.6 1.4 5.7 1.9 0.3 1.2 0.1 0.5 3.0

Highveld 52.3 25.8 4.9 1.7 6.3 1.8 0.2 1.0 0.1 0.6 3.9

Nongoma 45.1 25.9 10.1 1.5 5.7 0.5 0.3 1.8 0.0 0.9 2.5

5.4 Trace elements The concentrations of the selected trace elements (Table 4.7) are normalized with the Clarke values for hard coals, which are regarded as global averages (Ketris and Yudovich, 2009), and the concentration coefficients (CC) are reported in Table 5.2. The CC is the ratio between the element concentrations in the selected coal samples and the global hard coals. The CC is used to identify the extent to which certain trace elements are enriched or depleted compared to the global hard coals. This will give an indication as to whether these South African coal samples have higher, lower or comparable concentrations of trace elements relative to global averages for coals.

Table 5.2: Average concentration coefficient (CC).

Concentration (ppm) Concentration coefficient (CC) Coalfields Cr Mn Co Mo Cr Mn Co Mo Waterberg 24.7 163.8 7.7 1.8 1.5 2.3 1.3 0.9 Soutpansberg 23.9 161.9 15.1 7.6 1.4 2.3 2.5 3.8 Witbank 29.1 41.8 3.5 0.4 1.7 0.4 0.6 0.5 Highveld 35.2 184.2 5.2 0.9 2.1 2.6 0.9 0.5 Nongoma 2.0 92.9 8.2 1.0 0.1 1.3 1.4 0.5

For Cr, the Waterberg, Soutpansberg, Witbank, and Highveld coal samples have high CC (Table 5.2) and the Nongoma coal samples have low CC. Hence, the Waterberg, Soutpansberg, Witbank, and the Highveld coals are enriched in Cr compared to the Nongoma coals (Fig. 5.3). The Nongoma coals are depleted in Cr with concentrations ranging from 1.2–2.5 ppm. The Waterberg, Soutpansberg, and the Highveld Coalfields are enriched in Mn compared to the Witbank Coalfield and global hard coals. The Waterberg, Soutpansberg, and Nongoma Coalfields have a high CC of Co (Table 5.2); hence they are enriched in Co (Fig. 5.3) compared to the Witbank coal samples. The Soutpansberg samples report the highest CC for Mo, thus enriched compared to the other coals (Fig. 5.3).

4,5

3,5

2,5 CC

1,5

0,5

0 Waterberg Soutpansberg Witbank Highveld Nongoma Coalfields

Cr Mn Co Mo

Figure 5.3: Select trace element concentrations normalized with the Clarke coal values. Error bars are included to depict the range of values determined for each element determined per coalfield. The orange dotted line indicates the global average and in comparison to the specific coalfields. In Table 1.1, Cr concentrations have been previously reported to be high in South African coals (Cairncross et al., 1990; Faure et al., 1996; Wagner and Hlatshwayo, 2005; Bergh et al., 2011; Wagner and Tlotleng, 2012), and in this study, the Cr concentrations are high, except in the Nongoma coals (Table 5.3). The selected coal samples have high Mn concentrations (Table 5.3), and this is in agreement with Faure et al. (1996), Bergh et al. (2011), and Wagner and Tlotleng (2012). The Co and Mo concentrations for the selected coal samples are generally lower than the ones reported in Table 1.1, except in the Soutpansberg coal samples (Table 5.3).

Table 5.3: Chromium, Mn, Co, and Mo concentrations in various South African coals determined during this study compared to global averages for coal (ppm).

SA SA Av Global Global Clarke TE SA1 SA2 WBK3 WTB4 HV5 WBK6 WTB7 HV8 WTBc STBc WBKc HVc NGMc range1-8 1-8 range9 Av 10 values 11

Cr 12-63 25.0 28.0 64.0 70.5 41.4 43.5 47.7 12.0-64.0 45.4 0.5-60 10.0 17±1 24.7 23.9 29.1 35.2 2.0

Mn 30.0 nd nd 89.0 19.5 135.0 208.0 55.0 19.5-208 89.4 5.0-300 50.0 76±6 163.8 161.1 41.8 184.2 92.9

Co 7.8 3.3-14 7.9 9.0 6.3 11.0 13.1 4.5 3.3-14 8.7 0.5-30 5.0 6±0.2 7.7 15.1 3.5 5.2 8.2

Mo 1-2.7 <1-2.7 nd 3.0 1.2 2.1 2.3 2.6 1.0-3.0 2.1 0.1-10 5.0 2.1±0.1 1.8 7.6 0.4 0.9 1.0

TE: trace element, nd: not determined, SA1: South African coals (Watling and Watling, 1982), SA2: South African coals (Willis, 1983), WBK3: Witbank Coalfield No. 2 Seam (Cairncross et al., 1990), WTB4: Waterberg Coalfield (Faure et al., 1996), HV5: Highveld Coalfield No. 4 Seam (Wagner and Hlatshwayo, 2005), WBK6: Witbank Coalfield No. 4 Seam (Bergh et al., 2011), WTB7: Waterberg Coalfield (Wagner and Tlotleng, 2012), HV8: Highveld Coalfield No. 4 Seam (Kolker et al., 2017), SA Av1-8: South African average from studies1-8, Global range (Swaine, 1990), Global Av10: Global average (Zhang et al., 2004), Clarke values11: Coal Clarke values for hard coals (Ketris and Yudovich, 2009).

Coalfields considered for this study: -WTBc: Waterberg Coalfield; STBc: Soutpansberg Coalfield; WTBc: Witbank Coalfield No. 4 Seam; HVc: Highveld Coalfield No. 4 Seam; NGMc: Nongoma Coalfield.

5.5 Mode of occurrence The mode of occurrence of Cr was examined using correlation coefficients, density fractionation, and selective leaching. In the literature, the mode of occurrence of Cr in coals is noted as being problematic. Chromium is reported to occur in the organic matter and clay minerals, such as illite as discussed in Section 2.3.4.1.

5.5.1 Correlation coefficients Pearson’s correlation coefficient was used to determine the relationships amongst the selected parameters. Many correlations were tested (Appendix F), and those of Cr, organic matter, ash, and mineralogy, are discussed in order to explain the mode of occurrence of Cr. There is no apparent correlation between Cr and FC (r=-0.61) and also no correlation between Cr and ash (r=0.41) in the parent samples (Fig. 5.4A and 5.4B), suggesting a mixed organic-inorganic affinity. This is supported by Gluskoter et al. (1977), Dai et al. (2008), and Mastalerz and Drobniak (2012).

A positive correlation between Cr and clay minerals was calculated, suggesting that Cr may be associated with clay minerals, in agreement with (Mcdowell, 1983). In this study, there is a negative correlation (r=-0.75) between illite and Cr concentrations in the parent samples (Fig. 5.4C). This implies Cr is not associated with illite, which disagrees with the literature discussed in section 2.3.4.1. There is some correlation (r=0.69) between kaolinite and Cr, which implies Cr could be associated with kaolinite in the parent samples (Fig. 5.4D).

50,0 50 A 45,0 B 45 40,0 40 35,0 35

30,0 30 25,0 25

20,0 Cr (ppm) 20 Cr (ppm) 15,0 15 10,0 10 5,0 5 0,0 0 0 20 40 60 80 100 0 20 40 60 80 FC (wt.%) Ash

50 50 R² = 0,4732 C 45 R² = 0,5627 D 45 40 40 35 35

30 30 25 25

Cr (ppm) 20 Cr (ppm) 20 15 15 10 10 5 5 0 0 0 5 10 15 0 10 20 30 40 50 Illite Kaolinite

Figure 5.4: Correlation of Cr in parent coal samples. A: fixed carbon (FC) and Cr; B: Ash and Cr; C: illite and Cr; D: kaolinite and Cr concentrations.

5.5.2 Density fractionation Six samples with high Cr concentrations were selected for density fractionation and separated into 24 coal samples based on density, and they were used to investigate the mode of occurrence. From the density fractionated samples, there is a weak positive correlation (r=0.63) between the organic matter and Cr concentration (Fig. 5.5A). This implies Cr is associated with the organic matter in these coal samples. There is a negative correlation (r=-0.65) between the Cr concentrations and the ash content (Fig. 5.5B), which implies Cr is not associated with the mineral matter in the density separated samples. From the parent samples, an association of Cr with kaolinite was determined. For the density fractionated samples, there is no correlation between Cr and kaolinite (Fig. 5.6), but this needs to be investigated further since a limited number of samples were selected due to cost implications.

80 R² = 0,4019 A 70

Cr (ppm) 30

0 0 10 20 30 40 50 60 70 80 FC (wt.%)

B 70 60 R² = 0,4167

30 Cr (ppm) 20

0 0 10 20 30 40 50 60 70 80 Ash

Figure 5.5: Correlation between FC, ash, and Cr concentrations in density fractionated samples. A: FC and Cr; B: ash content and Cr concentrations

Cr (ppm) 30

0 0 2 4 6 8 Kaolinite

Figure 5.6: Correlation between kaolinite and Cr concentrations in density fractionated samples.

Sample SBU-F1.4 and S2M-F1.4 have the highest Cr concentrations (65.2 and 67.0 ppm respectively) (Fig 5.7) compared to the parent samples and the other 22 density separated samples (Table 4.11). High Cr concentrations were observed in the lighter fractions (Float 1.4). Chromium could be associated with the organic matter and this was also noted by Wagner and Tlotleng (2012) in the Waterberg Coalfield and Kolker et al. (2017) in the Highveld Coalfield.

40 Cr (ppm)

0 Parent F1.4 F1.7 F1.9 S1.9

WZ7 SBU S2M S4M S4B H1A

Figure 5.7: Chromium concentration in different density separated coal samples.

5.5.3 Selective leaching Six samples with high Cr concentrations were selected for selective leaching using four different solvents, as explained in Section 3.3.5.2. The total concentration leached from the selected samples was used to determine the percentage leached of each element. The calculated percentages were then used to quantify the modes of occurrence. The proportions of the element leached by each solvent and the residue are presented in Figure 5.8.

100%

90%

80%

70% Residue 60% HNO3 50% HF 40% HCl

30% CH3COONH4

20%

10%

0% WZ7 SBU S2M S4M S4B H1A

Figure 5.8: Chromium leached by different solvents and Cr remaining in the residue after sequential leaching (%).

Figure 5.8 shows the percentage of leachable Cr with the different solvents used.

There was no CH3COONH4 leachable Cr in the selected coal samples. Hydrochloric acid leached an average of 16.3% of the Cr in the different samples, which may have a minor carbonate or mono-sulfide affinity. Hydrofluoric acid leached an average of

41.3% of the Cr in these samples, and HNO3 leached about 10.3% of Cr in the different samples. The HF leachable Cr is certainly associated with silicate minerals, while the HNO3 leachable Cr could have been associated with sulphide minerals; this was noted by Querol et al. (2000) in the Illinois No. 6 coals. Chromium that was not leached could be in insoluble phases such as spinels or enclosed in the organic matrix, accounting for 31.7% of Cr remaining in these coal samples. The results of this study are in agreement with Huggins et al. (2000) and Finkelman et al. (2018). Chromium is most likely found in aluminosilicates minerals and organic matter, and this is supported by Davidson (2000) and others as discussed in Section 2.3.4.1.

5.5.4 SEM-EDX To further investigate the mode of occurrence of Cr, elemental mapping was used. Chromium does not appear to concentrate around the macerals (Fig. 5.9A) or silicate minerals (Fig. 5.9C) across the sample; see also Appendix E. Background counts were observed for Cr (Fig. 5.9B and D), and Cr is finely disseminated in the organic matter and clay minerals. SEM could not confirm the mode of occurrence of Cr since Cr was below the detection limit.

Figure 5.9: An example of BSE images and elemental mapping of Cr in sample S2M-F1.4. Figure 5.9B shows the elemental map of Cr for Figure 5.9A and Figure 5.9D shows the elemental map of Cr for Figure 5.9C.

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5.6 Why do South African coals contain high Cr? Possible source It is evident that the South African coals report high Cr values. In the literature, coals that have high Cr concentrations are often found near ultramafic deposits that contain abundant chrome-bearing minerals such as spinels. Such occurrences have been documented in Kosovo, Serbia (Ruppert et al., 1996), Washington, USA (Brownfield et al., 1995), and Indonesia (Finkelman, 1993). High Cr contents observed in the coal samples of the Kosovo Basin are related to Cr-bearing minerals such as spinels, chromite, and olivines, which were transported into the Kosovo paleoswamp by rivers that drained the serpentinized ultramafic bodies (Ruppert et al., 1996).

In a study by Brownfield et al. (1995) on coal samples from north-western Washington, it was also noted that detrital grains of Cr-bearing minerals contribute to the total Cr content. However, no chrome-bearing minerals were observed in the selected South African coal samples. Chromium concentrations in these South African coal samples are much lower (10’s of ppm) than those of coals formed nearby an ultramafic source (100’s of ppm Cr) (Huggins et al., 2000).

The Bushveld Complex in South Africa (Fig. 5.10) contains the largest reserve of Cr in the world (Naldrett, 2004). The Waterberg, Soutpansberg, Witbank, and Highveld Coalfields located close to the Bushveld Complex (Fig. 5.10) are enriched in Cr. The Nongoma Coalfield (Fig. 5.10) located further away from the Bushveld Complex is depleted in Cr. Chromium incorporated in South African coals could have originated from the Bushveld Complex. This is based on the present-day proximity of the Bushveld Complex to the coalfields.

The Bushveld Complex (2.05 billion years old) predates the Karoo Supergroup (280 to 190 million years old), and could have been exposed during the Permian and contributed to the sediments hosting the coal beds. Sediments derived from pre- Karoo hills were deposited into the low lying areas. For example, in the vicinity of the Witbank Coalfield, palaeocurrent directions and lithology have clearly indicated that pre-Karoo rocks located in the north have shed significant amounts of sediments into the MKB (Ryan and Whitfield, 1978). The fact that Nongoma coals have much lower Cr values and their location is further away is consistent with the model of a Bushveld sediment source. Chromium was subsequently dissolved from primary

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sources, such as chromite, and redistributed during coal formation as previously suggested by Wagner and Hlatswayo (2005).

Figure 5.10: A geological map showing the location of the Bushveld Complex and the selected coalfields in this study, (modified after Vorster, 2003 and Hancox and Götz, 2014).

Chromium is absorbed from the soil and transported into the plants. This could have been the case with the plants that formed coal in Southern Africa. As these plants decayed and formed peat, Cr could have remained in the decaying plants, and incorporated into macerals during degradation. Hence, an association of Cr with the macerals, but this aspect needs to be investigated further.

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CHAPTER 6: CONCLUSION

The main aim of this study was to investigate the occurrence of Cr in a variety of South African coals from different coalfields, and to determine the possible sources of Cr. Five coalfields were considered and the petrography, mineralogy, geochemistry, and mode occurrence were investigated. The aim and objectives of this study were achieved. The research questions posed are answered below and the conclusions made. Recommendations for future studies are subsequently discussed.

6.1 Summary and conclusion

1. The ROM Waterberg coals reported high petrographically observable mineral matter contents, dominated by clays and quartz. The Soutpansberg samples are vitrinite-rich, and the Witbank, Highveld, and Nongoma samples are inertinite-rich. The Waterberg, Soutpansberg, Witbank, and Highveld coal samples are characterized as medium rank bituminous and the Nongoma coal samples are characterized as high-rank anthracites based on the mean random vitrinite reflectance (Table 4.1). The Nongoma coal samples have the highest FC, and lowest VM and ash contents. The Waterberg coal samples have high ash content, and the other three coalfields have a variable VM, FC, and ash contents. The Nongoma and Waterberg coal samples are high in S, while the other coal samples have a low S content (Table 4.2). 2. The Waterberg coal samples contain kaolinite and quartz, while the Soutpansberg coals are dominated by quartz. The dominant mineral in the Witbank and Highveld Coalfield samples is kaolinite. The Nongoma coal samples differ and contain illite and chlorite, with some pyrite (Table 4.3). The presence of chlorite can be linked with the high rank of these coals. 3. During sample preparation, possible contamination from the mill was addressed. Milled samples and pulverized samples were compared. The samples that were milled have higher Cr values, indicating possible contamination from the mill. As a result, samples that were assessed were pulverized using a mortar and pestle (Section 3.2). It is possible that not all

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sources of Cr contamination were eliminated but all samples were treated in the same manner once received from the mine. 4. Microwave-assisted digestion was used to digest the coal samples in two stages, as discussed in Section 3.3.4. ICP-OES was unable to detect the trace element concentrations in the digested solutions. As a result, ICP-MS was used to analyse the trace element concentrations. For the verification of the results, certified reference material SARM 19 was included in the analyses and the recoveries were within the certified value. 5. The Waterberg, Soutpansberg, Witbank, and Highveld Coalfields are enriched in Cr compared to the Nongoma coals and global hard coals. The Waterberg, Soutpansberg, and Highveld Coalfields are enriched in Mn compared to global hard coals. The Waterberg, Soutpansberg, and Nongoma Coalfields are enriched in Co compared to the Witbank Coalfield. The Soutpansberg Coalfield is enriched in Mo compared to the other coalfields and the global hard coals (Table 5.3). 6. The mode of occurrence of Cr was determined using correlation coefficients, selective leaching, and density fractionation. A positive correlation between Cr and kaolinite exists, which means Cr could be hosted by kaolinite. From selective leaching, most of the Cr was leached using HF, indicating an association of Cr with clay minerals, specifically kaolinite (dominant clay form) in these coal samples. Some of the Cr was not leached and remained in the residue, indicating a possible association with the organic matter. High Cr concentrations were observed in the lighter fractions of the density separated samples, indicating an association of Cr with the organic matter; which is in agreement with literature. 7. South African coals are comparatively high in Cr compared to global hard coals, and the enrichment of Cr could be due to sediment source. The Waterberg, Soutpansberg, Witbank, and Highveld Coalfields located close to the Bushveld Complex have high Cr concentrations. The Nongoma Coalfield samples located further away from the Bushveld Complex has far lower Cr concentrations, and this could also be due to the absence of chrome-bearing minerals in these coal samples. The Bushveld Complex thus could have provided the sediments included into the peat swamps.

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The mode of occurrence of Cr remains inconclusive. This is in agreement with literature as discussed. A mixed association was determined; Cr is associated with both the organic and inorganic matter in these selected coal samples as shown by different techniques. In conclusion, it is likely that the Cr in these selected coal samples (apart from the Nongoma coal samples) originated from the Bushveld Complex, and was dissolved and re-distributed during coal formation, as suggested by Wagner and Hlatswayo (2005).

6.2 Recommendations From the results obtained in this study, the following recommendations are provided.

 High Cr concentrations are of concern if Cr is present as Cr6+ which is a carcinogen. One of the initial objectives for this study was to determine the Cr speciation using XAFS. Unfortunately, for this study XAFS was not available as the application for beam time at ESRF was unsuccessful. For future studies, XAFS should be used in order to identify the Cr speciation in South African coals. This will help understand if the high Cr in South African coals poses any health or environmental effects.  Due to time and financial constraints, limited samples were selected and used to investigate the mode of occurrence of Cr. The results obtained for the mode of occurrence of Cr are conflicting, although in agreement with literature, and imply multiple modes. There is still more work to be done to determine the definite mode of occurrence of Cr.  This study tried to fill the gap on the limited data that is available on trace elements in South African coals, and to also set an example future studies. The trace elements data pool for South African coals still remains limited as a limited number of samples were used. So there is still a lot of work that needs to be done in order to understand the origin and mode of occurrence of trace elements, as well as determining their hazard rating.

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APPENDIX A In this section, TGA graphs that show proximate results for the different samples are provided.

Figure A-1: Proximate results for sample WZ9.

Figure A-2: Proximate results for sample WZ7.

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Figure A-3: Proximate results for sample WZ5.

Figure A-4: Proximate results for sample FBU.

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Figure A-5: Proximate results for sample FMU.

Figure A-6: Proximate results for sample SBU.

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Figure A-7: Proximate results for sample SMU.

Figure A-8: Proximate results for sample S2T.

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Figure A-9: Proximate results for sample S2M.

Figure A-10: Proximate results for sample S2B.

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Figure A-11: Proximate results for sample S4T.

Figure A-12: Proximate results for sample S4M.

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Figure A-13: Proximate results for sample S4B.

Figure A-14: Proximate results for sample H1A.

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Figure A-15: Proximate results for sample H2B.

Figure A-16: Proximate results for sample N1A

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Figure A-17: Proximate results for sample N1B.

Figure A-18: Proximate results for sample N1C.

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Figure A-19: Proximate results for sample N2A.

Figure A-19: Proximate results for sample N2B.

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Figure A-19: Proximate results for sample N2C.

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APPENDIX B The application to use XAFS at ESRF for this study was not successful and below is the decision letter from the ESRF.

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APPENDIX C The XRD diffractograph patterns for the selected coal samples after LTA are shown in this appendix.

10000 9000 8000

7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-1: XRD diffractograph pattern for sample WZ9.

10000 9000 8000

7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-2: XRD diffractograph pattern for sample WZ7.

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10000 9000 8000 7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-3: XRD diffractograph pattern for sample WZ5.

10000 9000 8000

7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 degrees Two-Theta, Cu radiation

Figure B-4: XRD diffractograph pattern for sample FBU.

139

10000 9000 8000

7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-5: XRD diffractograph pattern for sample FMU.

10000 9000 8000

7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-6: XRD diffractograph pattern for sample SBU.

140

10000 9000 8000

7000 6000 5000 4000 3000 Intensity, counts 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-7: XRD diffractograph pattern for sample SMU.

10000 9000 8000

7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-8: XRD diffractograph pattern for sample S2T.

141

10000 9000 8000

7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-9: XRD diffractograph pattern for sample S2M.

10000 9000 8000

7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees two-Theta, Cu radiation

Figure B-10: XRD diffractograph pattern for sample S2B.

142

10000 9000 8000

7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-11: XRD diffractograph pattern for sample S4T.

10000 9000 8000

7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-12: XRD diffractograph pattern for sample S4M.

143

10000 9000

8000 7000 6000 5000

Intensity, counts, 4000 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-13: XRD diffractograph pattern for sample S4B.

10000 9000 8000

7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-14: XRD diffractograph pattern for sample H1A.

144

10000 9000 8000

7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-15: XRD diffractograph pattern for sample H2B.

10000 9000 8000

7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-16: XRD diffractograph pattern for sample N1A.

145

10000 9000 8000

7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-17: XRD diffractograph pattern for sample N1B.

10000 9000 8000

7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-18: XRD diffractograph pattern for sample N1C.

146

10000 9000 8000

7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-9: XRD diffractograph pattern for sample N2B.

10000 9000 8000 7000 6000 5000 4000

Intensity, counts 3000 2000 1000 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Degrees Two-Theta, Cu radiation

Figure B-20: XRD diffractograph pattern for sample N2C.

147

Counts 6000 UJ_1 4000 2000 0 UJ_5

5000

0 UJ_9

4000

0 UJ_13

5000

0 10000 UJ_17

5000

0 UJ_21 4000

0 10 20 30 40 50 60 70 80 90 Position [°2θ] (Cobalt (Co))

Peak List Kaolinite 1A; H4 Al2 O9 Si2 Calcite; C1 Ca1 O3 Quartz; O2 Si1 Dolomite; C2 Ca1 Mg1 O6 Alunite; H6 Al3 K0.72 Na0.28 O14 S2 Siderite; C1 Fe1 O3

Figure B-21: XRD diffractograph pattern for samples WZ7-F1.4, SBU-F1.4, S2M-F1.4, S4M-F1.4, S4B-F1.4, H1A-F1.4

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APPENDIX D The selected coal samples were analysed in triplicate using ICP-MS and the individual runs for each element are presented in the tables below.

Table C-1: Chromium concentrations for the selected samples. Standard Coalfields Sample Cr Average Deviation WB3 24,11 24,63 19,13 22,62 3,035 Waterberg WB4 27,94 30,04 26,81 28,26 1,637 WB5 22,01 24,92 22,47 23,13 1,562 FBU 20,85 25,04 19,69 21,86 2,812 FMU 5,09 5,25 3,78 4,71 0,809 Soutpansberg SBU 46,58 43,79 38,65 43,01 4,024 SMU 27,59 27,88 22,35 25,94 3,112 S2T 25,59 19,00 26,50 23,70 4,095 S2M 37,43 40,77 33,56 37,25 3,610 S2B 26,05 27,06 22,02 25,04 2,663 Witbank S4T 25,68 30,49 31,55 29,24 3,129 S4M 27,82 31,23 28,52 29,19 1,804 S4B 23,33 26,63 41,33 30,43 9,578 H1A 39,70 40,71 30,90 37,10 5,393 Highveld H2B 36,74 30,38 32,90 33,34 3,200 N1A 2,75 1,10 1,60 1,82 0,847 N1B 2,28 2,88 1,99 2,38 0,456 N1C 1,15 1,15 1,21 1,17 0,034 Nongoma N2A 1,76 1,82 1,69 1,76 0,065 N2B 2,74 2,02 2,37 2,38 0,360 N2C 2,46 2,43 2,49 2,46 0,030

Table C-2: Manganese concentrations for the selected samples. Standard Coalfields Sample Mn Average Deviation WB3 208,31 203,60 210,58 207,50 3,564 Waterberg WB4 150,79 138,38 145,32 144,83 6,219 WB5 129,98 139,95 146,85 138,93 8,485 FBU 137,19 157,95 141,52 145,55 10,955 FMU 164,75 185,38 175,95 175,36 10,328 Soutpansberg SBU 88,14 89,16 86,47 87,92 1,360 SMU 238,22 247,53 231,02 238,93 8,277 S2T 38,04 26,43 37,49 33,99 6,552 S2M 110,76 118,58 112,42 113,92 4,118 S2B 16,28 17,25 18,25 17,26 0,988 Witbank S4T 66,13 79,73 81,35 75,74 8,362 S4M 6,27 6,61 5,94 6,27 0,334 S4B 2,98 2,89 4,54 3,47 0,929 H1A 174,14 188,25 184,30 182,23 7,280 Highveld H2B 189,96 191,96 176,36 186,10 8,489 N1A 55,51 43,41 59,21 52,71 8,263 N1B 31,51 44,99 34,03 36,84 7,169 N1C 251,96 234,82 247,70 244,83 8,924 Nongoma N2A 96,28 93,52 93,38 94,40 1,638 N2B 49,89 47,90 46,96 48,25 1,497 N2C 82,47 75,08 83,56 80,37 4,612

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Table C-3: Cobalt concentrations for the selected samples. Standard Coalfields Sample Co Average Deviation WB3 8,92 9,92 10,11 9,65 0,644 Waterberg WB4 8,40 8,26 7,32 7,99 0,588 WB5 4,69 5,73 5,87 5,43 0,647 FBU 10,61 13,57 11,63 11,94 1,502 FMU 14,23 14,39 12,06 13,56 1,300 Soutpansberg SBU 24,20 21,99 22,97 23,05 1,108 SMU 12,18 12,61 11,30 12,03 0,668 S2T 4,27 2,97 4,06 3,77 0,701 S2M 2,58 2,70 2,43 2,57 0,135 S2B 7,06 7,38 6,86 7,10 0,263 Witbank S4T 2,23 2,45 2,40 2,36 0,119 S4M 1,61 1,69 1,44 1,58 0,126 S4B 2,73 3,24 5,00 3,66 1,186 H1A 4,99 5,26 5,01 5,09 0,149 Highveld H2B 5,38 5,69 4,79 5,29 0,459 N1A 9,23 7,28 9,84 8,79 1,337 N1B 5,69 2,67 3,07 3,81 1,642 N1C 8,10 7,47 7,81 7,80 0,316 Nongoma N2A 12,97 13,42 13,17 13,19 0,222 N2B 4,04 3,43 3,63 3,70 0,311 N2C 12,47 11,48 12,53 12,16 0,592

Table C-4: Molybdenum concentrations for the selected samples. Standard Coalfields Sample Mo Average Deviation WB3 2,78 3,01 2,78 2,86 0,133 Waterberg WB4 2,26 2,20 1,77 2,08 0,264 WB5 0,57 0,60 0,57 0,58 0,015 FBU 6,76 8,92 7,10 7,59 1,2 FMU 1,59 1,66 1,29 1,51 0,2 Soutpansberg SBU 18,40 16,56 16,10 17,02 1,2 SMU 4,43 4,39 3,80 4,20 0,4 S2T 0,83 0,63 0,62 0,69 0,118 S2M 1,06 0,85 0,64 0,85 0,206 S2B 0,23 0,22 0,18 0,21 0,028 Witbank S4T 0,54 0,36 0,11 0,34 0,213 S4M 0,24 0,21 0,17 0,21 0,034 S4B 0,39 0,41 0,42 0,41 0,017 H1A 0,89 1,00 0,91 0,93 0,058 Highveld H2B 0,99 1,01 0,87 0,95 0,076 N1A 1,09 0,76 0,64 0,83 0,233 N1B 0,27 0,38 0,14 0,26 0,118 N1C 2,63 2,29 2,26 2,39 0,207 Nongoma N2A 1,15 1,12 0,77 1,01 0,207 N2B 0,59 0,59 0,32 0,50 0,159 N2C 0,91 0,83 0,68 0,81 0,116

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APPENDIX E Elemental mapping of different particles in sample S2M-F1.4 is shown in this appendix.

Figure D-1: Elemental composition of macerals and clay minerals.

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Figure D-2: Elemental mapping across macerals and clay minerals.

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APPENDIX F

Mineral Volatile FixedC Vitrinite Liptinite Inertinite Matter Moisture Matter arbon ASH QTZ FLD CARB ILLITE IS KAOL CHLR PY OTHR SiO2 Al2O3 Fe2O3 TiO2 CaO MgO Na2O K2O MnO P2O5 SO3 BaO Cr2O3 NiO V2O5 Cr Mn Co Mo Vitrinite Pearson 1 -.495* -.607** -0,064 -.546* -0,240 0,246 -0,097 0,079 -0,127 .469* 0,249 0,178 -.598** -0,026 0,082 0,090 0,035 0,016 0,099 -0,085 -0,205 -0,317 -0,258 0,216 -0,049 -0,113 0,050 0,120 -0,261 -0,261 -0,126 -0,277 .442* .823** .640** Correlatio Liptinite Pearson -.495* 1 -0,280 .733** -0,022 0,406 -.814** .771** 0,238 0,223 -0,281 -0,400 -0,187 .686** -0,348 -0,343 -0,165 0,407 0,029 -.437* -0,105 -0,073 0,095 -0,066 -0,345 0,090 0,009 -0,117 0,013 -0,014 -0,014 -0,093 .465* 0,181 -0,304 -0,201 Correlatio Inertinite Pearson -.607** -0,280 1 -.753** .614** -0,203 .558** -.655** -.447* -0,282 -0,292 0,108 0,066 0,071 0,345 0,247 0,191 -0,395 0,010 0,356 0,231 0,192 0,156 0,295 0,044 0,031 -0,158 0,076 -0,138 0,276 0,276 0,162 -0,147 -.771** -.676** -.566** Correlatio MineralMat Pearson -0,064 .733** -.753** 1 -0,314 .454* -.898** .895** .503* .480* -0,017 -0,335 -0,221 0,389 -0,398 -0,363 -0,330 .453* -0,033 -.526* -0,220 -0,058 0,078 -0,149 -0,223 -0,010 0,322 -0,141 0,071 -0,126 -0,126 -0,091 0,407 .609** 0,177 0,185 ter Correlatio Moisture Pearson -.546* -0,022 .614** -0,314 1 0,190 0,168 -0,426 -0,314 0,062 -0,374 -0,166 -0,001 0,344 0,011 -0,011 0,360 -0,392 -0,154 -0,058 0,165 .673** .697** -0,012 -0,213 0,250 -0,051 0,361 0,170 .781** .781** 0,411 0,220 -0,287 -.561** -0,405 Correlatio VolatileMat Pearson -0,240 0,406 -0,203 .454* 0,190 1 -.624** 0,298 .479* 0,296 0,207 -.781** -0,275 .783** -.608** -.577** -0,265 .453* -0,064 -.544* -0,060 0,012 0,352 -0,165 -0,372 0,059 0,080 -0,040 0,256 0,187 0,187 0,291 .855** 0,086 -0,083 0,199 ter Correlatio FixedCarb Pearson 0,246 -.814** .558** -.898** 0,168 -.624** 1 -.923** -0,423 -.435* 0,004 .507* 0,216 -.655** .517* .520* 0,335 -.433* 0,002 .471* 0,122 0,092 -0,181 0,116 0,285 -0,023 -0,178 0,146 -0,137 0,042 0,042 -0,029 -.613** -0,406 -0,020 -0,135 on Correlatio ASH Pearson -0,097 .771** -.655** .895** -0,426 0,298 -.923** 1 0,347 0,370 -0,021 -0,254 -0,129 0,388 -0,355 -0,361 -0,341 0,383 0,047 -0,317 -0,141 -0,222 -0,043 -0,068 -0,147 -0,040 0,178 -0,213 0,031 -0,247 -0,247 -0,145 0,334 .473* 0,149 0,143 Correlatio QTZ Pearson 0,079 0,238 -.447* .503* -0,314 .479* -0,423 0,347 1 0,174 0,008 -0,024 -0,354 0,284 -.536* -0,322 -0,369 .640** -0,174 -.468* -0,414 -0,151 -0,018 0,048 -0,119 -0,144 0,087 -0,131 -0,117 -0,109 -0,109 0,073 0,264 0,206 0,083 0,142 Correlatio FLD Pearson -0,127 0,223 -0,282 .480* 0,062 0,296 -.435* 0,370 0,174 1 -0,198 -0,156 -0,073 0,175 -0,159 -0,102 -0,068 -0,101 0,002 -0,137 -0,028 0,163 0,094 0,151 0,001 -0,062 .597** -0,049 0,071 -0,073 -0,073 -0,132 0,227 0,376 -0,030 -0,027 Correlatio CARB Pearson .469* -0,281 -0,292 -0,017 -0,374 0,207 0,004 -0,021 0,008 -0,198 1 -0,241 -0,096 -0,108 0,020 -0,139 -0,189 0,118 0,051 -0,022 0,042 -0,242 -0,109 -0,116 0,279 0,113 -0,035 0,070 0,307 -0,160 -0,160 0,142 0,346 0,042 .589** .784** Correlatio ILLITE Pearson 0,249 -0,400 0,108 -0,335 -0,166 -.781** .507* -0,254 -0,024 -0,156 -0,241 1 0,145 -.680** 0,332 .521* 0,211 -0,306 0,006 .462* -0,097 -0,010 -0,264 0,149 0,351 -0,134 -0,120 0,002 -0,309 -0,093 -0,093 -0,175 -.722** -0,001 0,021 -0,154 Correlatio IS Pearson 0,178 -0,187 0,066 -0,221 -0,001 -0,275 0,216 -0,129 -0,354 -0,073 -0,096 0,145 1 -0,299 0,380 .747** -0,140 -0,229 -0,010 0,243 0,136 -0,120 -0,112 0,015 0,182 -0,212 -0,082 -0,223 -0,073 -0,050 -0,050 -0,091 -0,292 -0,074 0,192 -0,084 Correlatio KAOL Pearson -.598** .686** 0,071 0,389 0,344 .783** -.655** 0,388 0,284 0,175 -0,108 -.680** -0,299 1 -.556** -.527* -0,165 0,352 0,031 -.536* 0,032 0,074 0,361 0,007 -0,361 0,117 -0,051 0,025 0,239 0,213 0,213 0,186 .820** -0,146 -0,420 -0,118 Correlatio CHLR Pearson -0,026 -0,348 0,345 -0,398 0,011 -.608** .517* -0,355 -.536* -0,159 0,020 0,332 0,380 -.556** 1 .584** 0,295 -.518* -0,044 .520* 0,207 0,113 -0,195 0,071 0,210 0,058 0,263 0,022 -0,229 -0,109 -0,109 -0,198 -.545* -0,114 -0,136 -0,190 Correlatio PY Pearson 0,082 -0,343 0,247 -0,363 -0,011 -.577** .520* -0,361 -0,322 -0,102 -0,139 .521* .747** -.527* .584** 1 -0,050 -0,338 0,010 0,318 0,029 -0,056 -0,257 0,244 .521* -0,229 0,013 -0,129 -0,236 -0,148 -0,148 -0,205 -.541* -0,176 0,067 -0,201 Correlatio OTHR Pearson 0,090 -0,165 0,191 -0,330 0,360 -0,265 0,335 -0,341 -0,369 -0,068 -0,189 0,211 -0,140 -0,165 0,295 -0,050 1 -.585** -0,229 .434* -0,134 .532* 0,192 0,002 -0,050 .529* -0,037 .625** -0,200 0,294 0,294 0,085 -0,223 0,210 -0,321 -0,157 Correlatio SiO2 Pearson 0,035 0,407 -0,395 .453* -0,392 .453* -.433* 0,383 .640** -0,101 0,118 -0,306 -0,229 0,352 -.518* -0,338 -.585** 1 -0,079 -.584** -0,197 -.525* -0,189 -0,256 -0,342 -0,059 -0,375 -0,227 0,029 -0,340 -0,340 -0,047 0,353 0,146 0,115 0,075 Correlatio Al2O3 Pearson 0,016 0,029 0,010 -0,033 -0,154 -0,064 0,002 0,047 -0,174 0,002 0,051 0,006 -0,010 0,031 -0,044 0,010 -0,229 -0,079 1 -0,236 .732** -.478* -.515* 0,102 0,106 -.737** 0,055 -.592** .617** -0,328 -0,328 -.649** 0,111 -0,255 0,267 0,310 Correlatio Fe2O3 Pearson 0,099 -.437* 0,356 -.526* -0,058 -.544* .471* -0,317 -.468* -0,137 -0,022 .462* 0,243 -.536* .520* 0,318 .434* -.584** -0,236 1 -0,055 -0,006 -0,181 0,133 0,290 0,222 0,026 0,086 -.485* -0,120 -0,120 0,071 -.552** -0,151 -0,196 -0,278 Correlatio TiO2 Pearson -0,085 -0,105 0,231 -0,220 0,165 -0,060 0,122 -0,141 -0,414 -0,028 0,042 -0,097 0,136 0,032 0,207 0,029 -0,134 -0,197 .732** -0,055 1 -0,285 -0,190 -0,257 -0,165 -0,430 0,074 -0,402 .753** -0,111 -0,111 -0,285 0,165 -0,397 0,070 0,195 Correlatio CaO Pearson -0,205 -0,073 0,192 -0,058 .673** 0,012 0,092 -0,222 -0,151 0,163 -0,242 -0,010 -0,120 0,074 0,113 -0,056 .532* -.525* -.478* -0,006 -0,285 1 .810** -0,023 -0,097 .470* 0,289 .620** -0,123 .823** .823** .514* -0,035 0,154 -0,291 -0,185 Correlatio MgO Pearson -0,317 0,095 0,156 0,078 .697** 0,352 -0,181 -0,043 -0,018 0,094 -0,109 -0,264 -0,112 0,361 -0,195 -0,257 0,192 -0,189 -.515* -0,181 -0,190 .810** 1 -0,263 -0,361 .514* 0,038 .559** 0,073 .877** .877** .823** 0,333 0,035 -0,291 -0,072 Correlatio Na2O Pearson -0,258 -0,066 0,295 -0,149 -0,012 -0,165 0,116 -0,068 0,048 0,151 -0,116 0,149 0,015 0,007 0,071 0,244 0,002 -0,256 0,102 0,133 -0,257 -0,023 -0,263 1 .582** -0,164 0,175 0,041 -0,251 -0,193 -0,193 -0,268 -0,095 -0,279 -0,158 -0,202 Correlatio * ** K2O Pearson 0,216 -0,345 0,044 -0,223 -0,213 -0,372 0,285 -0,147 -0,119 0,001 0,279 0,351 0,182 -0,361 0,210 .521 -0,050 -0,342 0,106 0,290 -0,165 -0,097 -0,361 .582 1 -0,256 0,135 -0,079 -0,157 -0,232 -0,232 -0,263 -0,300 -0,101 0,350 0,134 Correlatio MnO Pearson -0,049 0,090 0,031 -0,010 0,250 0,059 -0,023 -0,040 -0,144 -0,062 0,113 -0,134 -0,212 0,117 0,058 -0,229 .529* -0,059 -.737** 0,222 -0,430 .470* .514* -0,164 -0,256 1 -0,207 .858** -0,238 0,295 0,295 .596** 0,177 0,235 -0,338 -0,115 Correlatio P2O5 Pearson -0,113 0,009 -0,158 0,322 -0,051 0,080 -0,178 0,178 0,087 .597** -0,035 -0,120 -0,082 -0,051 0,263 0,013 -0,037 -0,375 0,055 0,026 0,074 0,289 0,038 0,175 0,135 -0,207 1 -0,194 -0,025 -0,015 -0,015 -0,129 -0,037 0,127 0,039 0,062 Correlatio SO3 Pearson 0,050 -0,117 0,076 -0,141 0,361 -0,040 0,146 -0,213 -0,131 -0,049 0,070 0,002 -0,223 0,025 0,022 -0,129 .625** -0,227 -.592** 0,086 -0,402 .620** .559** 0,041 -0,079 .858** -0,194 1 -0,094 0,394 0,394 .574** 0,113 0,195 -0,212 0,038 Correlatio BaO Pearson 0,120 0,013 -0,138 0,071 0,170 0,256 -0,137 0,031 -0,117 0,071 0,307 -0,309 -0,073 0,239 -0,229 -0,236 -0,200 0,029 .617** -.485* .753** -0,123 0,073 -0,251 -0,157 -0,238 -0,025 -0,094 1 0,087 0,087 -0,058 .545* -0,154 0,364 .579** Correlatio Cr2O3 Pearson -0,261 -0,014 0,276 -0,126 .781** 0,187 0,042 -0,247 -0,109 -0,073 -0,160 -0,093 -0,050 0,213 -0,109 -0,148 0,294 -0,340 -0,328 -0,120 -0,111 .823** .877** -0,193 -0,232 0,295 -0,015 0,394 0,087 1 1.000** .639** 0,146 -0,087 -0,243 -0,112 Correlatio NiO Pearson -0,261 -0,014 0,276 -0,126 .781** 0,187 0,042 -0,247 -0,109 -0,073 -0,160 -0,093 -0,050 0,213 -0,109 -0,148 0,294 -0,340 -0,328 -0,120 -0,111 .823** .877** -0,193 -0,232 0,295 -0,015 0,394 0,087 1.000** 1 .639** 0,146 -0,087 -0,243 -0,112 Correlatio V2O5 Pearson -0,126 -0,093 0,162 -0,091 0,411 0,291 -0,029 -0,145 0,073 -0,132 0,142 -0,175 -0,091 0,186 -0,198 -0,205 0,085 -0,047 -.649** 0,071 -0,285 .514* .823** -0,268 -0,263 .596** -0,129 .574** -0,058 .639** .639** 1 0,269 -0,151 -0,195 0,060 Correlatio Cr Pearson -0,277 .465* -0,147 0,407 0,220 .855** -.613** 0,334 0,264 0,227 0,346 -.722** -0,292 .820** -.545* -.541* -0,223 0,353 0,111 -.552** 0,165 -0,035 0,333 -0,095 -0,300 0,177 -0,037 0,113 .545* 0,146 0,146 0,269 1 -0,042 -0,065 0,346 Correlatio Mn Pearson .442* 0,181 -.771** .609** -0,287 0,086 -0,406 .473* 0,206 0,376 0,042 -0,001 -0,074 -0,146 -0,114 -0,176 0,210 0,146 -0,255 -0,151 -0,397 0,154 0,035 -0,279 -0,101 0,235 0,127 0,195 -0,154 -0,087 -0,087 -0,151 -0,042 1 0,347 0,201 Correlatio Co Pearson .823** -0,304 -.676** 0,177 -.561** -0,083 -0,020 0,149 0,083 -0,030 .589** 0,021 0,192 -0,420 -0,136 0,067 -0,321 0,115 0,267 -0,196 0,070 -0,291 -0,291 -0,158 0,350 -0,338 0,039 -0,212 0,364 -0,243 -0,243 -0,195 -0,065 0,347 1 .787** Correlatio Mo Pearson .640** -0,201 -.566** 0,185 -0,405 0,199 -0,135 0,143 0,142 -0,027 .784** -0,154 -0,084 -0,118 -0,190 -0,201 -0,157 0,075 0,310 -0,278 0,195 -0,185 -0,072 -0,202 0,134 -0,115 0,062 0,038 .579** -0,112 -0,112 0,060 0,346 0,201 .787** 1 Correlatio

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