Effect of water quality on coal thermoplasticity

Feng Zhang

A thesis in fulfilment of the requirements for the degree of Master of Philosophy in Engineering

School of Minerals and Energy Resources Engineering Faculty of Engineering

June 2018

II III Acknowledgements

I would like to thank my supervisors Associate Professor Seher Ata and Dr. Ghislain Bournival for their support, guidance and encouragement throughout my research study. I am especially grateful to Seher, who was the supervisor of my undergraduate thesis, for trusting my ability to complete this research degree.

Special thanks to Professor Alan Buckley for his help with my understanding on coal surface chemistry and surface analysis. Also, many thanks to Mr. Noel Lambert, for providing valuable information related to this research topic, as well as preparing and transporting the coal and process water samples.

I appreciate Dr. Bin Gong, Dr. Yin Yao for their help with the XPS analysis and SEM analysis. I would like to thank Australian Coal Association Research Program (ACARP) for funding this research project and providing the coal and process water samples.

Lastly, thanks to my family and friends, especially to my girlfriend, Hongni Yin, for their endless support throughout these two years.

IV Abstract

Process water reuse is a common practice in Australian coal preparation plants. It is an effective solution to on-site water scarcity and to minimize environmental impacts. This study investigates the effect of inorganic mineral salts in the process water on the thermoplastic properties of metallurgical coals after a short period of mild oxidation. Two different Australian metallurgical coals were treated with the process water received from a coal preparation plant based in Queensland, AU, and exposed to air at ambient temperature. Artificial inorganic salt solutions and process water with adjusted salt dilution ratio were also used to treat the coal samples. The selection of the inorganic salt solutions was based on the concentrations of the major ions found in the process water. The inorganic salts used in the study include some sodium salts, e.g. sodium carbonate and sodium sulfate, as well as potassium and magnesium salts. Coal samples were treated with the different solutions following several different procedures respectively. The thermoplastic properties of the samples treated with designated solutions were measured using a Gieseler plastometer, which measures the fluidity of the coal samples. The results suggested that the fluidity of coal decreased as the concentration of inorganic salts in the solution increased. When different salt solutions were used, the degree of fluidity reduction was found to be similar at the same concentration. The effect of inorganic salt on coal thermoplastic behaviours was found to be dependent on the total salt concentration in the solution used in the treatment. Examination of the coal surface with a scanning electron microscope equipped with energy dispersive x-ray spectroscopic probe (SEM-EDS) showed the existence of both sodium and chlorine on the surface of process water treated coal. The surface analysis by X-ray photoelectron spectroscopy (XPS) indicated a relatively insignificant chemical reaction between the inorganic ions present in the water and the coal surface after rinsing the samples. Salts appeared to precipitate on non-maceral matter and act as an inert (non-plastic) additive.

V Table of Contents

Acknowledgements...... I

Abstract ...... V

List of Figures ...... VIII

List of Tables ...... XI

Chapter 1 Introduction ...... 1 1.1 Introduction to coal preparation, coke making and thermoplastic properties ...... 1 1.2 Statement of the problem ...... 4 1.3 Objectives ...... 6 1.4 Thesis structure ...... 8

Chapter 2 Literature review ...... 9 2.1 Introduction ...... 9 2.2 Coal thermoplastic properties ...... 10 2.2.1 Coal molecular transformations during pyrolysis ...... 12 2.2.2 Theories of coal thermoplastic properties ...... 17 2.3 Factors affecting coal thermoplastic properties ...... 21 2.3.1 Coal rank and maceral compositions ...... 22 2.3.2 Inorganic matter ...... 27 2.3.3 Oxidation ...... 33 2.3.4 Cross-linking ...... 39 2.3.5 Coal blend and additives ...... 42 2.3.6 Techniques for coal thermoplasticity examination ...... 44 2.4 Process water chemistry and coal thermoplasticity ...... 52 2.4.1 Process water chemistry ...... 52 2.4.2 Interaction between inorganic substances and coal surface ...... 53 2.5 Literature review summary...... 56

Chapter 3 Methodology ...... 58 3.1 General experimental considerations ...... 58 3.2 Characterisation of coal samples ...... 59 VI 3.3 Characterisation of water samples ...... 61 3.3.1 Process water sample ...... 61 3.3.2 Chemical solutions ...... 67 3.4 Experimental and analytical methods ...... 68 3.4.1 Salt solution soaking ...... 68 3.4.2 Oxidation ...... 68 3.4.3 Inorganic electrolyte removal ...... 70 3.4.4 Ash analysis ...... 71 3.4.5 Surface analysis ...... 73 3.4.6 Coal thermoplastic properties analysis...... 76

Chapter 4 Results and discussions ...... 79

4.1 Effect of Na2CO3 soaking on coal fluidity ...... 79 4.2 Effect of oxidation on coal fluidity ...... 83 4.3 Effect of oxidation on water chemistry ...... 87 4.4 XPS analysis of treated coal surface ...... 91 4.5 SEM-EDS Analysis of Oxidised Coal Surface ...... 96 4.6 Effect of total concentration of inorganic salt on coal fluidity ...... 101 4.7 Mechanism of fluidity reduction in coal preparation plant ...... 103 4.8 Concluding remarks ...... 105

Chapter 5 Conclusions and recommendations ...... 106 5.1 Conclusions ...... 106 5.2 Recommendations ...... 109

References ...... 110

Appendix ...... 125

VII List of Figures Figure 1.1 Illustration of coke degradation mechanisms in a blast furnace (Mousa et al., 2011)...... 2 Figure 2.1. Progressive changes of coal during the softening phase (Coetzee et al., 2014). 11 Figure 2.2. Schematic of mesophase evolution during coal pyrolysis (Fitzgerald, 1956). .. 12 Figure 2.3. Correlation between carbon content (wt.%) in coal with (a) maximum fluidity

(log10 ddpm) and (b) active methylene carbons (wt.%) (Kidena et al., 1996)...... 15 Figure 2.4. Schematic transformation of coal macromolecular structure during the thermoplastic phase (Spiro, 1981)...... 16 Figure 2.5. Coal fluidity as a function of pyrolysis time at different temperatures (Fieldner et al., 1931)...... 18 Figure 2.6. Structural differences in coal with varying rank (Kaneko et al., 2005)...... 23 Figure 2.7. Relationship between vitrinite reflectance and CSR index (Nakamura et al., 1977)...... 26 Figure 2.8. 3D mass spectrum graph of raw coal plotted against temperature and mass-to- charge ratio (m/z), measured by using (a) electron ionisation and (b) photoionisation methods (Li et al., 2018a)...... 30 Figure 2.9 Schematic representation of coal surface (Laskowski, 2013)...... 39 Figure 2.10. Standard chart for FSI (or CSN) profiles (American Society for Testing and Materials, 1999)...... 46 Figure 2.11. Schematic design of an Audibert-Arnu dilatometer (Australian Standards, 2002)...... 48 Figure 2.12. Schematic design of a Gieseler plastometer (Australian Standards, 1996). .... 50 Figure 2.13. Setup for Davis plastometer (Fieldner et al., 1931)...... 51 Figure 3.1. Crushed coal samples in vacuum sealed bags...... 60 Figure 3.2. Titration curves (solid line, left axis) and differentiated titration curves (dashed line with markers, right axis) of the process water sample (♦, sample 1; blue, diff. 1) and its replica (▲, sample 2; red, diff. 2)...... 65

Figure 3.3. Plunger setup used in the study to dewater the coal-solution mixture: (a) plunger head and filter mesh, (b) assembled setup with plunger handle and container...... 70

VIII Figure 3.4. pH curves from washing of Milli-Q treated (▲, blue) and 0.01 M Na2CO3 (●, green) treated coal samples (Note: A wash time of “0” refers to the supernatant before the first wash)...... 71 Figure 3.5. Coal B samples prepared for XPS analysis...... 74 Figure 3.6. Coal A samples prepared for SEM-EDS analysis...... 76 Figure 3.7. Gieseler plastometer setup (RMI, 2018)...... 77

Figure 4.1. Fluidity of coal A samples treated with Na2CO3 solutions without oxidation (Errors determined by analysing results from two independent runs)...... 80 Figure 4.2. pH of the supernant collected after each wash cycle and the corresponding cycle number for Milli-Q water-soaked coal (blue) and Na2CO3 soaked coal (orange), where wash cycle 0 refers to unwashed after the soaking treatment...... 82 Figure 4.3. Fluidity of washed and unwashed coal A samples treated with Milli-Q water and 0.01 M Na2CO3 (Errors determined by analysing results from two independent runs). 83

Figure 4.4. Log10 MF of coal A samples dried in air (filled) and under N2 (unfilled) (Errors determined by analysing results from two independent runs)...... 84

Figure 4.5. Log10 MF of coal A samples contacted in different 0.01 M inorganic salt solutions and oxidised for 7 days before drying under nitrogen gas (Errors determined by analysing results from two independent runs)...... 86 Figure 4.6. Titration curves (solid line, left axis) and differentiated titration curves (dashed line with markers, right axis) of the process water sample (♦, sample 1; blue, diff. 1) and its replica (▲, sample 2; red, diff. 2) after oxidation with coal...... 88

Figure 4.7. XPS wide energy spectrum of coal surface after 7-day oxidation with (a) process water (green), (b) 0.01 M Na2CO3 solution (red) and (c) Milli-Q water (blue)...... 92 Figure 4.8. O(1s) spectra of coal B samples oxidised in (a) process water, (b) 0.01 M

Na2CO3 solution and (c) Milli-Q water...... 93 Figure 4.9. C(1s) spectra of coal B samples oxidised in the presence of (a) process water

(green), (b) 0.01 M Na2CO3 solution (red) and (c) Milli-Q water (blue)...... 94 Figure 4.10. S(2p) spectra of coal B samples oxidised in the presence of (a) process water

(green), (b) 0.01 M Na2CO3 solution (red) and (c) Milli-Q water (blue)...... 95

IX Figure 4.11. Si(2p) spectra of coal B samples oxidised in the presence of (a) process water

(green), (b) 0.01 M Na2CO3 solution (red) and (c) Milli-Q water (blue)...... 96 Figure 4.12. Back-scatter image for coal A samples: (a) untreated surface and (b) process water treated surface...... 97 Figure 4.13. SEM results for (a) carbon surface and (b) non-carbon surface from process water treated coal A sample...... 98 Figure 4.14. Spectra from elemental scanning of (a) untreated surface and (b) process water oxidised surfaces...... 99 Figure 4.15. EDS analysis of the process water treated coal A surface showing: (a) original image of the surface, (b) Cl and (c) Na...... 100

Figure 4.16. Log10 MF of coal samples treated with process waters with different dilution ratios...... 103

X List of Tables Table 2.1. Coal behaviour during coking and the corresponding temperatures (Loison et al., 1963a)...... 9 Table 2.2. Formation of coal surface functional groups subjected to oxidation...... 38 Table 2.3. Common inorganic species in process water in Australian coal processing plants (Ofori et al., 2009, Bournival et al., 2017)...... 53 Table 3.1. Types and purities of reagents used in this study...... 59 Table 3.2. Characterisation of the coal samples: proximate analysis, ultimate analysis and Gieseler plastometer measurements...... 61 Table 3.3. Major ions in the process water sample used in the present study compared to typical process water in Australian coal processing plants (Ofori et al., 2009, Bournival et al., 2017)...... 63 - 2- - Table 3.4. Criteria for HCO3 , CO3 and OH determination...... 65 - 2- - Table 3.5. Calculated CaCO3 equivalent concentrations of HCO3 , CO3 and OH in the process water...... 65 Table 3.6. Process water samples with adjusted total salt concentration...... 66 Table 3.7. Inorganic salt solutions and their concentration used in this study...... 67 Table 3.8. Gieseler fluidity results of the dry coal A samples oxidised for different periods...... 68 Table 3.9. Ash analysis of untreated coal sample, process water-treated coal sample and HCl-treated coal sample...... 72

Table 4.1. Gieseler plastometric measurements of coal A samples treated with Na2CO3 solutions without oxidation...... 81 Table 4.2. Gieseler plastometric measurements of washed and unwashed coal A samples treated with Milli-Q water and 0.01 M Na2CO3...... 83 Table 4.3. Gieseler plastometric measurements of oxidised and unoxidised coal A samples when Milli-Q water, 0.01 M Na2CO3 and process water were present...... 85 Table 4.4. Gieseler plastometric measurements of coal A samples contacted in different

0.01 M inorganic salt solutions and oxidised for 7 days before drying under N2 gas...... 87 Table 4.5. Inflection points of each sample identified from the titration data...... 89

XI Table 4.6. Changes in concentration of process water before and after treatment with coal A sample for the major elements...... 90 Table 4.7. Atomic % of major ions on the surfaces of oxidised samples from XPS analysis...... 92 Table 4.8. Gieseler plastometric measurements of coal samples treated with different salt concentrations...... 102 Table A1. Full ICP-OES analysis results...... 125 Table A2. Titration results for process water and replica...... 126

XII Chapter 1 Introduction

1.1 Introduction to coal preparation, coke making and thermoplastic properties

Coal is a natural fuel that has been used for centuries. It is formed as a result of heat, compaction and decay of buried organic material. The organic material first forms a wet, spongy sediment layer, called peat, is dried over time, and then compressed and buried by tectonic activity. Since the source material, the original plant bodies, in the peat varies from origin to origin, coal has great variety in its chemical and physical properties. As coal was understood and used more, these differences became useful criteria for coal characterisation and classification (Rao and Gouricharan, 2016a).

Coal can be categorised into two main types: metallurgical coal and thermal coal. Metallurgical coal has the same origin as thermal coal. However, they are vastly different in end uses and applications. Thermal coal is mainly used to generate steam to provide an energy source to run turbines and generate electricity for public use. In contrast, metallurgical coal is used to produce coke, which is then used by industry in steel casting and iron making (Osborne, 2013).

Generally, the differentiation of thermal coal and metallurgical coal is based on the composition of the coal itself. Coal containing very high carbon content and low mineral matter and moisture content can be classified as metallurgical coal (Miller, 2013). In comparison, thermal coal has more impurities such as sulfur and inorganic salts. However, the most significant difference between the two is the thermoplastic property, which is the softening behavior of coal under pyrolytic condition. Thermal coal does not exhibit any thermoplastic property when heated in an oxygen-deficient atmosphere, as the coal itself is burnt off and mineral matter is turned into ash; whereas metallurgical coal swells, softens, melts and eventually resolidifies into coke during the whole heating process (Riazi and Gupta, 2015).

The thermoplastic properties of metallurgical coals are used in the production of coke, which is a fuel and a reducing agent in the iron blasting industry. Coke is produced by

1 heating metallurgical coal in a refractory oven to about 1100 °C in an oxygen-deficient atmosphere (Vasko et al., 2005).

Coke is the essential material for ironmaking through blast furnaces. As the most expensive material in the blasting furnace, coke has many critical roles during a blasting operation (Mousa et al., 2011). Figure 1.1 illustrates how coke works in the blasting furnace. Thermally, it provides heat energy in the blasting process; chemically, it works as a reducing agent to reduce iron oxide ore; and mechanically, it provides a strong bed for supporting the ore and preventing collapse in the furnace due to up-flowing gases (Gupta et al., 2014). Coke collapse is a severe safety issue in industrial operations. In 1996, two steelmaking plants in the US had coke collapse accidents, resulting in hot steam burst and rupture of the blast furnace and the death of two workers (The Times of Northwest India, 2001).

Figure 1.1 Illustration of coke degradation mechanisms in a blast furnace (Mousa et al., 2011).

2 In a blast furnace (Figure 1.1), coke passes through the stack at about 200 °C to the hearth at more than 1600 °C. Coke experiences a range of harsh reaction conditions that weaken and break, generating fines which are known to be detrimental to bed permeability (Mousa et al., 2011). Several factors affect coke fines generation including mechanical stress, high- impact abrasion, solution loss reaction, thermo-mechanical impact and high temperature chemical attack including recirculating alkalis (Gupta et al., 2008). At a high supplementary fuel injection rate, the coke-to-ore volume ratio decreases, resulting in a lower thickness of coke layer and longer periods of mechanical, thermal and chemical stress. Therefore, both mechanical strength and strong chemical resistance are important at higher coke injection rates.

The run-of-mine (ROM) coal must go through a few unit operations in coal handling and preparation plants (CHPPs) to upgrade product quality, known as coal beneficiation. These operations normally include crushing, screening, gravity separation, froth flotation and dewatering. The beneficiation processes separate coal from ash-associated impurities such as mineral matter by methods such as gravity separation and flotation (Laskowski, 2001). Since most of the beneficiation processes require a large amount of water, water scarcity has become one of the leading challenges for the mineral and coal industries. Fresh water supply issues are forcing many coal plants to find alternative sources of water (Klima et al., 2012).

To overcome the water challenge, a strategy of recycling process water is commonly practised in CHPPs. This strategy not only fulfils the need for the enormous amount of water required for coal processing, but also reduces cost and environmental impact. Recycled water can be sourced from various processes on site including tailings dams, concentrate and the overflow from tailings thickeners (Slatter et al., 2009). Flocculants are used to improve the agglomeration of solid particles suspended in the process water, to enhance the clarity of the water (Alam et al., 2011). However, although the recycled process water becomes visually much clearer after the flocculation, the contamination level of dissolved salts and organic matter remains fairly high (Rao and Finch, 1989b, Liu et al., 2013, Leong et al., 2014). The effects of such contamination are discussed shortly.

3 1.2 Statement of the problem

The thermoplastic properties of coal can be affected by several factors categorised as inherent factors (e.g. mineral matter, rank and macerals) and external factors (e.g. oxidation, inorganic salts and plastic additives) (Mochizuki et al., 2013, Khan and Jenkins, 1986b, Rhoads et al., 1983, Ignasiak et al., 1974, Price et al., 1992). Inherent factors such as macerals and inherent mineral matter were found to affect coal thermoplasticity to various extents (Grigore et al., 2006, Grigore et al., 2007, Li et al., 2017a). In addition, thermoplasticity of coal is also dependent on coal rank, as it does not only involve the inherent minerals and macerals, but also includes volatile matters and fixed carbon (O'Keefe et al., 2013, Yoshida et al., 2000, Yu et al., 2013). This study is focused on the external factors, as most of the inherent factors are purely determined by the origin, source plant and coalification maturity of the coal (Ibarra et al., 1996, Taylor et al., 1998). It is generally agreed that oxidation reduces coal thermoplasticity (Rhoads et al., 1983, Ignasiak et al., 1974, Vega et al., 2017, Kus and Misz-Kennan, 2017). However the effect of oxidation on coal cannot be generalised because the composition of coal varies from seam to seam, which sets limits on maintaining the consistency of composition of coal samples (Banerjee et al., 2016). In addition, the effect of oxidation on coal depends on the oxidation environment. For example, the effect of oxidation on coal was found to be proportional to the duration and temperature of oxidation (Vega et al., 2017, Kus and Misz-Kennan, 2017, Arisoy and Beamish, 2015).

The thermoplastic properties of coal have been extensively investigated by many authors using various techniques (Patrick and Shaw, 1972, Khan and Jenkins, 1986a, Crewe et al., 1975). Patrick and Shaw (1972) used a Gieseler plastometer to investigate coal fluidity, which is a parameter of coal thermoplasticity that indicates the softening behaviour of a coal. The addition of dry sodium carbonate (Na2CO3) of 5 wt.% to coal weight reduced the fluidity of the coal by up to 90%, and the plastic range was reduced while the maximum fluidity temperature was unchanged (Patrick and Shaw, 1972). When 5 wt.% of sodium carbonate was mixed with the seven different coal samples separately, the reductions in coal fluidities were different but significant for all coal samples (Patrick and Shaw, 1972).

4 The swelling property of coal is also often examined in coal thermoplasticity investigations as it is an important indicator of the coal’s thermoplasticity and quality. A coal with greater swelling property usually has better thermoplasticity and produces better coke. Khan and

Jenkins (1986b) reported that 2 wt.% of dry potassium carbonate (K2CO3) and potassium hydroxide (KOH) significantly reduced coal swelling while potassium chloride (KCl) showed no effect. Crewe et al. (1975) indicated that the swelling properties of coal samples could be sharply reduced by mixing coal with inert sand, dry NaOH, and dissolved NaOH solution respectively. It was pointed out that inert sand acts as an inert additive, where the NaOH, in either dry or dissolved form, was retained on the coal surface and worked as a reactive additive. However, the negative impact from NaOH was found to be reversed by soaking the treated coal in distilled water. From the findings of previous works (Marsh and Walker, 1979, Li et al., 2017b, Patrick and Shaw, 1972, Crewe et al., 1975), it seems that the presence of inorganic salts does have some negative impacts on the thermoplastic behaviours of coal. The addition of either dry or dissolved inorganic salts was found to influence thermoplasticity. However, dry and dissolved inorganic salts might each interact with coal differently.

Metallurgical coal goes through extensive processing to remove some of the impurities that reduce the quality and calorific value of coal. Coal processing requires a significant quantity of water and recycling is general practice in most wash plants. Recycled process water contains high concentrations of inorganic electrolytes, organic material and flocculants. Typical contaminants in the reused water are pyrite oxidation products (Fe2+, 3+ 2 2+ 3+ 2+ Fe and SO4 ), soluble inorganic ions released from raw coal (Ca , Al and Mg ), humic acids and residual flotation reagents. These contaminants in the process water, especially the accumulated inorganic substances, might have a profound influence on the thermoplastic properties of coal. However, so far, no investigation has been conducted on the relationship between process water constituents and coal thermoplasticity. However, the effects of the abovementioned chemical species in recycled process water on coal processing have been extensively studied (Rao and Finch, 1989b, Liu et al., 2013, Leong et al., 2014), and it was found that the use of recycled process water can have a profound impact on coal surface chemistry. For example, metal ions such as calcium, iron and

5 aluminum can alter the hydrophobicity of the coal surface and lead to reduced flotation due to pH dependent precipitation of these species on the coal surface (Xu et al., 2003).

Aplan (1976) suggested that the coal particles have an overall positive surface charge at low pH, and a negative charge at high pH. Laskowski and Parfitt (1989) found that the fine clay particles tend to attach to the negatively charged coal particles, which not only increases the coal surface charge, but also increases the ash content of the coal. Metal ions can also affect the magnitude of the surface charge causing agglomeration that can be detrimental to coal flotation (Xu et al., 2003). It was reported that the surface charge of coal decreases when polyvalent cations are added into the slurry, and the maximum flotation recovery occurs when the coal is at its isoelectric point (Fuerstenau et al., 1983, Diao and Fuerstenau, 1991). Besides the externally added organic and inorganic reagents, coal flotation is affected by precipitation or adsorption of the dissolved mineral species released from the coal itself during grinding and pulping (Somasundaran et al., 2000). Liu et al. (1994) suggested that the concentrations of dissolved iron (Fe), aluminium (Al), calcium (Ca) and magnesium (Mg) decrease as the pH increases, with the mode of alkali addition being irrelevant. If the pH increases during coal processing, there will be precipitation of metal ion species whereas if the pH decreases, there will be dissolution of mineral species. As the process water usually exhibits a higher pH, it is very likely that the coal surface chemistry is changed during processing as a result of precipitation or adsorption. However, the effects of such changes on coal thermoplastic properties are yet to be investigated.

1.3 Objectives

Process water recycling is a common practice in Australian CHPPs, however there is insufficient knowledge on the impacts of using recycled process water on the thermoplastic properties of metallurgical coals. Previous investigations have studied the interaction between the coal surface and inorganic salts in process water and saline water (Laskowski, 1965, Pugh et al., 1997, Yoon and Sabey, 1982, Yoon, 1982), and have also examined the difference in coal thermoplastic behaviours where the coal was conditioned with inorganic salts (Marsh and Walker, 1979, Khan, 1989, Patrick and Shaw, 1972, Laskowski, 1965, Pugh et al., 1997, Yoon and Sabey, 1982, Yoon, 1982). However, no investigation has been

6 carried out on the connection between the two phenomena. In addition, most studies on the interaction between process water and coal were based on flotation, while in the CHPPs coal is in contact with reused water in other separation units, such as heavy medium separation (Rao and Gouricharan, 2016b, Riazi and Gupta, 2015). Therefore, the interaction between coal and process water still requires more research to be fully understood.

It is hypothesised that treatments with process water will induce interactions between inorganic materials in the process water and coal surface, which will result in changes in coal thermoplasticity. Based on this hypothesis, investigations on the effects of recycled process water on coal thermoplasticity and the interactions between inorganic electrolytes in the process water and coal surface during a designated period of oxidation are carried out. The mechanisms of changes in coal surface chemistry and the subsequent change in thermoplastic properties are also explained. Specifically, the objectives of this research are to:

 explore the mechanisms of interactions between inorganic electrolytes and the coal surface during the designated treatments  investigate the effects of oxidation in the presence of inorganic salts on coal thermoplastic properties  Understand on the role of inorganic salts during the coal thermoplastic stage.

The investigations address these objectives and fill the gaps in current knowledge. In this study, the fluidity and surface chemistry of two Australian coal samples, from Queensland and New South Wales respectively, under various treatment conditions were examined. The coal samples were treated with process water from one of the coal preparation plants, as well as with the artificial inorganic salt solutions. Other treatments such as oxidation and washing with Milli-Q water were also employed in the present study. The outcome of this study is expected to greatly contribute to the understanding of the negative impacts of process water on coal thermoplastic properties and the mechanisms of these impacts, as well as development of corrective suggestions and improvements to the current industry practices.

7 1.4 Thesis structure

This thesis is organised in five chapters and three appendices, as follows:

Chapter 1 presents the background and the motivation to the study. The background includes a general introduction to coal thermoplastic properties, and coal preparation and coke production procedures. The problem statement and the structure of the thesis are also included in the chapter.

Chapter 2 provides a detailed review of literature on fundamentals of coal thermoplastic properties; factors affecting coal thermoplasticity; and process water chemistry and coal thermoplasticity.

Chapter 3 describes the materials and experimental setups used in the study. It also includes characterisation of the coal and process water samples, fluidity examination and surface analysis of the weathered, washed and raw coal samples.

Chapter 4 presents the results and discussion of findings from the study. A possible mechanism of how coal interacts with inorganic salts in the process water and how this interaction may affect coal fluidity is also explained.

Chapter 5 summarises the main findings and the conclusions of the study. Suggestions and recommendations are also made.

8 Chapter 2 Literature review

2.1 Introduction

Coke making involves carbonisation of metallurgical coal in a coke oven under an inert atmosphere (Rao and Gouricharan, 2016b, Osborne, 2013). Coal experiences a thermoplastic stage as the temperature increases during coke making. Table 2.1 shows the behaviour of coal during coking as the temperature increases. When entering the thermoplastic stage, the coal first softens, and then swells because of the release of volatilised constituents. As the temperature continues to increase, the coal becomes liquid and starts to resolidify. The coke product is the resolidified material that remains in the oven (Osborne, 2013). The thermoplastic properties of coal are important parameters for coke quality estimation. Although extensive studies have been conducted to investigate coal thermoplastic properties, the mechanism of the solid-liquid-solid transformation is still not fully understood.

Table 2.1. Coal behaviour during coking and the corresponding temperatures (Loison et al., 1963a). Temperature Stage Status Behaviours of coal (°C) mild changes on surface, vaporisation 0~300 - Solid of moisture Softened and solid softens, and swells as a result of 300~800 Softening swollen solid gas escaping, eventually turns into a ~ liquid liquid liquid component resolidifies and 800~1100 Resolidification Liquid ~ solid forms coke

Many coal preparation plants in Australia reuse process water in their coal processing circuit to solve issues such as water scarcity and negative environmental impacts (Rao and Finch, 1989a, Bournival et al., 2017, Ofori et al., 2009). However, the reuse of process water causes the accumulation of both inorganic and organic materials in the circuit (Gupta et al., 2012, Wang and Peng, 2014a). Although researchers have focused on the interactions between process water and the coal surface and consequent effects on the upgraded coal 9 product, the effects of these interactions on coal thermoplastic properties are yet to be investigated.

This chapter reviews previous works in the areas related to the present study. Section 2.2 is focused on the fundamentals of coal thermoplastic properties and the major theories explaining the thermoplastic behaviours of coal. Section 2.3 explores the factors that were found to influence coal plasticity during pyrolysis and their mechanisms. Previous investigations on the interaction between process water constituents and the coal surface are reviewed in section 2.4.

2.2 Coal thermoplastic properties

It is important to understand the fundamentals of coal thermoplastic properties in order to study the mechanism of how foreign elements affect the properties. The thermoplastic properties are representative of a coal’s ability to transform through the solid-liquid-solid phase during pyrolysis, which is the process of coal decomposition at high temperature in the absence of air (Solomon et al., 1988). A schematic representation of the progressive transformation of coal during the softening stage (300 °C to 650 °C) is shown in Figure 2.1. As can be seen, the coal sample expands its volume significantly within the temperature range of 300 °C to 390 °C, which is the primary pyrolysis stage of coal (Coetzee et al., 2014). Starting from around 350 °C, coal begins to lose its moisture content and forms an unstable plastic phase (metaplast). Above 400 °C the primary release of volatile matters and formation of semicoke take place until around 450 °C. At higher temperatures (450 °C to 550 °C), the density of the semicoke increases forming the final coke product. In the meantime the secondary release of volatile matters takes place progressively, resulting in losses of methane and hydrogen gases (Habermehl et al., 1981).

10 Figure 2.1. Progressive changes of coal during the softening phase (Coetzee et al., 2014).

Pyrolysis occurs in all major coal conversion processes. At lower temperature (below 225 °C), only mild changes occur in coal structure. For instance, there might be disruption of hydrogen bonds, vaporisation and release of non-covalent bonded molecules. The primary pyrolysis starting from around 350 °C to 400 °C. The weaker aliphatic bridges connecting the larger aromatic clusters in the coal matrix are cleaved producing molecular fragments. These fragments are often referred to as metaplast (Figure 2.2), an unstable plastic phase which is a mixture of gas, liquid and solid (Fitzgerald, 1956, van Krevelen, 1993). Coal then experiences the secondary pyrolysis when it undergoes further reactions in the gas phase. At temperatures from 800 °C to 1100 °C, functional groups and side-chains attached to the aromatic rings in the tar thermally decompose to release additional gases, usually comprising CO, CO2, light hydrocarbons, H2 and heteroatom species. At this stage, the semicoke has converted into solid coke, with the process accompanied by shrinking and consequent cracking of the material (Rouzaud et al., 1988). Nomura and Thomas (1998) suggested that the thermoplastic phenomenon of coal is a consequence of chemical changes in the macromolecular structure, accompanied by physical changes including generation of gases and subsequent development of microstructure.

11 Figure 2.2. Schematic of mesophase evolution during coal pyrolysis (Fitzgerald, 1956).

The maximum fluidity of coal is reached when the concentration of metaplast is at a maximum (Rouzaud et al., 1988). Meanwhile, the fragments from the cleavages of the aliphatic bridges are released as tar, which is composed of one or multiple aromatic ring structures. Functional groups and labile bridges attached to those aromatic clusters are also released at the same time in the form of light gases, such as CO and CO2, and light hydrocarbons. When the temperature reaches around 500 °C to 600 °C, coal starts to lose its fluidity, and a condensation reaction takes place as aromatic structures repolymerise together to form larger aromatic structures. Anisotropic ordered semicoke is formed by solidification of the metaplast.

2.2.1 Coal molecular transformations during pyrolysis

The thermoplastic behaviours of coal during pyrolysis were reported to be closely related to the transformation of its molecular structure (Komaki et al., 2005). Komaki et al. (2005) investigated a range of structural properties of two coal samples: Witbank (South Africa) low fluidity coal (WIT) and Goonyella (Australia) high fluidity coal (GNY). They found that the transferable hydrogen in the two samples behaved differently during the pyrolysis process, which resulted in different thermoplastic behaviours. The lower fluidity of WIT coal consumes most of its transferable hydrogen before reaching the softening temperature, causing insufficient hydrogen supply in later conversion; whereas the higher fluidity GNY sample consumes its transferable hydrogen effectively throughout the plastic range. The difference in transferable hydrogen reaction is due to the difference in the amount of tar production, as the WIT coal produces much more tar before reaching softening temperature,

12 which caused the over-consumption of transferable hydrogen in the early stage (Komaki et al., 2005). The size difference in aromatic clusters within the two coal samples correlates to the differences in their fluidity and swelling ratio (Komaki et al., 2005). In their coal- maceral analysis, inertinite-rich samples with larger aromatic clusters displayed almost no fluidity, and the survival of a small amount of plastic organics was also observed beyond the resolidification temperature (Komaki et al., 2005). This finding coincides with Osborne (2013)’s description of the chemical structures and thermal behaviours of vitrinite and inertinite.

Marsh and Neavel (1980) and Grint et al. (1985) explored the importance of transferable hydrogen species in coal during the thermoplastic phase, suggesting that the transferable hydrogen stabilises the free radicals split from bond cleavage and transforms these radicals into solvating species. The effects of aromatic moieties and transferable hydrogen consumption on coal’s plastic properties are also discussed by Kidena et al. (2004) who suggested that coal plasticity is not only dependent on the size of the aromatic clusters, but also the amount and the efficiency of the consumption of transferable hydrogen.

Neavel (1982) suggested that the thermoplasticity of coal is a liquefaction process. The chloroform solubles are considered as a mobile hydrogen donor source and a solvating fluid. The thermoplasticity is encouraged by the addition of low molecular weight solvating species that come from thermally produced free radical fragments. Oxley and Pitt (1958), on the other hand, reported that the role of chloroform soluble components is not decisive in determining the kinetics of fluidity. The theory by (Neavel, 1982) was later supported by the findings of Yokono et al. (1984), Clemens and Matheson (1992) and Clemens et al. (1989). These later studies described measurements of transferable hydrogen in different coals and the close relationship between coal plasticity and transferable hydrogen. Furthermore, Clemens et al. (1989) reported more supportive findings that the fluidity of coal is dependent on the transferable-hydrogen source to cap radical species and generate low molecular weight solvating species during heating. Yu et al. (2013) summarised in their review paper that the thermoplastic properties of coal are determined by both aliphatic structure and hydrogen bonds. The hydrogen bonds provide the most dominant binding force in coal pyrolysis, and they tend to form associative or even supermolecular structures 13 when associated together. However, the metaplast formed is primarily from the aliphatic groups. For the associative structure formed by hydrogen bonds, only those with appropriate molecular weight can be broken and changed into metaplast, whereas the others are vaporised and released from the coal structure directly.

Ouchi et al. (1989) used the solvent extraction method with pyridine and quinoline to investigate plastic properties of a bituminous coal. The extractable component and the residue after the pyridine extraction and quinoline extraction were examined by thermogravimetric analysis using an infrared image furnace. They found that the lower molecular weight material in the coal was responsible for the higher fluidity due to the possibility of greater solvent extractability. The coal plasticity was significantly reduced under the influence of oxidation, but the addition of a solvent-soluble fraction of coal and/or polyaromatic hydrocarbons to the oxidised coal seemed to help in restoring coal plasticity. Kirov and Stephens (1967) investigated solvent extraction of carbonized coals with chloroform and related the release of chloroform extract to coal fluidity at a slow heating rate (3 °C/min). They stated that some liquid formed by physical melting when the temperature was increased above a critical point, and a further increase in temperature would lead to pyrolytic reactions, i.e. bond-breaking reactions, that generate more liquid.

Kidena et al. (1996) used scanning electron microscopy (SEM) and carbon-13 nuclear magnetic resonance (13C NMR) to examine the surface of six coal samples at high temperatures and correlated the surface analysis with Gieseler fluidity measurements. They showed an indirect correlation between the concentration of methylene groups that link two aromatic moieties and the maximum fluidity, as a similar pattern with carbon content in coal was found as shown in Figure 2.3. This correlation suggests that coal fluidity could be associated with the concentration of methylene functional groups in coal.

14 (a) (b)

Figure 2.3. Correlation between carbon content (wt.%) in coal with (a) maximum fluidity

(log10 ddpm) and (b) active methylene carbons (wt.%) (Kidena et al., 1996).

Nomura and Thomas (1998) studied the molecular structural changes during coal carbonisation by Fourier transform infrared spectroscopy (FTIR) and found that the relative intensity of methylene groups (-CH2) in the aliphatic chain decreased with increasing temperature. More recent work by Fernandez et al. (2012) used bituminous extractions as coal fluidity modifiers and highlighted the importance of a higher degree of aromatic condensation in enhancing coal plasticity, because highly aromatic compounds appeared to stabilise the free radicals formed during coal decomposition and increase the thermoplasticity. In addition, the presence of oxygen, nitrogen and sulfur functionalities in the reaction system promotes condensation which facilitates resolidification of the system.

A majority of the volatile matters are driven-off as the coal becomes a complete liquid material at around 650 °C to 700 °C as previously described. Above 700 °C, the medium and low weight molecules that are produced from the decomposed macromolecular network start to recombine with each other and the activated carbon, leading to a rearrangement of the macromolecular network. Such reactions cause the metaplast to resolidify and ultimately form coke (Butterfield and Thomas, 1995, Fitzgerald, 1956). Figure 2.4 summarises the transformations taking place in the coal during pyrolysis (Spiro, 1981). Initially the molecules exist in lamellae with flat aromatic planes interrupted by aliphatic protrusions. The first group of bonds to be thermally cracked are the weak covalent bonds that are usually associated with protrusions. Next, pyrolysis results in enhanced parallelism of aryl planes accompanied by two-dimensional mobility. As a 15 significant number of the weak bonds are cracked, the remaining planar polycyclic structures, which were formerly locked in place by the aliphatic moieties, are free to slide over one another in two dimensions. The smaller aliphatic fragments are generated by the thermal-cracking, and they work as lubricates in between the planes to inhibit cross-linking and van der Waals π-π interactions between the aromatic char pieces. Finally, when lubricating fragments and gases diffuse from the planes, the char pore system of the semicoke develops.

Figure 2.4. Schematic transformation of coal macromolecular structure during the thermoplastic phase (Spiro, 1981).

16 2.2.2 Theories of coal thermoplastic properties

The mechanisms of coal’s thermoplastic behaviours have been extensively studied for decades, and three main theories have become widely accepted: the homogenous melting theory, the partial melting theory, and the metaplast theory. All of these theories aim to explain the mechanisms of coal softening and resolidification from either a physical or a chemical point of view. This section explains and reviews the three main theories in depth.

Homogenous melting theory

The homogenous melting theory suggests that the coal undergoes initial softening in a homogenous manner (Audibert, 1926, Gillet, 1951, Loison et al., 2014). According to the theory, the entire mass of coal experiences true melting homogenously while decomposition occurs at the same time, yielding infusible solids and gases. The product from homogenous melting is therefore a mixture of fusible and infusible products. Resolidification corresponds to the completion of the transformation into infusible products. Audibert (1926) suggested that the softening of coal above 350 °C is purely a physical phenomenon, where only little dependence was found between the initial softening temperature and the chemical composition of the coal (with volatile matter ranging from 20% to 38%). The heating rate was also noted to play an insignificant role in the softening temperature (Audibert, 1926). However, it was stated that a higher heating rate will increase the temperatures of pyrolytic reactions (maximum fluidity temperature and resolidification temperature), because the quantity of non-decomposed fused coal will increase when the heating rate is increased, resulting in greater plasticity. In other words, the homogenous melting theory suggests the plastic behaviour of coal depends on the difference between the softening temperature and decomposition temperature under the assumption that the melting mechanism is homogenous throughout the entire mass of coal. Nonetheless, the homogenous theory cannot account for the fact that the fluidity increases as a function of time when the temperature is set to constant (Fieldner et al., 1931). Samples of high volatile bituminous coal from Taggart bed from Virginia, United States were used in the investigations conducted by Fieldner et al. (1931). As shown in Figure 2.5, the fluidity of the coal sample, obtained using a Davis plastometer, varied while different

17 maximum temperatures held constant produced different profiles. It was found that the higher the temperature the faster the coal transforms and the higher the fluidity it can achieve. The differences in the maximum fluidity and the plastic range of the coal samples in Fieldner et al. (1931)’s study seemed to be too significant to fit the homogenous melting theory, especially when the coal samples and heating rates were the same and the only difference was the maximum temperature.

Figure 2.5. Coal fluidity as a function of pyrolysis time at different temperatures (Fieldner et al., 1931).

Furthermore, Loison et al. (1963a) pointed out that the homogenous melting theory is improbable because after melting, the fissuration, or the bubble structure, of the semicoke gives rise to a microscoporosity of the same order as that of the initial coal. Moreover, van Krevelen et al. (1956) stated that by extrapolating measurements at lower temperature, the fluidity of undecomposed fused coal would be too low to account for actual values. 18 Partial melting theory

The partial melting theory suggests that only a fraction of coal melts upon heating. The melting fraction was identified as the fraction of coal that is soluble in organic liquids such as benzene and chloroform (Pott et al., 1933, Kirov and Stephens, 1967). The benzene and chloroform soluble extracts were found to be very fusible, whereas the insoluble residue showed no plastic property. However, Dryden (1951) attempted to mix the solvent soluble extracts with the insoluble residue and found that the mixture does not have the exact same plastic properties as the initial coal. Later studies showed that the chloroform soluble compounds melt first and dissolve compounds with higher molecular weight, leading to the plasticity of the coal (Ouchi, 1961, Ouchi et al., 1983).

On the other hand, (Ng et al., 1982) found that when the chloroform soluble from a coking coal was blended with a high volatile, inertinite-rich non-coking coal and co-carbonised, the non-coking coal developed an anisotropic microtexture that is similar to a coking coal after carbonisation. However, although a correlation was found between the yield of soluble products and the thermoplastic properties of coal, this correlation cannot be applied in general due to the wide variation in the compositions of coal from different origins (Neavel, 1982). The addition of oil and controlled hydrogenation of coal were found to bring a similar increase in the yield of soluble products while the thermoplasticity was unchanged. Qin et al. (2010) found that the formation of metaplast is mainly determined by the caking components, while little or no metaplast is generated by the non-caking components. The metaplast is mostly generated from active components with medium- sized molecules and/or molecular structures as it displays good cross-linking and bonding points. Meanwhile, the non-caking residue yields gas and tar in the solid phase with little to no liquid phase.

Metaplast theory

The most widely adopted theory of coal thermoplastic behaviours is the metaplast theory (Maroto-Valer et al., 1997, Miura et al., 2005, Fong et al., 1986b, Fitzgerald, 1956, James and Mills, 1976). Pyrolysis transforms coal into fluid products either completely or partially, with these fluid products considered the cause of the softening and 19 thermoplasticity in the metaplast theory (Fitzgerald, 1956). The disappearances of these fluid products induced by thermal decomposition or evaporation are the reason for resolidification. The metaplast theory considers the fluid products from pyrolysis of bituminous coals as the metaplast, also called “thermobitumens”, signifying that they are only produced through thermal reactions (i.e. pyrolytic reactions) and do not pre-exist in coal. The mechanisms of coal softening and resolidification, according to the metaplast theory (Fitzgerald, 1956), are shown in Equations 1 to 4. In these equations, R1 and R2 refer to the primary solid residue and secondary solid residue respectively, whereas G1 and G2 refer to the primary gas and secondary gas generated. Coal generates fluid metaplast

(Metaplastliquid) and primary solid residue at first. As the temperature increases, the metaplast experiences pyrolytic reactions and forms primary gas (G1) and solid product

(Metaplastsolid), the solid products further decompose and generate more gas (G2) and the undecomposed solid material (R2) aggregates with R1 to form the semicoke.

Coal →Metaplastliquid + R1 1

Metaplastliquid →G1 + Metaplastsolid 2

Metaplastsolid → R2 + G2 3

R1 + R2 → semicoke 4

The assumption of the metaplast theory is that the metaplast is initially in a liquid state produced by depolymerisation reactions during thermal decomposition, and the majority of the metaplast is volatilised as gaseous products (i.e. G1 and G2) by thermal-cracking reactions and released from the coal mass (Fitzgerald, 1956, Chermin and Vankrevelen, 1957). Chermin and Vankrevelen (1957) pointed out that the depolymerisation and cracking reactions overlap in time, so that during the pyrolysis of coal, metaplast (liquid), undecomposed coal, solid residue and semicoke (solid), and gas exist simultaneously until all the metaplast products are released as gases, which results in complete resolidification and the formation of the final coke product. It was also suggested that the thermoplasticity of a coal and the metaplast that can be generated is proportional: the greater the plasticity, the more metaplast the coal can generate.

20 The metaplast theory also explains that metaplast is not formed in low rank coals due to the relative thermal instability of metaplast. Besides, rapid removal of metaplast under the vacuum condition destroys the plasticity. It also explains why fluidity increases under the isothermal heating condition. However, the theory fails to explain the non-plastic behaviour of the residue extracted from the coking coals.

2.3 Factors affecting coal thermoplastic properties

The thermoplastic properties of coal can be affected by various factors. Zhang et al. (2004) reported the relationship between various thermoplastic indices (caking index G, maximum thickness of plastic layer Y, total dilation, Gieseler fluidity) with coke strength after reactivation (CSR). The results suggest that the increase of coal rheology properties, to some extent, is beneficial to the CSR value. A fluidity range of the order of 400 ddpm is believed to be optimum for a high CSR coke (Prachethan Kumar et al., 2007). In addition, coal fluidity has a critical effect on controlling the size and shape of coke anisotropy, which affects the coke reactivity and hence coke strength (Lu et al., 1981).

Fluidity is frequently used as the indicator of thermoplastic properties for the coking industry and is usually measured using a Gieseler plastometer (see Figure 2.12 in Section 2.3.6). Once the coal sample is packed in the plastometer, it is heated at a constant rate of 3 °C/min, which is similar to that used in commercial coke ovens. The maximum fluidity (MF, ddpm) is recorded as coal reaches maximum fluidity. The maximum fluidity and plastic temperature range are used to predict the behaviour of the plastic phase of coal during carbonisation.

In order to understand the effect of coal properties on coal thermoplastic properties, it is imperative to understand the key aspects of coal composition. The formation of coal is the result of changes in the physical and chemical structure of an organic material deposit in response to temperature and time and is generally a consequence of thermal effects and compaction. Due to its heterogeneous nature, coal can be characterised by rank (vitrinite reflectance), maceral composition, thermoplastic property and chemical properties.

21 2.3.1 Coal rank and maceral compositions

Coal rank is an indication of coalification. A higher rank coal has normally experienced a longer period of coalification, which would be an indication of a higher carbon content

(%Cdaf). Therefore, the relationship between carbon content and coal rank is that the higher the carbon content the higher the rank of the coal, and vice versa. Figure 2.6 shows the differences of molecular structure in coal with varying rank (Kaneko et al., 2005). Medium rank coal generally contains volatile mater between 25% to 35%, whereas bituminous coal normally contains carbon content between 80% to 90%. (Speight, 2005). The mean vitrinite reflectance (Ro mean) of bituminous coal is often between 0.6% to 1.8%. Within this range, the maximum fluidity increases with increasing rank (Marsh, 1982). Lower rank coal, including lignite and subbituminous, as well as high ranked anthracite, is not easy to coke.

22 Figure 2.6. Structural differences in coal with varying rank (Kaneko et al., 2005).

On the other hand, Yoshida et al. (2000) proposed that maximum fluidity temperature (MFT) shifting to higher temperatures with increasing coal rank is caused by the increase in aromatic unit content. It was suggested that the MFT is dependent on the chemical structure of coal, which in turn suggests that coal fluidity itself is less rank-dependent. The authors pointed out that fluidity is more dependent on coal’s network structure, as well as the caking components. Macerals are the main constituent in coal. There are three main groups of macerals: vitrinite, liptinite and inertinite. According to van Krevelen (1993), liptinite turns into fluid when heated; inertinite shows no fluidity; and vitrinite’s fluid characteristic

23 stays in between liptinite and inertinite. The vitrinite group, including vitrinite, semi- vitrinite and exinite , are referred to as reactive macerals as they produce plastic mass in a temperature range of 330 °C and 360 °C and serve as a binder phase. On the other hand, inertinite generally exhibit negligible or no plasticity, and hence are referred to as inert macerals. Different maceral groups display different carbonisation behaviour due to differences in chemical and physical characteristics. In general, fluidity is controlled by the relative proportion of plastic components (vitrinite + liptinites) and inert components (inertinite + mineral grains) (Gray and Champagne, 1988).

Osborne (2013) has given similar descriptions from a chemical point of view. Vitrinite is moderately aromatic but with a significant amount of aliphatic side chains. Vitrinite can easily be broken down so that the aliphatic chains are released, leaving the remaining aromatic carbon-rich molecular structure to soften and swell. Inertinite is the most aromatic type which is more structurally ordered. Liptinite is predominantly aliphatic in terms of chemical structure, hence the extreme fluidity. Some studies (Yu et al., 2003, Kidena et al., 2002, Stanger et al., 2013) suggested that vitrinite-rich coals normally exhibit stronger exothermicity, greater extent of swelling, and better coke strength. In these studies, vitrinite-rich coals were found to display better swelling behaviours during heating in comparison to inertinite-rich coals and low vitrinite coals. In addition, two stages of tar and gas release events were observed, with the first one occurring prior to the swelling of coal, and the second one happening during the primary pyrolysis (Xie et al., 2013, Stanger et al., 2013, Tran et al., 2016).

According to Ryan (2000), fluidity increases with the amount of total reactive macerals in coal for a given rank, and it is most likely due to differences in the hydrogen levels of vitrinite grains of different coal rank. These findings are consistent with the theory proposed by Solomon et al. (1992), which explained the early-stage gas and tar release as a result of hydrogen bond disruption and non-covalent bond vaporisation at low temperature.

Inert macerals decrease the maximum fluidity as well as the plastic temperature range (Predeanu and Panaitescu, 2009). However, Kidena et al. (2002) found that the fluidity of coal was not significantly affected by the inertinites during the plastic temperature range,

24 explaining that it is due to inertinites’ larger aromatic cluster size, less substitution of alkyl- and oxygen functional groups in aromatic rings, and higher density of cross-linking. Coal fluidity influences the nature of interfaces between reactive and inert macerals (Barriocanal et al., 1996) and the size and shape of coke anisotropy (Lu et al., 1981).

Many other approaches and techniques to characterise the chemical nature of coal constituents have been attempted, but a clear correlation with the plastic properties is yet to be determined. Patrick and Walker (1989) found that the formation of oxygen cross-linkage during carbonisation limits the thermal depolymerisation, which consequently inhibits the development of suitable sized lamellae to maintain a fluidity system. Low rank coal does not form anisotropic carbon during softening as higher oxygen increases the reactivity resulting in isotropic structures (Marsh, 1992).

Extensive studies have been carried out describing the correlation between coke strength with the rank of parent coal. As shown in Figure 2.7, the CSR value reaches the maximum in the region of prime coking coal with mean reflectance of 1.2% to 1.3%, however falls towards coal of higher or lower rank (Nakamura et al., 1977). This change in the CSR could be attributed to the different coke texture formed, which has strong dependence on coal rank (Caldeira and Stainlay, 2002). However, some scattered points in this relationship suggest that rank, although being a dominant factor, is not the only factor that influences coke strength.

25 Figure 2.7. Relationship between vitrinite reflectance and CSR index (Nakamura et al., 1977).

In terms of coke mechanical strength, the abrasion strength (measured by M10 and I10 indices) and breaking strength (measured by M40 and I40 indices) improved as coal rank increased, except for those coals with poor plastic properties to allow bonding between the particles (Vogt et al., 1988).

Maceral compositions affect coke strength and reactivity as they control the type of texture formed in coke (Marsh, 1982, Mitchell and Benedict, 1983). Vitrinite is the main source of the anisotropic carbon phase in cokes. By using mercury porosimetry, Nishimura et al. (1996) found that coke matrix strength is increased with mean coal reflectance from 0.6% to 1.2%. Nishioka and Yoshida (1983) examined coal samples from Australia and Japan and found that micro-strength is highest when coal mean reflectance is around 1.2% and falls towards coal of higher or lower rank, which is similar to that of CSR vs reflectance. On the other hand, Gransden et al. (1991) had somewhat different results in their investigation of Canadian coal samples using microscopic techniques and free swelling tests. They found that larger amounts of vitrinite with reflectance > 1.1% produced cokes with much greater CSR than lower rank coal. The different results from Nishioka and Yoshida (1983) might be due to the different nature of the coal samples as well as the

26 methodology used. However, it seems likely that vitrinite in coal does have a pronounced effect on the strength of the coke product. Inertinite is responsible for the formation of isotropic components in the coke, and increases coke reactivity (Sakawa et al., 1982).

Increased presence of isotropic and fine mosaic texture increases coke reactivity with CO2 and hence decreases mechanical coke strength (Sharma et al., 2005).

2.3.2 Inorganic matter

Coal contains inorganic matter naturally, as it is an inevitable result of the coalification process (Ibarra et al., 1996). The pre-existing inorganic matter can be seen as inherent to the coal structure, and is closely related to the origin and coalification history of the coal (Rao and Gouricharan, 2016a). On the other hand, some studies have added inorganic salts into the coal blend to nullify the caking properties of coals during the gasification or liquefaction of coking coals (Speight, 2005). Such studies are relevant in flotation with the increasingly common practice of recycling process water with the accumulation of inorganic salts in the recycled process water (Gupta et al., 2012, Wang and Peng, 2014a). The following presents the effects of inherent minerals and added inorganic matter on coal thermoplastic properties.

Inherent minerals

The presence of inherent minerals normally has negative influences on coal quality, because the minerals do not exhibit significant calorific values nor plastic properties. These inherent minerals act as diluents that reduce the performance of a coal product, for both thermal use and coke production (Li et al., 2017a, Mahoney et al., 2002). In fact, the coal processing industry was developed to remove these diluents and therefore improve the carbon content in the coal product, a process often called coal upgrading or beneficiation (Riazi and Gupta, 2015, Rao and Gouricharan, 2016a).

The inorganic matter in coal is normally uniformly distributed and finely divided throughout the coal mass. Common minerals present in coal are quartz, kaolinite, illite, montmorillonite, smectite, chamosite, albite, siderite, pyrite, ankerite, dolomite, fluorapatite, calcite, gypsum, anhydrite, anatase, brookite and rutile (Mahoney et al., 2002, Quinn et al.,

27 2002, Grigore et al., 2007, Sakurovs et al., 2007). For coking coal, a large amount of mineral matter is undesired as the higher ash producing content associated with the mineral matter usually leads to a reduced plastic range, hence adversely affecting coke formation. However, the presence of minerals can occasionally benefit the expansion property in the coke oven (Wilson and Wells, 1950).

The inherent iron oxides affect coal fluidity, both physically and chemically. Physically, iron oxides can increase the total surface area of inert materials providing a “diluent” effect in the plastic phase while chemically they can catalyse the polymerisation reactions of the intermediate products to produce char at the expense of tar and light gases (Mochida et al., 1976, Cypres and Soudan-Moinet, 1980). In the case of low volatile matter bituminous coal, iron oxides (Fe2O3 and Fe3O4) decreased the thermoplastic properties but increased the solid yields (Khan and Jenkins, 1986a). Sulfur and Fe2O3 are reported to cause a large reduction of maximum fluidity (Price et al., 1992). Pyrite present in most coking coals is known to catalyse coke reactivity (Grigore et al., 2006, Gupta et al., 2008). The transformation of pyrite proceeds through the sequence of pyrrhotite, troilite and metallic iron (te Lindert and Timmer, 1991, Price et al., 1994). The decomposition of pyrite is controlled by temperature and the total sulfur gas pressure in the system (Hu et al., 2006).

In addition, oxides of minerals such as iron, calcium and potassium are known to catalyse coke reactivity (van der Velden et al., 1999, Price et al., 1994, Harris and Young, 1989, Vogt and Depoux, 1990, Kim et al., 2009). During carbonisation, the mineral oxides break down and react with other minerals to form new species in crystalline phase or in amorphous phases. Therefore, after carbonisation, these minerals do not exist as oxides in coke but rather occur in various mineral phases depending on the nature of coal mineral transformations during coking (Vogt and Depoux, 1990). Coke reactivity is influenced by the crystalline and amorphous phases such that the crystalline form is more effective catalysing the coke reactivity (Grigore et al., 2007). Coal minerals are classified based on their impact on the coke reactivity index (CRI), such that silicon bearing minerals including kaolinite, quartz and feldspars do not affect coke reactivity, while illite and montmorillonite moderately affect coke reactivity. Those carbonates, chlorides and iron bearing minerals have a catalytic effect on coke reactivity. Under an inert atmosphere, most minerals in coal 28 remain stable below 376.85 °C, while illite and montmorillonite begin to decompose at

396.85 °C (Kerkkonen et al., 1996).

Calcite is known to decompose during carbonisation and the lime derived from calcite exerts a strong catalytic effect on coke reactivity (Hermann, 2002). The decomposition of siderite forms metallic iron, magnetite and wustite (Gotor et al., 2000, Grigore et al., 2006). In addition, minerals were reported to influence anisotropic microtexture such that clays and pyrite decrease the size of anisotropic carbon in the surrounding regions, and therefore affect the coke reactivity (Gray and Champagne, 1988).

In recent studies by Li and colleagues (Li et al., 2017b, Li et al., 2018b, Li et al., 2018a), the effect of Na species on coal samples from Zhundong coalfield (Xinjiang, China) during pyrolysis were investigated extensively. The special characteristic of Zhundong coal is that it contains high concentrations of alkali and alkaline metallic (AAEM) species, with the most significant species being the Na species contributing 2% to 10% of the coal weight. According to another study (Zhang et al., 2013), the Na species in bituminous coal samples from Zhundong, China can be categorised into four forms: (1) the H2O-soluble form (e.g.

NaCl, Na2SO4, NaNO3); (2) the NH4Ac-soluble form which is usually associated with the carboxyl groups (i.e. -COONa) as part of coal’s organic structure; (3) the HCl-soluble form, which is covalently associated with –N or –O containing groups; and (4) the insoluble form associated with silicates or aluminosilicates. Li et al. (2018a) used three types of coal samples, including raw coal, water washed coal (by deionised water), and HCl washed coal, to conduct pyrolysis experiments and a subsequent thermal analysis coupled with mass spectrometry equipped with skimmer-type interface and with electron ionisation (EI) and photoionisation (PI) capability. The raw Zhudong coal produced 16 kinds of volatile gases as shown in Figure 2.8. The water-washing and HCl-washing did not affect the total pyrolytic yield significantly, but significantly altered the compositions and yields of volatile gases. The larger aromatic compounds of C7H8O, C9H12 and C10H14 disappeared as a result of the water-washing and HCl-washing pretreatments removing the H2O-soluble and HCl-soluble Na species. The HCl-washing significantly reduced the rate of production and total yield of volatile gases including H2, CO, CO2 and aromatic C6H6O and C8H10, but greatly improved the production of C3H6. Similar results were observed in Li et al. (2017b), 29 where the volatile gases generated during pyrolysis were closely related to the modes of sodium species that exist in the coal sample. The H2O-soluble Na was found to have a catalytic effect on the thermal-cracking reaction of the coal and volatilised more easily, whereas the exchangeable sodium species were more likely to form chars (i.e. infusible solids) through pyrolytic reactions.

Figure 2.8. 3D mass spectrum graph of raw coal plotted against temperature and mass-to- charge ratio (m/z), measured by using (a) electron ionisation and (b) photoionisation methods (Li et al., 2018a).

30 Sulfur exists in all coal in two forms: inorganic compounds (pyrite and sulfates) and organic matrix (Ibarra et al., 1996). Some previous studies (Clark et al., 1984) pointed out that the effect of sulfur content on coal fluidity during the carbonisation process can be significant.

According to Mochizuki et al. (2013), the sulfur content in coal composition may influence coal fluidity, as their analysis on coal maximum fluidity using the high-temperature in-situ 1H-NMR method showed a positive correlation between maximum fluidity and mobile components in sulfur-rich coal. This finding is consistent with the relationship between transferable hydrogen and coal fluidity discovered earlier (Kidena et al., 1996, Yokono et al., 1987) that the more the amount of transferable hydrogen, the higher the maximum fluidity. As sulfur is a hydrogen acceptor in nature, and coking coal is high in mobile hydrogen donors, it is hypothesised that the sulfur compounds may react with the hydrogen compounds and form H2S (Senftle and Davis, 1984, Rhoads et al., 1983, Mochizuki et al., 2013).

Mochizuki et al. (2013) suggested that sulfur has a significant impact on coal fluidity. They found that the maximum fluidity drastically decreased (from 145 ddpm to 22 ddpm) when elemental sulfur was added to the coal sample. They also noticed that the sample with the highest sulfur content displayed the lowest maximum fluidity, suggesting that the sulfur content in coal may work as an inhibitor for fluidity.

Inorganic matter addition

Some studies used inorganic salt additives in their carbonisation or flotation investigations which explored the interaction between coal and the additive and the consequent effects on coal fluidity (Patrick and Shaw, 1972, Marsh and Walker, 1979). Patrick and Shaw (1972) carried out extensive research on the effects of Na2CO3 on the softening behaviours of two British bituminous coal samples during the carbonisation process. They found that, when 5 wt. % of Na2CO3 was mixed with the coal samples, the fluidity reductions for were over 90%, from 275 ddpm to 15 ddpm and from 3960 ddpm to 240 ddpm. However, the addition of Na2CO3 did not have any effect on the temperature at which the maximum fluidity was reached, even though the plastic range was shortened. It was proposed that the underlying 31 mechanisms involved in such fluidity change could possibly be either the stabilisation of cross-links due to the addition of Na2CO3, where the cross-links are generally broken by the heat treatment to restrict the mobility of coal molecules; or the Na2CO3 provided alternative sources of cross-links that compensated for the broken cross-links from coal itself during heating. In either case, it seems that the addition of Na2CO3 chemically altered the macromolecular structure of coal during heat treatment and hence reduced fluidity. However, Patrick and Shaw (1972) did not conduct further experiments to investigate the effect of Na2CO3 on the macromolecular structure of coal, hence the exact effect of Na2CO3 on coal fluidity and the changes that it may bring to coal macromolecular structure remain unclear.

Some additives such as alkali salts have the ability to catalyse dehydrogenation which reduces the availability of transferable hydrogen in the coal and plasticity (Marsh and Walker, 1979). Crewe et al. (1975) observed a significant reduction in the coal plasticity (i.e. free swelling index) following the treatment of the samples using a 0.5 M NaOH solution. The treated coals were considered to be “decaked” as most of the thermoplastic properties were lost due to the NaOH treatment, leaving the coals almost non-coking. Subsequent “washing” experiments, where the samples were rinsed vigorously with deionised water to remove any residual NaOH, showed that the plasticity could almost be restored to its original state. Such high restorability of the plasticity strongly suggested that the salts may be attached to the coal surface predominantly by retention instead of through a chemical reaction.

Chiu and Hong (1985)’s investigation on the effects of Fe2O3 and K2CO3 on coke reactivity found that the microstructure of coal was not affected by the addition of Fe2O3 and K2CO3 while degradations on coke reactivity index (CRI) and coke strength after reaction (CSR) occur. It was the catalytic effect of Fe and K derived during pyrolysis that caused these degradations. Equations 5-10 below indicate the mechanisms of Fe2O3 and K2CO3 catalysis during the heat treatment. Equations 5 and 6 explain the cyclic oxidation-reduction mechanism of Fe-catalysed CO2 gasification (Laine et al., 1963); Equation 7 was considered as the main mechanism of Fe-catalyst formation during coal carbonisation (Chiu

32 and Hong, 1985); and Equations 8-10 explain the mechanism of K2CO3 catalysis in the CO2 gasification of coke (McKee and Chatterji, 1975).

5 xCO2 + yFe = FeyOx + xCO

6 FeyOx + xC = yFe + xCO

1000 °C 7 Fe2O3 + Ccoal → Fein coke + CO2

Carbonisation 8 K2CO3 + 2Ccoal → 2Kin coke + 3CO

1080 °C 9 2Kin coke + CO2 → K2Oin coke + CO

1080 °C 10 K2Oin coke + Ccoke → 2Kin coke + CO

Based on the mechanisms indicated by Equations 5-10, it can be seen that the addition of

Fe2O3 and K2CO3 will lead to cyclic oxidation-reduction reactions. The sources of carbon

(C) from the CO and CO2 gas evolutions during coal pyrolysis are mainly carbon in coal, which indicates that the carbon in coal would be reduced due to the cyclic reactions. Patrick and Shaw (1972) noted that when the temperature reaches about 800 °C the Na2CO3 additive would react with carbon and yield metallic Na and CO gases, where the Na is then completely volatilised at 1000 °C.

2.3.3 Oxidation

Oxidation can be critical to coal behaviour in many production and end-use processes. This phenomenon can and often does change coal’s calorific value, beneficiation, coking, liquefaction and gasification characteristics (Cox and Nelson, 1984). Upon coal oxidation under laboratory simulation at temperatures greater than 100 °C, carboxyl, carbonyl, ether and phenolic groups tend to form (Perry and Grint, 1983, Rhoads et al., 1983). However, when the temperature is below 70 °C, the oxidation mechanism can be quite different (Yohe, 1958).

33 Wu et al. (1988) found that mild oxidation does have some negative impacts on coal thermoplasticity. The results obtained from the Gieseler plastometer demonstrated the most significant response from coal maximum fluidity to weathering time. The maximum fluidity of coal samples showed significantly different behaviours at different temperature of weathering. At 80 °C, the maximum fluidity decreased rapidly within 50 days of weathering. Furthermore, it was reported that the maximum fluidity dropped below 1 ddpm after 50 days, suggesting nonexistence of testable fluidity. The maximum fluidity also showed significant but smaller decreases for the samples at 25 °C and 50 °C over the weathering times of 313 days and 268 days respectively. However, the maximum fluidity did not show significant changes over the first 120 days of weathering at 25 °C. Similarly, Jha et al. (2014) found that weathering, over a 2-month period, impacts both the free swelling index and pH of coal, suggesting a negative effect of weathering on coal’s thermoplasticity.

Clemens et al. (1989) conducted extensive research on the effect of mild oxidation on coal fluidity on a molecular basis and found that mild oxidation can be destructive to coal thermoplasticity, as oxidation depletes the supply of transferable hydrogen from C-H bonds which are the main hydrogen donors in coal (Senftle and Davis, 1984, Rhoads et al., 1983). Rhoads et al. (1983) examined the effect of low-temperature short-term oxidation on a high fluidity coal. The Gieseler plastometer test result for the coal sample oxidised for 48 hours at 140 °C indicated that oxidisation under such conditions can completely eliminate coal fluidity; whereas the same degree of fluidity loss takes a longer time for samples oxidised at 60 °C. FTIR analysis showed that the aliphatic C-H groups in the coal samples decreased with increasing oxidation time; while no significant change in the aromatic region was observed. The aliphatic C-H losses were significant at the initial stage of oxidation for samples oxidised at both 60 °C and 140 °C, and these changes correspond closely to the Gieseler fluidity measurements, indicating a correlation between fluidity reduction and loss of aliphatic C-H groups. In addition, a small proportion of the samples oxidised for 16 days at 140 °C were soaked with 1 N NaOH for one night, and the FTIR measurements on these NaOH treated samples showed a significant band shift (from 1690 cm-1 to 1575 cm-1)

34 compared to the untreated oxidised samples, strongly suggesting the formation of carboxylic acids (-COOH) during oxidation.

Joseph and Mahajan (1989) investigated the effect of weathering on coal aliphatic structure by using acid-catalysed transalkylation of coal with phenol and boron trifluoride and infrared spectroscopic monitoring. The concentration of aliphatic cross-links decreased during oxidative weathering, where the infrared analysis indicated that carbonyl groups were produced during weathering. Therefore, the authors suggested that the aliphatic cross- links were oxidised to carbonyl and/or carboxylic acid groups. Joseph and Mahajan (1989) gave an explanation of the possible oxidation mechanism during weathering: the transformation from methylene groups to carbonyl groups might involve autooxidation at the benzylic position, which can be initiated by the abstraction of a benzylic hydrogen atom by free radicals that were already in the coal and can cause the benzylic radical to react with oxygen and produce peroxy radicals. The peroxy radicals can then abstract hydrogen atoms from other molecules in the coal to form hydroperoxide, which can produce carbonyl compounds under various reactions.

Loison et al. (1963b) found that low rank and oxidised coal display significant less plasticity. For high rank coal, the metaplast formation increases and reaches its maximum at first, and then decreases as the hydrogen content is depleted. These observations suggest that coal plasticity development during pyrolysis is closely related to oxygen and hydrogen content in the parent coal. Senftle and Davis (1984) found that the thermoplastic properties of coal are extremely sensitive to oxidative weathering. Organic constituents in coal react with oxygen at ambient temperature, which leads to an increased oxygen content and decreased atomic hydrogen-to-carbon ratio of the coal (Schmidt et al., 1940, Brooks and Maher, 1957, Cox and Nelson, 1984).

The oxidation of organic constituents in coal has been well studied. Peroxides were found to be transient intermediates in the early stages of coal oxidation (Jones and Townend, 1949). The decomposition of the peroxides leads to the formation of oxygen-containing functional groups such as hydroxyl, carbonyl and carboxyl groups, and ether linkages were detected from wet chemical and spectroscopic studies. Great uncertainty remains on the

35 mechanisms of chemical structural changes of coal during oxidation. The oxygen molecule has two unpaired electrons, one on each oxygen atom forming a diradical (•O-O•). Cox and Nelson (1984) suspected the effectiveness of direct oxidation by exposing the coal surface to oxygen, as its diradical structure is too unreactive to oxidise the coal. However, since coal already contains carbon radicals, it is possible that the molecular oxygen and carbon radicals are from peroxy radicals (Ccoal-OO•) (Yokono et al., 1981, Essenhigh, 1981, Cox and Nelson, 1984). These peroxy radicals act as an initiator for free radical chain reactions which produce peroxides and their decomposition products. On the other hand, water, as a protic solvent, is usually present in coal, which might also serve as a source of oxidation on the coal surface (Crelling et al., 1979, Cox and Nelson, 1984). The decomposition of intermediate peroxides (formed from oxygen in water and carbon in coal) might also occur via ionic reaction mechanisms (Davies et al., 1976).

Previous research found that the thermoplastic properties of bituminous and subbituminous coal are often altered long before changes can be detected in the coal’s chemical composition (Marchioni, 1983, Cox and Nelson, 1984). The narrowing of fluidity range and reduced fluidity in oxidised coal was only detectable when heated (Maloney et al., 1982, Marchioni, 1983). It was suggested that the loss of thermoplastic properties is caused by the existence of highly cross-linked macromolecular structures which do not easily melt and flow upon heating.

Plasticity development during carbonisation is essential to produce metallurgical coke of good quality. Several parameters affect coal plasticity. Loison et al. (1963b) and Senftle and Davis (1984) found that the thermoplastic properties are extremely sensitive to oxidation. Neavel (1982) reported a sharp decrease in the fluidity of coal oxidised at mild temperatures (from 80 °C to 150 °C) over a period of up to 6 months. Such decreases could be related to the changes in the coal surface after oxidation, where the vitrinite reflectance was found to be sharply reduced (Goodarzi and Murchison, 1973, Goodarzi et al., 1975, Neavel, 1982). Previous studies suggested that oxidation causes an increase in the reactive oxygen groups on the coal surface, such as the hydroxyl group (-OH), carboxyl group (- COOH) and carbonyl group (C=O) (Szladow and Ignasiak, 1976, Wachowska et al., 1979). In addition, oxidation-induced decreases of aliphatic, alicyclic carbon and hydrogen content 36 were also observed (Ignasiak et al., 1972, Mazumdar and Chatterjee, 1973). Such changes limit the availability of a hydrogen donor source to produce free radicals during carbonisation. The decrease in fluidity found in Neavel (1982) seems to be largely attributed to the increased concentration of oxygen content and the reduced transferability of hydrogen as explained by other authors (Szladow and Ignasiak, 1976, Mazumdar and Chatterjee, 1973, Ignasiak et al., 1972, Wachowska et al., 1979).

On the other hand, Painter et al. (1981) proposed that the ester cross-links form during the condensation of aromatic carboxylic acids and the phenolic groups generated during the carbonisation process are responsible for the deterioration of coal thermoplastic properties. Joseph and Mahajan (1989) found that the concentrations of aliphatic cross-links decrease with oxidation time while the fluidity, as measured by the Gieseler plastometer, also decreases. The decrease in coal fluidity after oxidation could be attributed to the formation of hydroperoxide during the oxidation process due to the abstraction of hydrogen atoms onto the peroxy radicals produced from oxidation of the benzylic radicals that were already present in the coal. It should be mentioned that most oxidation studies were carried out using traditional or modified laboratory oxidation procedures including laboratory drying and low-temperature oxidation (temperatures below 150 °C), which are a means of exposing coal to oxygen with oxygen acting as an oxidising agent under different temperatures (Pisupati and Scaroni, 1993, Mastalerz et al., 2009). Table 2.2 below provides a summary of findings from research discussed in this section.

37

(Perry and [6]

,

d spectroscopy (IR)

(Gethner, 1987a) ray photoelectron spectroscopy (XPS) - Analytical method Titration with ferrous thiocyanate solution Infrare Fourier transform infrared spectroscopy (FTIR) Fourier transform infrared spectroscopy (FTIR) Fourier transform infrared spectroscopy (FTIR) X , [5]

200 °C - 180 °C 140 °C - - (Rhoads et al., 1983) Temperature 50 140 Ambient °C 60 100 °C 100 °C , [4],

(Liotta 1983)et al., [3] volatile bituminous

Coal type Bituminous and anthracite Bituminous High High volatile bituminous High volatile bituminous Bituminous Formation of surfacecoal functionalgroups subjected to oxidation. . 2 . 2

[5] ) 1 - ) Table 1 (Adams and Pitt, 1955) - [2]

, [2] ) [4] 1

- ) 1

-

) ) 1

) 1 [1]

- - 1

- ) )

[6] 1 1

) 1150 cm ) - -

1 - 1 ) [3] - - 90, or ~1020 cm 1 ) )

- ) 1 1 - - 1 - 3000 cm - H (1300 - (Jones and Townend, 1945) O and O - Identified functionalgroups Peroxygen (peroxides) Phenol (3500 Carbonyl (1690 cm C Phenol (~3400 cm Ether (~1100 cm Anhydrides (~1840 cm Esters (~1770 cm Carboxyl (~1690 cm Carboxylic acid (~1320 cm Carbonyl (~1670 cm Ketones (~1725 cm Esters (~1750 cm Aldehydes (~1600 cm Phenolic carboxylic acids (~1400 cm Ethers (~1260, ~10 Ether hydroxylor (286.2 eV) Carbonyl (288 eV) Carboxyl (289.2 eV) [1] Grint, 1983)

38 2.3.4 Cross-linking

The cross-linking reactions in coal during pyrolysis have been extensively studied. As a highly cross-linked polymer, coal consists of a number of stable fragments connected by relatively weak cross-links. These cross-links are affected by a number of factors including coal rank, heating rate, functional groups, minerals and oxidation (Butterfield and Thomas, 1995, Deshpande et al., 1988, Solomon et al., 1990). Cross-linking reactions are also crucial to coal conversion as they control the ultimate tar yield, the tar molecular weight distribution, and char’s fluidity, molecular order, surface area and reactivity (Solomon et al., 1990).

Effect of oxidation and surface groups

Coal has a heterogeneous structure, as it can be considered as a matrix of hydrocarbons that contains various functional groups such as carboxyl groups (-COOH), hydroxyl groups (-

OH) and methyl groups (-CH3). Besides, the surface of coal consists of mineral matter and pores as a result of coalification. Figure 2.9 shows a schematic representation of the coal surface (Solomon et al., 1990).

Figure 2.9 Schematic representation of coal surface (Laskowski, 2013).

Previous studies have suggested that oxidation increases cross-linking reactions in coal during heat treatments (Deshpande et al., 1988, Solomon et al., 1990). Solomon et al. (1990) observed early cross-linking reactions and lower tar evolutions in oxidised coal samples at temperatures above 300 °C. The reason is believed to be that oxidation increased oxygen content in coal, and therefore increased the concentration of oxygen functional groups (carboxyl and hydroxyl). Similar results were reported by Ignasiak et al. (1972). The higher 39 concentrations of carboxyl groups in oxidised coals encourage early cross-linking reactions during pyrolysis, hence fluidity reduction is observed in oxidised coal. On the other hand, based on the four coal samples tested, Ibarra et al. (1991) found that coal with low concentrations of carboxyl and hydroxyl groups tends to start cross-linking above 400 °C, while the coal containing high concentrations of carboxyl and hydroxyl groups shows significant cross-linking prior or at 400 °C. However, because tar and methane are also released above 400 °C, the authors failed to identify the main reason for increased cross- linking reactivity.

Effect of coal rank

Coal is ranked based on the proportion of each composite it contains, such as volatile matter, ash and carbon (O'Keefe et al., 2013). The relationship between cross-linking in coal during pyrolysis and the rank of coal has always been of research interest. This section discusses previous studies and their findings on the dependency of cross-links on coal rank.

For oxidised or low rank coal, which contains high carboxyl groups, a large amount of CO evolution at low temperatures was observed. Such evolution indicates the early cross- linking between oxygen and carbon in carboxyl group-rich coal, which is essentially low rank or oxidised coal (Deshpande et al., 1988). On the other hand, it was reported that cross-linking reaction is rank-dependent (Solomon et al., 1990). The low rank lignite was observed to show early cross-linking as the temperature reached 200 °C, which is much lower than that of bituminous coal. The higher rank bituminous coal only showed slight increases in cross-linking reactions at the beginning of the heat treatment, and no more increases were observed until 400 °C was reached. This behaviour was attributed to the breakup of the covalent bonds during the developments of tar evolution, fluidity and rapid liquefaction. In addition, primary weight losses of bituminous coal with high volatile matter during pyrolysis occurred at temperatures where the cross-link densities were low, hence the volatiles were less restricted to be released and more weight losses were observed. On the other hand, Butterfield and Thomas (1995) found that the weight losses for coal with lower volatile matter were due to the fewer volatile contents and more cross-linked structures in low rank coal.

40 Solomon et al. (1990) indicated that there are at least two distinct cross-linking events: (1) low rank coal exhibited cross-linking reactions prior to the bridge-breaking reactions at low temperatures; (2) high rank coal exhibited cross-linking reactions as a result of bridge- breaking reactions at moderate temperatures. However, Ibarra et al. (1991) pointed out cross-linking is not dependent on coal rank, since early cross-linking reactions do not necessarily occur for low rank coal, and it is the type, amount and contents of the functional groups that determine the cross-linking reactivity.

Cross-linking and coal thermoplasticity

A decrease in cross-linking reactions could lead to increased fluidity as evident from the work of Deshpande et al. (1988). The authors showed that fluidity of the lignite char sample was almost non-existent when heated at 600 °C/s, whereas the same sample heated at a 20000 °C/s heating rate exhibited signs of more significant fluidity development, such as pore development and swelling. The reason an increased heating rate can enhance fluidity development is that cross-linking reactions were minimised when coal is heated with an extremely high heating rate (rapid pyrolysis).

Butterfield and Thomas (1995) conducted several experiments to investigate the changes in coal macromolecular structure during heat treatment with four coal samples. Four experiments including dilatation, carbonisation, solvent swelling with pyridine, and gas permeability were undertaken, where the gas permeability measurements were taken during dilatation using a modified dilatometer. The setup enabled measurements of gas permeability while coal was transforming in the dilatometer, hence providing results that are consistent with the dilatometric measurements in terms of temperature. The cross-link density started decreasing around the initial softening temperature until the caking point (maximum fluidity) of coal was reached, and then it started increasing as the heating continued until about 575 °C when the experiments ended. The changes in cross-link density showed identical trends with the trends recorded in changes in gas permeability.

41 2.3.5 Coal blend and additives

Blending of coal

Coal blending is often practised in the thermal coal industry to reduce environmental impacts by adding biomass or plastic wastes to coal charge (Vivero et al., 2005, Shih and Frey, 1995, Sami et al., 2001). On the other hand, sufficient supplies of good coking coal have always been a challenge in certain countries (Mitchell, 1984, Graham et al., 1999, Dıez et al., 2002, Kiselev and Liskovets, 2007, Miura et al., 1981). In those countries, it is necessary to use a mixture of high volatile coal and low volatile coal due to the limited supply of good coking coal (Park et al., 2010, Marsh and Neavel, 1980, Dıez et al., 2002, Nomura et al., 2004, Vasko et al., 2005). However, because the characteristics of coal vary significantly by origin, most of the findings from previous investigations are not quantitatively representative, and a general rule to create a coal blend to produce coke with certain characteristics cannot be established due to the great variety of coal (Dıez et al., 2002, Graham and Wilkinson, 1978). Nonetheless, a general understanding on the effect of coal blending has been obtained from decades of research. When a high volatile coal, H, is co-pyrolysed with a low volatile coal, L, the strength and hardness of the coke yield is dependent on the proportion of the low volatile coal in the coal blend. As the proportion of L increases from 0% to 100%, the strength and hardness of the coke produced increases rapidly up to 20% to 30% and then more slowly, until reaching the 50% to 60% mark where the coke strength stops improving. Another interesting finding is that if a different L coal is added to the same H coal, the quality of coke varies greatly with the characteristics of the L coal. Therefore, it was recommended that it is important to select the most suitable L coal for each type of H coal (Loison et al., 2014, Loison et al., 1963a). Kumar et al. (2008a) found that adding non-coking coal to hard coking or semi-coking coal drastically decreased their plasticities. The significant reduction was due to the less reactive components and more inertinite in non-coking coal, reducing the overall reactivity in the coal blend. The maceral composition of the non-coking coal sample used by Kumar et al. (2008a) consisted of 33% inertinite, which is approximately 10 times more than that of all other coking coal samples; and the vitrinite content was only 55%, which is approximately a third less than that of all the coking coal samples. Since other compositional contents are 42 almost identical between the non-coking coal and coking coal, the coal blend’s overall aliphatic contents (C-H bonds) would be decreased after the addition of non-coking coal, therefore less transferable hydrogen was supplied for fluidity development of the blends (Senftle and Davis, 1984, Rhoads et al., 1983).

Adding different plastic wastes as additives for coal carbonisation decreased coal fluidity to different degrees (Gayo et al., 2016). The correlation was linked to the effect of low molecular weight species on coal fluidity. It is suggested that the plastics worked as inhibitors to coal fluidity development. The low molecular weight species produced during the decomposition of plastics react with oxygen functional groups in coal, causing the decrease in coal’s ability to become fluid within the expected range of softening temperatures, and therefore a shift of coal’s softening temperature to a higher range. The findings of Gayo et al. (2016) are in line with the results from previous research on the effect of organic sawdust on coal’s thermoplasticity (Diaz-Faes et al., 2007), which demonstrated that the extent of the reduction of coal fluidity is dependent on the thermoplastic properties of the coal, and the molecular structure, the thermoplastic behaviour and the hydrogen-donor and hydrogen-acceptor capacities of the additives.

Addition of inert substances

The addition of inert substances in coal charge influences both the thermoplastic properties of coal and the formation of semicoke. There are some materials that naturally exist in the coal structure, for example mineral matter and inertinite, a non-plastic maceral. The addition of external inert substances is generally carried out intentionally to improve coke quality or for research purposes. Common practice in the industry uses coke breeze, lean coals, semicoke or anthracite as the inert additive, also called “inerts”. The addition of any of these inerts will reduce the fluidity, free swelling, dilatation measurements, and caking indices of the coal. The degree of change in the thermoplastic properties varies with the type of the inerts and the type of coal. However, an inert additive, at a certain concentration, can completely eliminate the thermoplastic properties of a coal where the concentration is dependent on the type of coal. The effect of the inert additive on coal softening is greater when its particle size is finer (Nandi et al., 1977, Kök et al., 1998, Duffy et al., 2010) and/or

43 when its porosity is higher (Georgiadis and Gaillard, 1954, Bridgman, 1952, Kaiho and Toda, 1979).

The mechanism of an inert additive reducing the thermoplastic properties of a coal was discussed by Dorman et al. (1957). It was pointed out that the inert additive neither softens nor swells during the co-carbonisation process, hence increasing the viscosity of the swelling coal mass and decreasing the overall swelling. In other words, the plasticity of the coal mass is diluted by the non-plasticity of the inert additive. A mathematical approach was developed and Equation 11 was used to explain the relationship between the swelling of coal mass and the swelling of the coal-inert mixture. 훿 and 훿0 refer to the volumetric expansions of the coal-inert mixture and the coal mass respectively, whereas 푥 represents the percentage volume of inert additive in the mixture.

x 11 δ = δ (1 − ) 0 100

However, the actual expansion observed was less than the calculated value. Therefore, the proposed mechanism that inert additives work as a diluent of thermoplasticity in the mixture might be inadequate or insufficient at least. The equation might be an oversimplification as the impacts from the porosity, type and particle size of the inert additive were not considered. For certain types of inert additives, their chemical natures play a significant role in the thermoplastic properties of the coal-inert mixture. Iron oxides, such as hematite or limonite, are reducible at low temperature which markedly reduces the fluidity of a coal in a plastometer as well as the free swelling properties due to their ability to induce the oxidation of coal during its plastic region (Khan et al., 1988, Khan, 1989). The exception is magnetite, which only reacts with coal above the resolidification temperature. At temperatures below the resolidification temperature magnetite behaves like an infusible inert such as quartz (Barking and Eymann, 1952).

2.3.6 Techniques for coal thermoplasticity examination

The thermoplastic properties of coal can be examined in numerous ways by using different apparatus (Loison et al., 1963a). Tests such as fluidity, free swelling and dilatation were

44 devised to enable the thermoplastic behaviour of coal to be characterised by numerous indices that are not generally physical constants but serve as reference points (Gieseler, 1934, Brewer, 1945, Audibert, 1926, Loison et al., 1963a). Microscopic techniques are often used to examine coal thermoplastic properties as well, and the results from microscopic observations are sometimes combined with results from other tests to establish relationships between the thermoplastic behaviours and the observed structural changes (McCartney et al., 1971, Ergun et al., 1959, Maroto-Valer et al., 1998, Vivero et al., 2005, Duffy et al., 2010). In this section, the different techniques for the examination of coal thermoplastic behaviours are discussed and related to the present study.

Free swelling test

In free swelling tests, a small fraction of finely powdered coal is placed in a standardised crucible or tube without any external compaction and heated at a fixed heating rate of no less than 300 °C/minute until a temperature of 800 °C is attained. The result from a free swelling test is recorded as free swelling index (FSI), also known as the crucible swelling number (CSN) (American Society for Testing and Materials, 1999). As shown in Figure 2.10, the FSI measurements range from 9 to 0 with intervals of 0.5, where a FSI of 9 indicates a strong swelling behaviour and 0 indicates that the sample does not have any swelling property. If a coal is fusible, the particles soften and fuse together as the temperature increases. The only resistance that the swelling mass will meet is the walls of the crucible or tube (Loison et al., 1963a). If a coal is infusible under the heating condition used in the test, it distils without undergoing a physical change and remains in powdered form. Such behaviour will mark the coal with an FSI of 0 which indicates the sample is non-swelling. Generally, coal with an FSI below 2 is considered not suitable for coke production due to its poor swelling property. However, if a coal has an FSI greater than 8, it generally needs to be blended with lower fluidity coals, as the coke produced will be too weak to provide sufficient support in the blasting furnace in future uses (Gupta et al., 2005).

45 Figure 2.10. Standard chart for FSI (or CSN) profiles (American Society for Testing and Materials, 1999).

However, the final product remaining in the crucible from the free swelling test is the least coke-like product compared to the residues from plastometer and dilatometer tests (Reifenstein et al., 2000). The free swelling test residuum has much higher porosity than the coke oven product, and it also exhibits morphological elements that are more similar to char derived from drop tube furnaces rather than coke ovens. These changes are mainly caused by the extreme high heating rate in free swelling tests, as the high heating rate changed the rate of volatiles evolution and pore developments. Therefore not only is the macromolecular structure of coal altered, but also the mechanisms of coal-to-coke conversion (Reifenstein et al., 2000). Free swelling measurements are sensitive to such factors as oxidation, heating rate and the excess of fine coal in the analysis sample (Chelgani et al., 2016). These factors would contribute to errors in the FSI determination because they influence the composition (mainly the oxygenated groups) of the coal sample and cause variations in the coal’s thermoplastic behaviours (Chelgani et al., 2011, Khorami et al., 2011, Speight, 2005).

46 Dilatation test

In dilatation tests, a dilatometer is used to monitor the thermoplastic behaviour of coal during heating. There are two phenomena of interest in dilatation tests: (1) the softening and swelling, resulting in the formation of semicoke and taking place in the temperature range 300 °C to 500 °C; and (2) the behaviour of semicoke above the resolidification temperature (Fu et al., 2007). However, while most dilatometers were designed to be used for low temperature ranges (<600 °C) (Loison et al., 1963a), the Chévenard-Joumier dilatometer is capable of achieving temperatures up to 1000 °C (Lemaitre and Delmon, 1977).

Among the many types of apparatus, the Audibert-Arnu dilatometer is the most common one used. Figure 2.11 shows the schematic design of the Audibert-Arnu dilatometer (Australian Standards, 2002) . According to the Australian standard (Australian Standards, 2002), about 2 g of pulverised coal sample is mould-compressed and slightly tapered to form a cylindrical pencil shape (diameter = 6.5 mm) and placed in the retort tube (inner diameter = 8 mm). A piston (diameter = 7.8 mm) is inserted in the retort tube above the “pencil”. The piston is connected with a dilatation pointer moving along a graduated scale. The weight of the piston and the dilatation pointer together is about 150 g. The tube is placed in an electrical furnace containing a salt bath or aluminium block to ensure uniform temperature.

47 Figure 2.11. Schematic design of an Audibert-Arnu dilatometer (Australian Standards, 2002).

The initial softening temperature in the Audibert-Arnu dilatation tests is poorly defined (Griffin and Storch, 1937). The inaccuracy is attributed to the sample-loading mechanism.

48 Because a gap of approximately 1.5 mm between the “pencil” and the wall of retort tube is intentionally kept, the swelling coal sample tends to expand laterally and fill the retort tube first rather than expanding upwards to push the piston. Therefore, the initial softening temperature recorded by the Audibert-Arnu dilatometer is higher than the actual initial softening temperature of the coal sample. Similarly, Reifenstein et al. (2000) reported that the Audibert-Arnu dilatometer tends to show a lower resolidification temperature compared with the Gieseler plastometer test results on the same coal sample, due to the force exerted on the sample from the piston of the dilatometer. That is, the mass of the piston applies a force on the expanding coal sample that may be equal to the expanding force from the coal sample onto the piston.

On the other hand, all dilatation tests involve high confined pressures of up to 290,000 kPa because the samples must be mould-compressed to minimise the inter-particle voids in order to accurately measure the expansion and contraction of the sample. However, a high coking pressure would cause the coal to behave differently than a lower pressure (Roberts et al., 2003). The high pressure restricts the escape of the gases evolved during the thermoplastic stage. The gases are trapped within the plastic layer (i.e. the molten coal) and form micropores in the semicoke formed. Thus, the final product from the dilatation is rather a different material than what is produced by a typical coke oven and the dilatation result might not be precisely representative of how the coal actually behaves during pyrolysis.

Plasticity test

Plasticity tests use a plastometer to study the resistance on the rotation of a movable unit in the middle of a mass of pulverised coal subjected to a specified heating rate. There are two types of plastometers: constant-torque plastometer and variable-torque plastometer as shown in Figure 2.12 (Loison et al., 1963a). A typical constant-torque plastometer is the Gieseler plastometer (Gieseler, 1934), where the movable unit is subjected to a very weak constant torque about 100 g/cm, and the rotational speed of the unit is measured as an indication of the fluidity of the coal sample. The Gieseler plastometer test measures coal fluidity in dial division per minute. According to the Australian standard (Australian

49 Standards, 1996), 5 g of coal fines (-40 mesh) is compacted into a cylindrical capsule (diameter = 1~2 cm, height = 2~4 cm) equipped with a stirrer where the axis of the stirrer is subjected to a constant torque as discussed earlier. The coal sample is heated to 300 °C rapidly first and then a constant heating rate of 3 ± 1 °C/minute is applied for 10 ± 2 minutes. Before the softening of coal starts, the movement of the stirrer is blocked by that compacted coal sample, so the rotation only begins when the coal reaches certain plasticity (Gieseler, 1934). The Gieseler plastometer test is able to provide the changes in coal fluidity from the initial softening stage to the end of the resolidification stage of coal.

Figure 2.12. Schematic design of a Gieseler plastometer (Australian Standards, 1996).

50 The Gieseler plastometer only measures the fluidity when coal starts to soften without decomposition and release of volatiles (Reifenstein et al., 2000). Once the coal sample starts to swell, i.e. softening with decomposition, the stirrer arms push the materials in their path away, either upwards, downwards or outwards, leaving voids behind the paths, and the Gieseler plastometer tends to measure the amount of material that is not in the path of the stirrer arms instead of measuring the viscosity of the material. During the test, less expansive samples produce denser froths while swelling, creating more resistance for the movement of stirrer arms; vice versa, more expansive high volatile samples, due to the thinner pore walls and greater amount of pore structure generated, create less resistance for the stirrer arms to rotate.

Davis (1931) developed the first variable-torque plastometer and described its mechanism. In the Davis plastometer (Figure 2.13), 8 g of -20 to -40 mesh coal is placed in a horizontal cylindrical steel retort (diameter = 22 mm, length = 127 mm) rotating at a constant speed of 2 rpm driven by a synchronous motor. A shaft with rabble arms, fixed to the apparatus by means of a spring of a very slight weight, is located inside the retort. The deformation of the spring enables the measurements of torque acting on the shaft. Similar to the Gieseler plastometer, the retort is placed in a furnace and rapidly heated (+7 °C/minute) to 375 °C, and then the heating rate is maintained at 3.4 °C ± 0.2 °C/minute. The torque (kg/cm) acting on the spring is measured continuously and plotted as a function of temperature.

Figure 2.13. Setup for Davis plastometer (Fieldner et al., 1931). 51 2.4 Process water chemistry and coal thermoplasticity

Water scarcity is one of the leading challenges in the mineral and coal industries. A shortage of fresh water supply is forcing many coal plants to find alternative sources of water (Wang and Peng, 2014a). To overcome this challenge, a strategy of recycling process water has become common practice in coal preparation plants. The water recycling strategy not only fulfils the need for the large amount of water required for coal processing, but also reduces cost and environmental impact. Recycled water can be sourced from various processes on site including tailings dams, concentrate and the overflow from tailings thickeners. One of the consequences of process water reuse is that the salinity and the presence of other chemicals in the recycled water accumulate over time (Wang and Peng, 2014b, Gonzalez et al., 2010). The accumulation of these chemicals in the reused process water in coal preparation might have a negative impact on the thermoplastic properties of coal and coke quality as previously discussed in Section 2.2.2.

2.4.1 Process water chemistry

In a coal preparation plant, typical contaminants in the reused water are pyrite oxidation 2+ 3+ 2- products (Fe , Fe , SO4 ), soluble inorganic ions released from the inorganic matter in the coal (Ca2+, Al3+, and Mg2+), humic acids and residual flotation reagents. Table 2.3 shows the inorganic constituents that are commonly found in process water from Australian coal processing plants, some of which can have a profound effect on coal processing (Leong et al., 2014, Liu et al., 1994). For example, metal ions such as calcium, iron and aluminium can alter the hydrophobicity of the coal surface and cause reduced flotation due to pH dependent precipitation of these species on the coal surface. There is no study focused on the effects of these metal precipitations on coal thermoplasticity. However, iron and calcium are known inhibitors of coal thermoplasticity as explained in Section 2.2.2. Therefore, it is important to investigate whether the precipitation of these inorganic materials would affect coal thermoplastic behaviours.

52 Table 2.3. Common inorganic species in process water in Australian coal processing plants (Ofori et al., 2009, Bournival et al., 2017).

Minimum Average Maximum Inorganic element mg/L (M)

Na 385 (0.017) 1178 (0.051) 3100 (0.135) K 3.4 (0.0001) 19 (0.0005) 54 (0.001) Fe 0 (0) 6.814 (0.0012) 30.8 (0.0005) Si 2.62 (0.0001) 11.9 (0.0004) 49.4 (0.0018) Ca 6 (0.0001) 85 (0.002) 365 (0.009) Mg 3 (0.0001) 72 (0.003) 180 (0.007) Cl 333 (0.009) 1092 (0.031) 2360 (0.067)

SO4 57 (0.0006) 1097.6 (0.011) 4800 (0.050)

On the other hand, using saline water in flotation compared with fresh water could increase the combustible recovery, however such benefits are rarely observed in most Australian coal flotation plants, where a 75% recovery is often achieved (Wang et al., 2013). Besides, no consensus on a mechanism of recovery increases attributed to the use of saline water has been reached (Yoon and Sabey, 1982, Yoon, 1982, Laskowski, 1965, Kurniawan et al., 2011, Wang and Peng, 2013). Increasing the salinity of flotation process water impacts both bubble and froth properties and coal surface chemistry (Hancer et al., 2001, Cao et al., 2011, Pushkarova and Horn, 2008, Chen and Elimelech, 2007). However, the effect of saline- water-processing-induced coal surface changes on coal thermoplastic properties is yet to be investigated. Furthermore, the mechanism of how inorganic materials from the process water reside on the coal surface is not yet well understood, and the way these materials interact with coal during the thermoplastic phase is yet to be determined.

2.4.2 Interaction between inorganic substances and coal surface

When a solid such as coal is submerged into a liquid, it develops a surface charge as a result of the dissociation of functional groups (COOH, C=O, COH) on its surface, or as a result of ions in solution adsorbed onto the solid surface (Fuerstenau et al., 1988). Liu et al. (1994) conducted flotation of a bituminous coal from Pittsburgh No. 8 seam (Ohio, United 53 States) under the condition that the pH of the coal slurry was controlled by the addition of NaOH and HCl solutions. Two batches of experiments were carried out with one sample being coal washed with distilled water and the other sample being unwashed. The same procedure was practised and the flotation recovery results were compared to distinguish the effect of dissolvable substances on the coal surface on the flotation performance. The floatability of the washed coal was significantly higher than that of the unwashed coal under the same condition when dissolved NaOH was added to the flotation cell. The decrease in the recovery of the unwashed coal might be caused by the ionisation of the surface carboxylic functional groups (Liu et al., 1994). Fuerstenau et al. (1987) suggested that when present in aqueous solutions, carboxylic groups become ionised and influence the coal surface electrical charge. An increase in carboxylic content may result in more available oxygen ions for water molecules to hydrogen bond. Since the double bonds in carboxylic groups display a strong polar characteristic and they often attract water molecules, the floatability can therefore be reduced if the contents of carboxylic groups are increased. It was found that oxidised coal and lower rank coal contain higher oxygen functional groups which caused the formation of more negatively charged sites on the coal surface (Wen and Sun, 1977).

On the other hand, washing the coal three times with distilled water was reported to dissolve a significant amount of multivalent metal ions that were originally on the coal surface, including Fe, Al, Mg and Ca species. Fe, Al, Mg and Ca species were found to be predominant in the supernatant from washing (Liu et al., 1994). The authors suggested that in the pH range of 4 to 10, the metal ion species tend to precipitate on the coal surface if the pH is adjusted to increase, and there will be dissolution of mineral species if the pH is adjusted to decrease. In other previous research (Firth and Nicol, 1981, Lai et al., 1989), organic species, especially humic substances, were also found to be dissolved and released from coal. The amount of the organic species released was reported to be more significant from oxidised coal. However, the organic species are not of concern in the present study.

The effect of saline water on coal flotation has been extensively studied in recent years (Wang et al., 2013, Wang and Peng, 2013, Wang and Peng, 2014b, Kurniawan et al., 2011, Wang and Peng, 2014a). In general, salts with divalent and trivalent cations or anions gave 54 a high flotation response, while monovalent cations or anions gave a medium and low flotation response (Pugh et al., 1997). The effect of concentration on flotation response is complex. At high concentration (0.1 M and higher), the floatability of coal increases with increase in salt concentration. In this case, the electrostatic interaction between bubbles and particles is the major factor controlling the flotation process. While at low concentration, the hydrophobicity of the coal is the controlling factor for the flotation process and the floatability of coal decreases with increase in salt concentration (Li and Somasundaran, 1993, Harvey et al., 2002). Three mechanisms were proposed to explain the increased mineral flotation in saline water primarily on coal flotation, but are contradictory in some circumstances (Wang and Peng, 2014a).

Wang and Peng (2014b) reported that the interaction between saline water and clay minerals is synergistic which stabilises the froth and recovers combustible matter by true flotation. Such stabilisation of the froth occurred because saline water promoted the formation of association of clay platelets that were sustained in the dynamic flotation condition, entered the flotation concentrate and altered the froth property and coarse coal flotation behaviour. Flotation experiments were conducted with artificial saline water + 2+ + + 2+ - - (dissolving Na , Mg , Ca , K , SO4 , Cl and HCO3 in deionised water following the procedure provided by Ofori et al. (2009) and used SEM-EDS analysis to examine the surface of concentrates from flotation. It was found that the flotation concentrates using saline water showed significantly increased clay mineral entrainment. An explanation of this phenomenon was that the associated clay minerals are a result of the attractive van der Waals forces and stronger compression of the electrical double layers (Nasser and James, 2006).

At high salt concentration, the flotation circuit may be operated without the addition of a frother or collector. The salt may contribute to the compression of the electrical double layer and inhibition of bubble coalescence (Craig et al., 1993b, Craig et al., 1993a, Prince and Blanch, 1990, Weissenborn and Pugh, 1995). The electrical double layer around the particles is compressed, resulting in the opening of more hydrophobic surface sites and then the attachment probability between particles and bubbles increases (Laskowski, 1965). At the same time, the bubble coalescence is inhibited, leading to more small bubbles present in 55 the flotation system, which increases the collision probability between the particles and the bubbles (Henry and Craig, 2008, Henry et al., 2007). It is worth noting the improvement of collision and attachment probability are all beneficial for improving the flotation efficiency. In addition, the high salt concentrations can also promote particle aggregation in flotation and therefore affect the recovery of both coal and clays (Pawlik et al., 2004, Harvey et al., 2002, Wang and Peng, 2013). At low salt concentrations, the electrostatic repulsive forces can be easily overcome by long-range hydrophobic attraction leading to weakly coagulated structures (Xu and Yoon, 1990). So the hydrophobic interaction becomes the predominant controlling force (Li and Somasundaran, 1993) and gas nuclei formed by gas solubility will have a significant effect on hydrophobic coagulation (Zhou et al., 1996).

The findings from these studies clearly showed that the presence of dissolved inorganic salts in the coal flotation process has some influences on the surface of coal. When salt is present during flotation, the hydrophobicity of the coal surface is altered and the flotation recovery is also increased, along with an increase in the recovery of ash (Ofori et al., 2009, Laskowski, 1965). Although an increase in ash alone could reduce the thermoplasticity of the coal (Yoshida et al., 2000), the retention of the salt after flotation might also negatively impact coal’s thermoplasticity (Crewe et al., 1975). However, the correlation between coal thermoplasticity and inorganic salts in process water has never been explored. Therefore, the present study addresses this research gap by adopting an experimental approach.

2.5 Literature review summary

This chapter reviewed the previously established fundamentals on the mechanism and theories of coal thermoplastic properties. The advantages and disadvantages of the current techniques for examining coal thermoplasticity were also reviewed. In addition, due to the increasing use of process water recycling, concerns on its potential negative impacts on coal were identified, and the interaction between coal and reused process water and its possible effect on coal thermoplastic properties was elaborated. It is noted that, although there are available methods to examine the thermoplastic properties of a coal, none of those methods can accurately represent what is actually happening in the coke oven. In other words, the examination methods might not be as representative as they are intended to be.

56 The theories on the mechanism of coal’s behaviours during the thermoplastic stage show that understanding has deepened progressively over the last century, however the mechanism is yet to be fully understood as no confirmed explanation could be drawn. In addition, little work has been done to address the differences in coal thermoplastic behaviour when salts are introduced to coal in both dry and dissolved forms. It is very important to understand the difference in the mechanisms as to whether coal adsorbs or absorbs salts before pyrolysis, and the consequent results on coal thermoplastic properties during pyrolysis, so more research effort is required to determine the underlying mechanisms. The discussion on the chemistry of process water and its interactions with coal showed the possibility of inorganic materials being retained on the coal surface and influencing coal fluidity during coking.

Furthermore, the inorganic species, such as Na salts, were found to significantly affect the volatile yield and gas evolution during coal pyrolysis. Meanwhile, the addition of Na salts and other alkali and alkaline earth metal (AAEM) species were also reported to have different effects on coal thermoplasticity. However, there is an unexplored knowledge gap about whether processing coal with reused water will indeed affect its thermoplastic properties because no previous study has established any relationship between the two subjects. Although previous studies confirmed some effects of inorganic salts on coal thermoplastic properties, the effect of different ways of taking up these salts, such as from coal processing instead of direct treatment, is yet to be explored.

57 Chapter 3 Methodology

An experimental approach was taken to achieve the research objectives. The coal samples and process water samples were characterised first, followed by coal treatments using the process water taken from the coal preparation plants as well as some artificial inorganic salt solutions. The treated coal samples were then tested for fluidity using a Gieseler plastometer. The analyses of the coal surface chemistry were carried out with spectroscopic and microscopic methods. Details of the abovementioned experimental setups, sample characterisations, and experimental procedures are explained in this chapter.

3.1 General experimental considerations

Various experimental techniques were used in the study. One recurring operation was the cleaning of glassware and plasticware. The glassware was cleaned using an alcohol- hydroxide-bath technique (Hill, 1983). The glassware was soaked in a mixture of alcohol- hydroxide solution for 20 minutes. The solution was made of 60 g of sodium hydroxide (NaOH) (Sigma-Aldrich, reagent grade) dissolved in 60 g of Milli-Q water. The aqueous solution was added to 500 mL of ethanol (Chem-Supply, ≥ 99.8%). The labware was then thoroughly rinsed with water. The water used throughout this study for making up solution and cleaning labware was, unless mentioned otherwise, dispensed from a Merck Milli-Q Integral Water Purification System and had a resistivity of 18.2 mΩ m and surface tension of 72.6 mN m-1 at 21 °C. Other reagents used in this study included some inorganic salts, which were all analytical grade from Ajax Finechem, and hydrochloric acid (HCl) of 32% (v/v) concentration from EMSURE®. Table 3.1 summarises the chemicals used.

58 Table 3.1. Types and purities of reagents used in this study.

Reagent Purity Use

Na2CO3 99.8%, Ajax Finechem Salt solution soaking, oxidation

K2SO4 99%, Ajax Finechem Oxidation

Na2SO4 99%, Ajax Finechem Oxidation HCl 32%, Sigma-Aldrich Oxidation

CaCl2 97%, Ajax Finechem Oxidation

MgCl2 98%, Ajax Finechem Oxidation

CaCO3 99%, Ajax Finechem Oxidation

3.2 Characterisation of coal samples

The samples used in the study were run-of-mine coal taken from two separate coal preparation plants in Australia, Plant A in Queensland and Plant B in NSW (referred to as coal A and coal B). The samples were received in lumps of +10 mm in size to avoid extensive exposure to air. Once received, the coal A sample was crushed and sieved using a hammer crusher to produce particles of -250 +125 µm fraction, while the coal B sample remained as coarse lumps. The coal A was split using a rotary sample divider and individual samples were immediately placed in vacuum sealed plastic bags (see Figure 3.1) and kept at a temperature of 4 °C to minimise any possible oxidation.

59 Figure 3.1. Crushed coal samples in vacuum sealed bags.

Coal A and coal B had similar rankings and properties as shown in

Table 3.2. The characterisation of the coal samples was done by conducting an ultimate analysis and proximate analysis. Gieseler fluidity tests were also carried out to determine the original fluidity of the coal samples.

Table 3.2 shows the results from the ultimate analysis, proximate analysis and fluidity tests. Coal A and coal B have very similar compositions; however, coal B exhibited a much lower maximum fluidity. The initial softening temperature (IST) and maximum fluidity temperature (MFT) for coal B were both 3 °C lower than coal A while the resolidification temperature (RST) was 8 °C lower than coal A, indicating a significant earlier resolidification.

60 Table 3.2. Characterisation of the coal samples: proximate analysis, ultimate analysis and Gieseler plastometer measurements.

Coal sample Coal A Coal B IM 2.3 2.8 Ash 8.2 9.1 Proximate analysis (%) VM 34.0 35.8 FC 55.9 52.1 S 0.6 0.7 C 74.7 72.9 Ultimate analysis (%) H 5.1 5.1 N 2.0 1.6 O 6.8 7.9 IST (°C) 397 393 MFT (°C) 436 433 Gieseler plastometer measurements RST (°C) 471 463 PR (°C) 74 70 MF (ddpm) 2790 426 Note: IM: inherent moisture; VM: volatile matter; FC: fixed carbon; IST: initial softening temperature; MFT: maximum fluidity temperature; RST: resolidification temperature; PR: plastic range; MF: maximum fluidity

3.3 Characterisation of water samples

3.3.1 Process water sample

The water samples used in this study were taken directly from the thickener overflow at Plant A. Ion chromatography was used to qualitatively determine the species present in the water. The analysis was followed by inductively coupled plasma optical emission spectrometry (ICP-OES) to obtain a quantitative analysis. The total alkalinity in the water was determined by titration using the standard procedure of Eaton et al. (2008). 61 ICP-OES analysis

The ICP-OES analysis was conducted to obtain information on the concentration of each electrolyte in the process water sample. The ICP-OES was a Perkin Elmer OPTIMA 7300 and WinLab 32™ for ICP (v. 4.0) software was used to simultaneously record the analyte wavelengths. The instrument was calibrated before use by using the ASTM type I water acidified with nitric acid as the calibration blank. For the determination of total recoverable analytes in aqueous samples, a 100 mL aliquot from a well-mixed, acid-preserved sample was transferred to a 250 mL beaker, and 2.0 mL nitric acid and 1.0 mL of hydrochloric acid were added. The beaker was placed on a hotplate for solution evaporation. The hotplate was located in a fume hood and was previously adjusted to provide evaporation at a temperature of approximately but no higher than 85 °C. The beaker was covered with an elevated watch glass to prevent sample contamination from the fume hood environment. The volume of the sample aliquot was reduced to about 20 mL by gentle heating at 85 °C without boiling. The lip of the beaker was covered with a watch glass to reduce additional evaporation and gently refluxed for 30 minutes and cooled. The sample solution was quantitatively transferred to a 50 mL volumetric flask and made up to the mark with ASTM type I water and allowed to settle overnight.

An ICP torch was mounted in an axial orientation in the instrument’s torch box to provide selectable views of element observation. The read time was set for a variable range from 2 s to 5 s using the auto-integration mode of the equipment (Sarojam, 2010). The ICP-OES analysis result of the process water sample used in this study was compared to the results obtained in previous studies for a number of Australian sites (Ofori et al., 2009, Bournival et al., 2017) and are presented in Table 3.3.Most of the ions in the process water used in this study had relatively low concentrations within the range of minimum to average, while the concentration of K was very close to the average, and only Na and SO4 had concentrations above the average.

62 Table 3.3. Major ions in the process water sample used in the present study compared to typical process water in Australian coal processing plants (Ofori et al., 2009, Bournival et al., 2017).

Typical plant Present study Analyte Symbol Minimum Average Maximum mg/L (mM) Calcium Ca 19.1 (0.5) 6.0 (0.1) 85.0 (2.0) 365.0 (9.0) Potassium K 16.3 (0.4) 3.4 (0.1) 19.0 (0.5) 54.0 (1.0) Magnesium Mg 28.9 (1.2) 3.0 (0.1) 72.0 (3.0) 180.0 (7.0) Sodium Na 1453.3 (63.2) 385.0 (17.0) 1178.0 (51.0) 3100.0 (135.0) Silica Si 1.7 (0.1) 2.6 (0.1) 11.9 (0.4) 49.4 (1.8) Chlorine Cl 638.3 (18.0) 333.0 (9.0) 1092.0 (31.0) 2360.0 (67.0)

Sulfate SO4 1555.5 (16.2) 57.0 (0.6) 1097.6 (11.0) 4800.0 (50.0) Iron Fe 0 (0) 0 (0) 6.8 (1.2) 30.8 (0.5)

Alkalinity titration analysis

The alkalinity analysis was conducted by using a titration method outlined by Eaton et al. (2008) using a standardised HCl solution as titrant. The HCl solution used in this study was standardised (normality = 0.0861 N) by using a Na2CO3 solution (normality = 0.05 N). The process water sample used in each titration was 50 mL contained in a 150 mL glass beaker, which was transferred from a sample bucket using a 10 mL glass pipette. The HCl solution was added to the process water by using the pipette where each addition was 0.2 mL. A pH meter (Eutech Instruments, Thermo Fisher Scientific) was used to measure the pH of the solution.

The pH of the water sample was measured before the titration and then after every 0.2 mL addition of HCl. The titration was finished when the pH of the process water sample became steady and the HCl addition no longer caused any pH reduction.

63 The titration curves had two inflection points where the trend of the pH change was sharply changed (see Error! Reference source not found.). The normality (N) and volume of acid addition (A) at these two inflection points were used to calculate the alkalinity in terms of mg CaCO3/L as follows A × N × 50000 Alkalinity = 12 V where A is the amount of titrant (HCl acid solution) used in the experiment in mL, N is the normality of the HCl acid solution and V is the volume of water sample solution used in the experiment in mL. Using Equation 12, the alkalinity was calculated for the first and second inflection points shown in Error! Reference source not found. and denoted as total - alkalinity (T) and phenolphthalein alkalinity (P). The mg CaCO3/L equivalents of HCO3 , 2- - CO3 and OH were calculated based on the calculated T and P values according to the criteria of Table 3.4. Table 3.5 shows that the process water sample used in this study - 2- - contains HCO3 , CO3 and OH that are equivalent to 486.7 mg CaCO3/L, 31.4 mg

CaCO3/L, and 0 mg CaCO3/L, respectively. Results of full pH measurements are in

64 Table A2 in the Appendix.

10 0.5 9 0 8 -0.5 7 -1 6 -1.5

5 -2 pH

4 -2.5 pH Diff. 3 -3 2 -3.5 1 -4 0 -4.5 0 2 4 6 8 10 12 Addition of HCl (mL)

Figure 3.2. Titration curves (solid line, left axis) and differentiated titration curves (dashed line with markers, right axis) of the process water sample (♦, sample 1; blue, diff. 1) and its replica (▲, sample 2; red, diff. 2).

- 2- - Table 3.4. Criteria for HCO3 , CO3 and OH determination.

mg CaCO3/L Result of titration - 2- - OH CO3 HCO3 P = 0 0 0 T-2P P < (1/2) T 0 2P T-2P P = (1/2) T 0 2P 0 P > (1/2) T 2P-T 2(T-P) 0 P = T T 0 0 Note: T=Total alkalinity; P=Phenolphthalein alkalinity

- 2- - Table 3.5. Calculated CaCO3 equivalent concentrations of HCO3 , CO3 and OH in the process water.

mg CaCO3 /L Anion Replicate 1 Replicate 2 Average 65 - HCO3 486.7 486.7 486.7 2- CO3 31.4 31.4 31.4 OH- 0 0 0

Process water total salt concentration adjustment

The total salt concentration in the process water sample was adjusted to five different concentrations as shown in

Table 3.6. The increases of total salt concentration in the process water were achieved by boiling the process water sample at 100 °C on a hotplate until the calculated volume was reached, whereas the decreases of total salt concentration were carried out by diluting the process water sample with Milli-Q water until the calculated volume was achieved. These adjusted process water samples were used in the oxidation experiments in order to determine whether the fluidity of coal was affected by changes in total salt concentration, thus establishing a relationship between the total salt concentration and coal fluidity

Table 3.6. Process water samples with adjusted total salt concentration.

Salt concentration Dilution ratio Sample ID (mM) (%)

PW10 12 10

PW50 60 50

PW100 125 100

PW150 185 150

66 PW200 250 200

3.3.2 Chemical solutions

This study also used Milli-Q water (see Section 3.1) and artificially created solutions of inorganic salts. Table 3.7 summarises the type and concentration of all salt solutions used in this study. The selection of the salts was based on the major ions present in the process water sample as well as the results from soaking experiments with Na2CO3, as explained in Section 4.1.

Table 3.7. Inorganic salt solutions and their concentration used in this study.

Salt Concentration (M) 0.1

Na2CO3 0.01 0.001

K2SO4 0.01

Na2SO4 0.01

CaCl2 0.01

MgCl2 0.01

CaCO3 0.01 HCl 1.21

As well as the inorganic salt solutions, an HCl acid solution was prepared for oxidation. Another purpose of using HCl solution in the oxidation experiment was to demineralise the coal sample, as HCl acid solution was previously reported to be an efficient demineraliser for coal (Wijaya and Zhang, 2011, Bolat et al., 1998). The effect of the demineralisation from oxidation with HCl acid was determined by conducting ash analysis on the HCl- treated coal samples. The HCl solution was dissolved to 10% (v/v) concentration from 32% HCl. The molarity of the diluted HCl solution is 1.21 M as shown in Table 3.7. According to Bolat et al. (1998), HCl solution of 10% has the best demineralisation result by removing the most ash content without damaging the coal surface.

67 3.4 Experimental and analytical methods

3.4.1 Salt solution soaking

The experiments were first conducted by mixing the coal A sample with Milli-Q water, process water and a Na2CO3 (0.01 M) solution. The aim of these experiments was to determine the influences from the abovementioned solutions on the fluidity of coal A. Therefore, oxidation was deliberately avoided by conducting the experiments in a sealable glove-bag filled with N2 gas. Known quantities of coal (40 g) and water (240 g) were added in test tubes in a Wcoal:Wwater = 1:6 ratio. After stirring the mixture to completely wet the coal particles, the slurries were centrifuged for 1 hour at 9000 rpm to separate the particles from the liquid mixture. The coal particles were vacuum filtered and dried under a continuous flow of nitrogen in a desiccator. The fluidity of the dried coal samples was tested using a Gieseler plastometer.

3.4.2 Oxidation

Oxidation of dry coal

In order to examine the sensitivity of the coal A sample to oxidation, the untreated coal A samples were oxidised under dry conditions and ambient temperature in the laboratory for different periods and tested for fluidity. For this experiment, a coal A sample of 50 g was divided into five small fractions with equal weights (10 g each). One of the fractions was directly vacuum sealed without any oxidation, this sample was denoted as A0, while the other four samples were placed on a bench in exposure to air for 2 hours, 8 hours, 48 hours and 72 hours, denoted as A2, A8, A48 and A72, respectively. After the designated periods of oxidation, the coal samples were collected and vacuum sealed for Gieseler fluidity tests. Table 3.8 shows the Gieseler fluidity test results of the coal samples. The fluidity of the coal samples did not show a significant change until 72 hours of oxidation although some minor decrease of 200 ddpm maximum fluidity was observed on A48. After 72 hours of oxidation, the fluidity reduced by about 42% from 1900 ddpm to 1100 ddpm.

Table 3.8. Gieseler fluidity results of the dry coal A samples oxidised for different periods.

68 Oxidation period MF Sample ID (hours) (ddpm)

A0 0 1900

A2 2 2100

A8 8 1900

A48 48 1700

A72 72 1100

Oxidation of solution-treated coal

Oxidation was carried out with the Milli-Q water, process water and salt solutions mentioned in the previous section to determine if the oxidation of coal alone and in the presence of various inorganic salts had any impact on coal fluidity. Known quantities of coal (40 g) and the water sample (240 g) were added in capped polyvinyl chloride cylindrical containers in a Wcoal:Wwater ratio of 1:6. Some experiments were conducted with a Wcoal:Wwater ratio of 1:3. The solution was manually stirred to break any agglomerates that may have been present in the solution and coal particles were left in the solution for 1 hour. The particles were then separated from the solution in the containers in which the slurries were held. The excess solution was removed by a plunger (Figure 3.3) with a Whatman® 100 µm filter paper attached on the mesh. The experiment was designed to have coal samples being oxidised while holding approximately 30% moisture (water) content. Moisture content close to 30% is often found in dewatered coal in real operations (Le Roux et al., 2005). The samples were kept in a desiccator (in the absence of a desiccant), which could be purged to create an air- or nitrogen-rich environment. After a period of 7 days under either air or nitrogen, the samples were vacuum filtered and dried in the same container under a continuous flow of air or nitrogen. The fluidity of the dried coal samples was tested using a Gieseler plastometer.

69 (a) (b)

Figure 3.3. Plunger setup used in the study to dewater the coal-solution mixture: (a) plunger head and filter mesh, (b) assembled setup with plunger handle and container.

3.4.3 Inorganic electrolyte removal

To study the effect of residual electrolytes and leached materials on coal thermoplastic properties, some samples were washed with Milli-Q water to remove any leached materials and residual electrolytes from the coal subsequent to the oxidation process. In a typical experiment, 35 to 40 g of treated coal was first equally split in four test tubes of 50 mL capacity. Water was then added to each tube to obtain a coal to water (Wcoal:Wwater) ratio of 1:6. The slurry was centrifuged by using a Allegra X-30 Centrifuge (Beckman Coulter) for 1 hour at 9000 rpm and the supernatant was decanted from the solid. Water was then added into each tube in a Wcoal:Wwater ratio of 1:6 and the test tubes were centrifuged for a further 30 minutes. This process was repeated until the pH of the supernatant became steady. The coal samples were removed from the tubes, vacuum filtered, and dried under N2 gas. The dried samples were tested for fluidity using a Gieseler plastometer.

The pH curves from washing of the coal samples treated with Milli-Q water and Na2CO3 are shown in Figure 3.4. The pH of the supernatant from the Na2CO3 treated sample was 70 significantly reduced after the first six wash cycles and started to become steady. Meanwhile, the Milli-Q water treated coal sample showed some minor decrease in the pH of the wash-water and the pH stabilised after only two wash cycles. The results suggested that Na2CO3 can be retained on the coal surface and the complete removal of it required washing for at least eight cycles. On the other hand, the coal sample itself might contain a small amount of dissolvable substances that can be released after washing with Milli-Q water.

11

10

9

8 pH

7

6

5 0 1 2 3 4 5 6 7 8 9 10 Number of washes

Figure 3.4. pH curves from washing of Milli-Q treated (▲, blue) and 0.01 M Na2CO3 (●, green) treated coal samples (Note: A wash time of “0” refers to the supernatant before the first wash).

3.4.4 Ash analysis

Ash analysis was carried out on samples oxidised after treatments with process water and HCl solution. The treated coal samples were burnt in a Labec ashing muffle furnace. The ash contents of the treated coal samples were determined by following the Australian Standard (Australian Standards, 2000). The coal samples were first loaded in the silica crucibles where each sample had a weight of 1.00 g ± 0.1 mg. Once all samples were

71 loaded, the crucibles were transferred into the furnace and the temperature in the furnace was increased to 500 °C and maintained for 30 minutes. After 30 minutes of heating at 500 °C the temperature was increased to 815 °C and kept heating for 1 hour. After 1 hour of heating at 815 °C the crucibles were removed from the furnace and cooled to room temperature. The cooled crucibles containing coal were weighed to the nearest 0.1 mg. The ash was calculated by using Equation 13 for each sample.

m − m Ash = 1 3 × 100 13 m2 − m3

Where m1 = mass of crucible plus ash, g m2 = mass of crucible plus sample, g m3 = mass of crucible, g

The results of ash analysis are in Table 3.9. Ash analysis of untreated coal sample, process water-treated coal sample and HCl-treated coal sample.

Samples tested ash % 10% HCl 7.25 Process water 7.71 Untreated 7.27

. The treatment with HCl solution was not very effective in demineralising the coal sample. Furthermore, the treatment of process water and oxidation did not have any significant impact on the ash content of the coal sample. Nonetheless, the treatment with HCl did have a significant impact on the fluidity of the coal sample as noticed in a subsequent fluidity test. The fluidity of the coal sample was reduced by almost 85% to 269.5 ddpm. The fluidity result of the HCl-treated coal sample is discussed in detail in Section 4.4 along with other fluidity result discussions.

Table 3.9. Ash analysis of untreated coal sample, process water-treated coal sample and HCl-treated coal sample.

72 Samples tested ash % 10% HCl 7.25 Process water 7.71 Untreated 7.27

3.4.5 Surface analysis

Oxidation might induce changes in coal surface properties, such as change in chemical composition and/or enlargement of the pores (Xia et al., 2014). As reviewed in Chapter 2, the increased oxygenation on the coal surface is already known to play a significant role in coal thermoplasticity by disabling the transfer of hydrogen and promoting early cross- linking reactions (Arisoy and Beamish, 2015, Bouwman and Freriks, 1980, Clemens et al., 1989, Fuerstenau et al., 1987, Furimsky et al., 1983). The enlargement of pores, on the other hand, can result in an increase in the surface area, therefore it might lead to increased adsorption of electrolytes from the water samples on the surface of coal particles by providing a larger contact surface area (Gregg et al., 1967). An x-ray photoelectron spectrometer (XPS) was used to determine changes in the coal surface before and after the treatments. The surface of the coal treated with salt was also examined using a scanning electron microscope (SEM) to determine possible salt retention on the coal surface. Subsequently, energy dispersive x-ray spectroscopic (EDS) analysis of the surface was conducted to determine the presence of mineral salt precipitates. The purpose of these surface analyses was to investigate whether the oxidation process induced any chemical reaction, salt absorption or ion adsorption on the coal surface. The samples used for XPS analysis were fractions of the coarser coal B sample, and the coal A sample was used for

SEM-EDS analysis. Coal A samples were dried under N2 gas in a sealed desiccator immediately after the 7-day oxidation, and coal B samples were cleaned with a generous amount of water after the experiments and dried under N2 gas in a sealed desiccator for XPS analysis.

73 XPS analysis

XPS was used to determine the changes induced on the surface treated with 0.01 M Na2CO3 solution and process water. Fresh surfaces were prepared for XPS analysis in the following way: coal lumps were cleaved into small pieces and inspected to find a shiny and smooth surface for the XPS analysis. The selection of shiny surfaces was to avoid oxidised sites, mineral grains or clays as they normally would appear dull. The other faces on the coal pieces were polished using sandpaper to shape the samples to fit in the instrument. Each sample was thoroughly cleaned with Milli-Q water to remove any contaminants that may have come from sanding. Figure 3.5 shows the samples prepared for the XPS analysis using the method described earlier.

Figure 3.5. Coal B samples prepared for XPS analysis.

XPS analysis was conducted using an Escalab 250 Xi manufactured by Thermo Scientific. The background vacuum was adjusted to 2×10-9 mbar, and the source of x-ray was from mono-chromated Al K alpha (energy 1486.68 eV). The take-off angle of the photoelectrons was 90° and the spot size was 500 μm. The spectra of survey scan were recorded with the pass energy of 100 eV whereas 20 eV was used for region scans; the energy step size was

74 1.00 eV for the survey scan and 0.1 eV for the regional high-resolution scans. The binding energies were corrected by setting the C(1s) hydrocarbon peak at 284.8 eV. These parameters are typically used in the analysis of coal by XPS (Buckley, 1994a, Gong et al., 1998).

SEM-EDS analysis

A FEI Quanta 230 SEM was used to analyse the surface morphology of oxidised coal. The magnification was fixed at 2000 and 10000 times. The coal samples for the SEM-EDS analysis were the coal samples collected from the designated oxidation treatment using a

Wcoal:Wwater ratio of 1:6. Figure 3.6 shows the coal samples prepared for the SEM-EDS analysis. Untreated samples were analysed and used as a benchmark for the oxidised samples. The surfaces of both treated and untreated samples were examined to identify the differences on surface compositions. Both the untreated and treated samples were dried under N2 gas. Before the analysis, the treated coal sample was divided into two fractions, and only 5 to 10 coal particles from each sample were selected to be examined. The selected particles were pressed between two pieces of glass microscope slides to achieve a relatively even particle size. The selected particles were sputter-coated with a layer of platinum and carbon for better image quality and more accurate EDS detection, respectively. The SEM-EDS analysis only required approximately 0.1 g from each sample.

75 Figure 3.6. Coal A samples prepared for SEM-EDS analysis.

3.4.6 Coal thermoplastic properties analysis

The techniques commonly used to determine coal thermoplasticity have been discussed in detail in Section 2.3.6. Although all techniques were previously found to have some limitations, fluidity was found to be the most representative measurement of coal thermoplasticity. Furthermore, the Gieseler plastometer is also the most commonly used method for determining the thermoplastic properties of coal in the Australian coal industry. Therefore, the present study used a Gieseler plastometer, which measures coal fluidity in dial division per minute (ddpm), to investigate the fluidity of the coal samples.

76 Figure 3.7. Gieseler plastometer setup (RMI, 2018).

The fluidity of the treated coal sample was examined using a Gieseler plastometer and the procedure was performed according to the Australian Standard (Australian Standards, 1996) except for the size of the particles, which was as described earlier. The coal particle size requirement for the Gieseler plastometer is 425 µm (Australian Standards, 1996). The particle size of the coal samples used in the present study was 125-250 µm which is smaller than the particle size used in the standard Gieseler plastometer. The flotation products contributed a significant portion of the stockpiles in the treatment plants involved in this study and the selected size fraction was considered a good reflection of industry practices.

The reduced particle size could result in weakened thermoplastic behaviours because the smaller particles would be more easily melted and volatilised during pyrolysis, which would ultimately cause resolidification to occur earlier (Kök et al., 1998). Furthermore, the effect of process water chemistry on the fluidity of coal samples used is expected to be more pronounced, because of the greater surface area of the smaller particles. However, it is believed that the impact of the reduced particle size on coal fluidity does not affect the conclusions drawn in this study, because all the coal samples used for fluidity tests had the

77 same particle size and the maximum fluidity results were normalised using logarithm (i.e.

Log10 MF) before making any comparison.

For each test, 5 g of coal particles were compacted into a cylindrical capsule (diameter = 1~2 cm, height = 2~4 cm) equipped with a stirrer where the axis of the stirrer is subjected to a constant torque as discussed earlier. The coal sample was first rapidly heated to 300 °C and then a constant heating rate of +3 °C/minute was applied. Before the softening of coal starts, the movement of the stirrer was blocked by the compacted coal sample, so the rotation only begins when the coal reaches a certain plasticity (Gieseler, 1934). The change in coal fluidity from the initial softening stage to the end of the resolidification stage was monitored and the critical temperatures (i.e. initial softening temperature, maximum fluidity temperature and resolidification temperature) and maximum fluidity (MF) were recorded.

78 Chapter 4 Results and discussions

4.1 Effect of Na2CO3 soaking on coal fluidity

The process water sampled from the plant contained a variety of inorganic electrolytes and + Na , among all the analytes, had the highest concentration. Among sodium salts, Na2CO3 has been shown to influence coal thermoplasticity (Patrick and Shaw, 1972, Crewe et al.,

1975). Therefore, experiments using Na2CO3 solutions were carried out where coal A samples were contacted with Na2CO3 solutions of different concentrations and then tested for fluidity. To avoid any possible combined effects of Na2CO3 and oxidation, the coal-

Na2CO3 mixture was dewatered and dried immediately after the treatments under N2 gas. As shown in Error! Reference source not found., there was no significant change in

Log10 maximum fluidity for all samples except samples treated with 0.1 M Na2CO3. The fluidity of the coal samples treated with Na2CO3 solutions of 0.01 M and 0.001 M seemed to be unaffected, and statistical analysis of the results compared to those from the coal sample treated with water did not show any significant differences. The difference in the fluidity of the coal samples treated with the 0.1 M Na2CO3 solution, on the other hand, was found to be statistically significant when compared to the other three sets of results. Thus, it is evident that the fluidity reduction occurred only on the coal sample treated with the 0.1

M Na2CO3 solution. The reduction in fluidity can be correlated with Patrick and Shaw

(1972)’s work, in which a 5 wt.% addition of dry Na2CO3 was able to reduce fluidity by as much as 90%. Furthermore, adding 2 wt.% of dry Na2CO3 was also reported to significantly reduce fluidity, however the authors did not provide details on the degree of reduction and any further explanation. In the present study, the weight of Na2CO3 from the 0.1 M solution corresponded to 6.36% of the weight of coal sample whereas the weight of

Na2CO3 from the 0.01 M and 0.001 M solutions corresponded to 0.64% and 0.06% of the weight of each coal sample, respectively. After dewatering to 30% moisture, the relative weights of Na2CO3 retained in the coal sample were calculated to be 1.9%, 0.19% and 0.019% for solutions of 0.1 M, 0.01 M and 0.001 M used respectively. which is consistent with the findings from previous work (Patrick and Shaw, 1972). However, it is not common to find such high concentration of Na2CO3 in the real coal preparation process (Ofori et al.,

79 2009), therefore the solution with lower concentration of 0.01 M was used in all subsequent experiments. 4.0

3.5

3.0 MF (ddpm) MF

10 2.5 Log

2.0

1.5 Untreated 0.001 M 0.01 M 0.1 M Na2CO3 Na2CO3 Na2CO3

Figure 4.1. Fluidity of coal A samples treated with Na2CO3 solutions without oxidation (Errors determined by analysing results from two independent runs).

The Gieseler plastometric results can be seen in Table 4.1. The sample treated with 0.1 M

Na2CO3 showed a significant upward shift in the IST (+7 °C) while the MFT and RST were maintained to similar levels with that of other samples. Due to the increased IST and unchanged RST, the PR was shortened for 0.1 M Na2CO3 treated sample. However, an interesting observation was that the maximum fluidity of the 0.1 M Na2CO3 treated sample sharply reduced while the MFT was unchanged. This result suggests that the coal sample followed the same thermoplastic transformation mechanism as the untreated coal except that the initial softening was delayed. Most volatile matter was driven off and yield of metaplast achieved its highest at 435 °C. However, the total amount of metaplast yield was sharply reduced in this case, which seemed to be caused by the existence of Na2CO3 on coal surface after the treatment. On the other hand, other samples treated with lower concentrations of Na2CO3 and Milli-Q water seemed to be unaffected.

80 Table 4.1. Gieseler plastometric measurements of coal A samples treated with Na2CO3 solutions without oxidation.

IST MFT RST PR MF Log10 MF Treatment condition °C ddpm

0.1M Na2CO3 405 435 468 64 238 2.38

0.01M Na2CO3 398 434 468 70 1607 3.21

0.001M Na2CO3 398 435 469 71 2264 3.33 Untreated 397 436 471 74 2790 3.45

It was hypothesised that the reduction in coal fluidity was the result of changes in coal surface chemistry after the treatment. A method for washing the treated coal (as described in section 3.4.3) was used to determine whether the fluidity, which is affected by changes in coal surface chemistry, can be restored to its original state. The pH of the supernatant after each cycle was measured and the washing was repeated until a steady pH was reached so that any Na2CO3 from the solution absorbed (or weakly adsorbed, i.e. physisorbed) on the coal surface may be removed. Figure 4.2 shows the pH of the supernant measured by using a pH meter after each wash cycle. It can be seen that the pH of the supernant from washing the Na2CO3 soaked coal samples reduced sharply for the first six wash cycles and became steady, reaching a plateau after the eighth wash cycle. Such observation suggested that the inorganic electrolytes that attached to the coal surface during the soaking treatments were successfully removed. The Milli-Q water-soaked coal sample did not show any significant change in supernant pH, which indicated that no dissolvable material or dissolvable salts with neutral pH was released from the coal surface.

81 12

11

10

9 pH

8

7

6 0 2 4 6 8 10 Number of wash cycles

Figure 4.2. pH of the supernant collected after each wash cycle and the corresponding cycle

number for Milli-Q water-soaked coal (blue) and Na2CO3 soaked coal (orange), where wash cycle 0 refers to unwashed after the soaking treatment.

Error! Reference source not found. and Table 4.2 show the results from Gieseler plastmometric measurements of the coal A samples before and after washing. The results show no significant changes in maximum fluidity nor temperature history suggesting that the thermoplastic properties of coal were not affected by washing. The results cannot be correlated with what was reported previously (Crewe et al., 1975) where the plasticity of the coal was found to be restored to almost its original level after washing with distilled water. The difference might be due to the differences in the methods used as the previous study examined the free swelling index whereas the present study investigated fluidity. The free swelling index is usually conducted by rapid heating of coal to 825 °C ± 5 °C in 2.5 min, whereas in Gieseler plastometry the coal sample is heated slowly from 300 °C to 500 °C at a rate of 3 °C/min (Fong et al., 1986a). However, since both parameters represent the thermoplastic properties of coal, it is unlikely that the treatment was only effective in one method (Ignasiak et al., 1974). The result might be due to the differences in the coal samples used in the two studies. It is also possible that the concentration of salt solution

82 used in the present experiment was too low. Since the coal treated with 0.01 M Na2CO3 solution did not show a significant fluidity reduction initially, removing the electrolytes did not necessarily restore fluidity.

4.00

3.50

3.00

MF MF (ddpm) 2.50 10 Log 2.00

1.50 Milli-Q water Milli-Q water 0.01 M 0.01 M unwashed washed Na2CO3 Na2CO3 unwashed washed

Figure 4.3. Fluidity of washed and unwashed coal A samples treated with Milli-Q water and

0.01 M Na2CO3 (Errors determined by analysing results from two independent runs).

Table 4.2. Gieseler plastometric measurements of washed and unwashed coal A samples

treated with Milli-Q water and 0.01 M Na2CO3.

IST MFT RST PR MF Log10 MF Treatment condition °C ddpm Milli-Q washed 398 438 471 73 1532 3.19 Milli-Q unwashed 398 438 470 72 1571 3.2

0.01M Na2CO3 washed 398 437 470 72 1544 3.18

0.01M Na2CO3 unwashed 400 436 473 73 1458 3.16

4.2 Effect of oxidation on coal fluidity

Oxidation is known to greatly affect coal fluidity, however, there is no general rule to distinguish the effect of oxidation on fluidity because the properties of coal may exhibit considerable variations from seam to seam, which may affect the response of coal to 83 oxidation (Gethner, 1987b, Diaz-Faes et al., 2007). Therefore, to investigate the effect of oxidation on coal fluidity, coal samples were placed under pure nitrogen gas and air for 7 days, prior to fully drying the samples under N2 gas. The experiments were conducted with three oxidation additives: Milli-Q water, process water sample and a Na2CO3 solution of 0.01 M.

Error! Reference source not found. shows the fluidities of the treated coal samples. There was no significant change between the maximum fluidities of samples exposed to air and

N2 with all three water samples tested. In addition, variations in the plastic temperature history (see

Table 4.3) among the samples tested were found to be negligible (< 3 °C), suggesting that the coal’s thermoplastic behavior was not affected by temperature in the plastic phase. A paired t-test showed no statistical significance between the samples placed under air or under N2 gas. Thus, all subsequent experiments were conducted without the use of N2 gas, except for drying the samples.

4.00

3.50

3.00 MF(ddpm)

10 2.50 Log 2.00

1.50 Air Milli-Q N2 Air N2 Air Process N2 water Milli-Q 0.01 M 0.01 M water Process water Na2CO3 Na2CO3 water

Figure 4.4. Log10 MF of coal A samples dried in air (filled) and under N2 (unfilled) (Errors determined by analysing results from two independent runs).

84 Table 4.3. Gieseler plastometric measurements of oxidised and unoxidised coal A samples

when Milli-Q water, 0.01 M Na2CO3 and process water were present.

IST MFT RST PR MF Log10 MF Treatment condition °C ddpm Milli-Q - air 398 438 471 73 1691 3.23

Milli-Q - N2 396 437 471 76 1844 3.27

0.01M Na2CO3 - air 398 437 470 72 1150 3.06

0.01M Na2CO3 - N2 399 436 469 70 1304 3.12 Process water - air 399 436 468 69 980 2.44

Process water - N2 398 438 471 73 1691 3.23

However, the additives did have different effects on coal fluidity. Compared with the coal sample treated with Milli-Q water, samples treated with 0.01 M Na2CO3 solution only showed a slight decrease in fluidity, while the fluidity was significantly reduced in samples treated with process water. There was no combined effect from oxidation and treatment with 0.01 M Na2CO3 solution and it may be concluded that the process did not reduce fluidity significantly. On the other hand, the treatment with the process water and oxidation sharply reduced the fluidity of the coal sample, which may be attributed to the presence of other salts in the process water (

Table 4.3). The concentration of Na in the process water sample is 0.06 M, which is much higher than the concentration of Na (0.01 M) in the Na2CO3 solution used. Furthermore, the approximate concentration of all salts in the process water was calculated to be 0.125 M, which contains more than 10 times the salt present in a 0.01 M Na2CO3 solution and could have affected the fluidity. Subsequent experiments were carried out to examine the effect of individual salts in the process water on coal fluidity. The results are shown in Error! Reference source not found. where the salts selection was based on the species present in the process water sample and a concentration of 0.01 M was maintained for consistency with experiments in the Na2CO3 solution.

85 4.00

3.50

3.00 MF (ddpm) MF

10 2.50 Log

2.00

1.50 Milli-Q K2SO4 Na2SO4 CaCl2 MgCl2 CaCO3 water

Figure 4.5. Log10 MF of coal A samples contacted in different 0.01 M inorganic salt solutions and oxidised for 7 days (Errors determined by analysing results from two independent runs).

All salt solutions used were found to have similar degrees of effects on coal fluidity and the degree of fluidity reduction was statistically significant compared to the result from the coal sample treated with water. The temperature histories of oxidised coal samples are shown in

Table 4.4. It was found that all the samples oxidised with the presence on inorganic salt solution showed an upward shift in their ISTs by 4-7 °C. In addition, although the MFTs seemed to be affected insignificantly, the PR was shortened as the RSTs became lower. The shortening of the PR and delaying of initial softening stage coincides with the findings introduced in section “Effect of Na2CO3 Concentration on Coal Fluidity”, where 0.1 M solution treatment alone (without oxidation) showed similar effect on coal thermoplastic behaviors. It seems that the dominant factor in the fluidity reduction was not the type of salt, but the total amount of salts that was present on the coal surface. It might be simply a result of the retention of salt solution during the oxidation process, where the mechanism is similar to how moisture is trapped by the pores and cracks on the coal surface (Subba-Rao and Gouricharan, 2016). The ability of coal to retain moisture was studied by Stanmore et 86 al. (1997), and a similar retention of NaOH solution was reported by Crewe et al. (1975) and could explain the reduction in fluidity. The entrapped salt precipitates in the pores and works as either inert diluents that inhibited the plasticity of coal or reagents that react with the free radicals produced from coal thermal decomposition. The products formed from these reactions are non-plastic hence reducing the fluidity. Such reactions could be promoting early cross-linking reactions, which is the dominant type of reaction during the re-solidification stage (Patrick and Shaw, 1972, Solomon et al., 1990). Or, the reactions delay the thermal decomposition of coal as a result of the salt bonding with the free radicals released from coal thermal decomposition and forms denser materials that stabilise the mobility of the coal fluid (Patrick and Shaw, 1972, Crewe et al., 1975, Marsh and Walker, 1979). Subsequent investigations were carried out on the water chemistry and its effect on coal surface.

Table 4.4. Gieseler plastometric measurements of coal A samples contacted in different 0.01

M inorganic salt solutions and oxidised for 7 days before drying under N2 gas.

IST MFT RST PR MF Log10 MF Salt solution °C ddpm

K2SO4 403 439 468 65 485 2.69

Na2SO4 404 437 468 64 470 2.67

CaCl2 402 439 467 65 468 2.67

MgCl2 403 439 469 66 455 2.66

CaCO3 401 437 468 67 521 2.72 Milli-Q 397 437 471 74 1767 3.25

4.3 Effect of oxidation on water chemistry

The oxidation procedure was repeated with the coal and process water samples without dewatering the coal. The process water sample was collected after coal oxidation. The collected water samples were analysed with titration and ICP-OES to determine the alkalinity and the ionic composition of the water samples, respectively. The chemical composition of the collected process water was then compared with the previously determined composition of the raw process water sample (section 3.3.1). The aim of this

87 analysis was to compare the changes in the concentrations of the major anions and cations in the process water sample.

The alkalinities of the process water samples were determined by conducting titration following the process described by Eaton et al. (2008). The titration curves can be found in Error! Reference source not found., the inflection points of each sample were identified from the graphs and shown in Error! Reference source not found.. The total alkalinity, T and phenolphthalein alkalinity, P, were then calculated by using Equation Error!

Reference source not found. given in section 3.3.1, and the CaCO3 equivalent - 2- - concentrations of HCO3 , CO3 , and OH were determined in accordance to the criteria provided earlier. It was found that there was no OH- in all the process water samples. The - 2- calculated concentrations of HCO3 and CO3 are shown in Table 4.6.

10 0 9 -1 8 7 -2 6

5 -3 pH

4 pH Diff. -4 3 2 -5 1 0 -6 0 1 2 3 4 5 6 7 8 9 10 Addition of HCl (mL)

Figure 4.6. Titration curves (solid line, left axis) and differentiated titration curves (dashed line with markers, right axis) of the process water sample (♦, sample 1; blue, diff. 1) and its

replica (▲, sample 2; red, diff. 2) after oxidation with coal.

88

50 5.6 0.09

PW PW 554.52 (replica) oxidation

50 5.6 2nd point 0.09

PW PW 554.52 oxidation

50 61 0.4 0.09 39.

PW PW (replica) oxidation

50 0.2 1st point 0.09 19.8 PW PW oxidation

6 0.09 50.1

592.94 PW raw PW (replica)

50 6.2 2nd point 0.09 613.93 PW raw PW

0.8 0.09 50.1

79.06 PW raw PW (replica)

. Inflection points of each sample identified from the titration the data. sample identified from . Inflection points of each

1st point 50 5 0.8 . 0.09 4 PW PW raw 79.22

Table Table /L) (mL) 3 Sample Sample Alkalinity volume(mL) Acid addition Acid Normality(N) (mg CaCO

89 The ion concentrations of the process water samples before and after the treatment of coal A with salt solution are reported in Table 4.6. Major changes (difference greater than 0.1 mg/L) were found only for a few ions.

Table 4.6. Changes in concentration of process water before and after treatment with coal A sample for the major elements.

Concentration before Concentration after Sensitivity Analyte oxidation oxidation

mg/L B 0.19 0.31 0.12 Ca 18.08 17.8 0.28 K 16.34 15.37 0.97 Mg 28.93 25.9 3.03 Na 1453.3 1451 2.3 S 536.34 554.15 17.81 Si 1.73 2.12 0.39 Cl 638.32 699 60.68

SO4 1555.54 1616 60.46

CO3 31.4 60 28.6

HCO3 486.7 495 8.3

As shown in Table 4.6, most of the cations did not show significant changes after the oxidation process. Among all analytes, the biggest increase was found in sulfur (S), which increased by 3.32%. On the other hand, the most significant decrease in concentrations of cations was found in magnesium (Mg), with a 10.47% decrease. There was not much change in the concentrations of anions. All anions showed slight increases except the carbonate (CO3), indicating almost a doubled concentration after oxidation. This increase is more likely to be caused by the dissolution of CO2 in the coal process water slurry rather than a result of the dissolution of inorganic matter (Weiss, 1974). The change in the ion concentration would be more significant if the mineral electrolytes were bonded with the coal surface through an exchange reaction (Tyler and Schafer, 1980, Murakami et al., 1995) or if ion adsorption occurred. It is possible that the degree of reaction or adsorption was too small to be observed from the analysis of water chemistry. Note that absorption would not 90 change the concentration of salts since the amount of water trapped is proportional to the amount of salts.

Since the interactions between inorganic electrolytes and coal surface cannot be confirmed without analysis of the coal surface after treatment, additional investigations were carried out using XPS to examine the surface of the oxidised coal B samples, which were crushed to a coarser size.

4.4 XPS analysis of treated coal surface

Figure 4.7 presents the XPS wide energy spectra of coal B after being treated with process water, Milli-Q water or 0.01 M Na2CO3, and Table 4.7 shows the atomic % of four of the major ions on the surfaces of the coal samples. The spectra in Figure 4.7 follow similar trends and no shift in binding energy was observed. The intensities of the three spectra suggest that there is a decrease in the C(1s) peak for the process water treated sample, while the C(1s) peaks for the other two samples remained on the same level. The atomic % of C(1s) (see Table 4.7) on the surface of the process water oxidised sample is 81.65%, being the lowest among the three samples. The intensity of O(1s) for the process water treated sample was found to be slightly higher (12.60%) as indicated in Table 4.7. On the other hand, the changes in the peaks of Si(2p) and S(2p) were not very obvious.

91 C(1s) O(1s) c N(1s)

Intensity b

a

1400 1200 1000 800 600 400 200 0 Binding energy (eV)

Figure 4.7. XPS wide energy spectrum of coal surface after 7-day oxidation with (a)

process water (green), (b) 0.01 M Na2CO3 solution (red) and (c) Milli-Q water (blue).

Table 4.7. Atomic % of major ions on the surfaces of oxidised samples from XPS analysis. C(1s) O(1s) Si(2p) S(2p) Sample % Process water 81.65 12.60 0.41 0.30

Na2CO3 87.98 8.78 0.35 0.27 Milli-Q 88.60 8.49 0.49 0.21

Figure 4.8 shows the intensity of O(1s) spectrum on coal B samples treated with process water, 0.01 M Na2CO3 and Milli-Q water respectively. It was found that the O(1s) intensity on the process water treated surface is greater compared to the other samples whereas the Milli-Q water oxidised coal showed the lowest oxygen concentration. It might be possible that the increased O(1s) intensity on coal surfaces treated with salt solutions is caused by the formation of oxygen-containing functional groups. However, the O(1s) spectrum for coal is usually broad and structureless. The component from water that was physically adsorbed at a higher binding energy (533 eV in this study) is unresolved from components

92 of other oxygen-containing species. Therefore, it is difficult to determine the cause of the change in the O(1s) spectrum without further investigations (Buckley, 1994b).

O(1s) a b

c Intensity

538 536 534 532 530 528 Binding energy (eV)

Figure 4.8. O(1s) spectra of coal B samples oxidised in (a) process water, (b) 0.01 M

Na2CO3 solution and (c) Milli-Q water.

Figure 4.9 shows the C(1s) spectra of samples oxidised in process water, 0.01 M Na2CO3 solution and Milli-Q water. According to the C(1s) spectrum and atomic % (see Table 4.7) of the process water oxidised sample, carbon contributed to a relatively lower proportion of the examined surface area compared with that of the other samples. The decrease in the atomic % of C(1s) is probably due to the increased concentration of oxygen species on the oxidised coal surface. It is unlikely that carbon was released from coal through reaction(s) with substances from the process water because carbon in coal is normally released in gaseous forms through combustion or carbonization (Gibbins and Chalmers, 2008).

It is observed that at lower binding energy the line widths of the hydrocarbon components from coals oxidised in the presence of 0.01 M Na2CO3 solution and Milli-Q water are similar, whereas the coal treated with process water showed a significant decrease in intensity reflecting a lower concentration of hydrocarbon components. Additional intensity was observed between 286 eV and 290 eV for coals treated with 0.01 M Na2CO3 solution

93 and process water. The increases might be caused by the formation of oxygen containing C- O, C=O, and COOH groups, which are known products of coal oxidation (Xia et al., 2014, Fuerstenau et al., 1987, Ruberto and Cronauer, 1978). It seems that formation of the oxygen containing functional groups was accelerated by the presence of salt during the oxidation process, as more of these groups were formed in samples oxidised in the presence of the

0.01 M Na2CO3 solution and process water. The intensity of O(1s) spectrum in the process water treated sample was found to be inversely proportional to the C(1s) spectra in Figure 6 (see Supporting information), which may be attributed to the functional groups.

CH C(1s) c

b

a

C-O Intensity

C=O COOH

290 288 286 284 282 280 Binding energy (eV)

Figure 4.9. C(1s) spectra of coal B samples oxidised in the presence of (a) process water

(green), (b) 0.01 M Na2CO3 solution (red) and (c) Milli-Q water (blue).

It is clear that Na presented at a very low concentration in the sample treated with process water. Humic acids or/and fulvic acids are abundant in process water and coal. These acids contain –COOH functional groups which would be possible adsorption sites for Na. Therefore, a small amount of Na might be adsorbed on either the acids that were already on the coal surface or the –COOH on the coal surface. It is also possible that the sample surface has been oxidised during the treatment or contains existing oxidised sites, exposing available –COOH sites for Na to adsorb. However, the degree of such adsorption is too low 94 to be considered as the cause of the drastic fluidity reduction observed in the samples used in this study compared to the literature (Crewe et al., 1975).

On the other hand, the treatment with process water seemed to have little to no effect on the S(2p) and Si(2p) spectra, as there was no significant fluctuation in the spectra of these two elements. Because the atomic % of these S(2p) and Si(2p) were too minor (<0.1%) on the surfaces analysed, it was considered negligible and unlikely to induce a significant fluidity reduction.

S(2p)

Intensity

176 174 172 170 168 166 164 162 160 158 Binding energy (eV)

Figure 4.10. S(2p) spectra of coal B samples oxidised in the presence of (a) process water

(green), (b) 0.01 M Na2CO3 solution (red) and (c) Milli-Q water (blue).

95 Si(2p)

Intensity

110 108 106 104 102 100 98 96 Binding energy (eV)

Figure 4.11. Si(2p) spectra of coal B samples oxidised in the presence of (a) process water

(green), (b) 0.01 M Na2CO3 solution (red) and (c) Milli-Q water (blue).

The experimental results indicate that salt absorption likely causes a fluidity reduction in the coal sample, which could be related to coal’s naturally porous surface structure. Other surface analyses were conducted to confirm this hypothesis.

4.5 SEM-EDS Analysis of Oxidised Coal Surface

SEM-EDS analysis was used to determine the presence of inorganic salts on the coal surface after oxidation with salt. The samples were prepared as described earlier by using platinum coatings for SEM surface texture analysis and carbon coatings for EDS elemental analysis. Figure 4.12 shows the back-scatter images of the untreated and treated coal surfaces from SEM. The difference in brightness indicates the difference in chemical composition of the surface. In the figure the darker grey color represents the carbon contents, whereas the lighter colored area represents non-carbon species. More non-carbon

96 species are present on the oxidised coal surface as shown by the back-scatter images. However, the EDS analysis suggests that these non-carbon species are mostly composites of Al, Si and O.

a

b

Figure 4.12. Back-scatter image for coal A samples: (a) untreated surface and (b) process water treated surface.

The carbon and non-carbon regions on the process water oxidised coal surface were scanned with magnification increased to 10,000 times. The difference in morphology can be clearly seen in Figure 4.13. The carbon surface remained relatively smooth, whereas the 97 non-carbon region showed a distinct increase in surface roughness. Furthermore, it appears that the non-carbon material was composed mainly of fine particles unevenly distributed on the coal surface. As the majority of the non-carbon components on both coal samples were found to be a composite of Al, Si and O, it is very likely that this substance belongs to the aluminosilicate group, which is commonly seen in coal mineralogy. In other words, this substance was probably already in the coal.

a

b

Figure 4.13. SEM results for (a) carbon surface and (b) non-carbon surface from process water treated coal A sample.

98 Figure 4.14 compares the spectra from elemental scanning between the untreated coal sample and process water oxidised coal sample. It is evident that Na, Cl and a tiny amount of K were on the coal surface after oxidation with process water. The spectra of Na and Cl correspond to the existence of the solid grains on the coal surface, indicating that NaCl precipitated after the evaporation of moisture. Although K was also detected according to the spectrum, there was no solid K compound observed on the coal surface, which might be due to the insufficient amount of K absorption on the coal surface.

C

O Al Si Na S Cl K Intensity b

a

0 0.5 1 1.5 2 2.5 3 3.5 4 Binding energy (eV)

Figure 4.14. Spectra from elemental scanning of (a) untreated surface and (b) process water oxidised surfaces.

The most interesting finding from the SEM and EDS analyses is that both Na and Cl appeared to be present on the surface of the process water treated coal sample. Figure 4.15 shows the scanned image of (a) the oxidised coal surface in the presence of process water and the elemental scans of (b) Cl and (c) Na on that surface. It is obvious that the light- colored substance is a composite of Na and Cl, which is very likely NaCl. This substance was not observed on the surface of the untreated coal sample, as shown in the elemental scanning spectra of untreated and treated coal (see Supporting information).

99 a

b

c

Figure 4.15. EDS analysis of the process water treated coal A surface showing: (a) original image of the surface, (b) Cl and (c) Na. 100 The composite of Na and Cl observed was located in the aluminosilicate grain in a dry form (as shown in Figure 4.15), rather than in the pores on the coal surface. Thus, a possible interaction between the NaCl in the process water and the coal surface could be that the process water was first retained on the coal surface in the gaps of aluminosilicate particles during sample preparation, and then the water in the retained moisture was released due to natural evaporation and N2 drying, leaving the dried concentrate on NaCl on the coal surface as precipitates.

4.6 Effect of total concentration of inorganic salt on coal fluidity

In the previously established experimental method, inorganic salts were retained in the coal as a result of incomplete moisture removal (approximately 30% remaining moisture). The remaining moisture (i.e. salt solution) was dried with the coal under N2 gas, where the water was evaporated leaving solid salt grains on coal surface. The proposed mechanism considers these remaining salt grains as inert additives to coal that might cause the fluidity reduction. In addition, it was found that each inorganic salt has similar effect of fluidity reduction, and the effects were non-comparable to what was observed on the samples treated with process water, where the fluidity was reduced much more. It was suspected that the effect of inorganic salts on coal fluidity is not only similar but also accumulative when they are working as inert additives. In other words, the relationship between coal fluidity inorganic salts is more likely to be dependent on the total amount of all salts rather than the type and concentration of individual salt(s).

To justify the hypothesis that coal fluidity is affected by the concentration of total inorganic salts, subsequent oxidation experiments were carried out with the process water samples with adjusted total salt concentrations. Table 4.8 shows the Gieseler plastometric measurements of coal samples treated with different salt concentrations. It can be seen that fluidity of the coal sample decreases as the total concentration of inorganic salts increases. However, the temperature histories and PR seemed to be unaffected.

101 Table 4.8. Gieseler plastometric measurements of coal samples treated with different salt concentrations.

IST MFT RST PR MF Log MF Diluted process water 10 °C ddpm

10% 403 439 472 69 670 2.83

50% 402 438 469 67 690 2.84

100% 403 438 470 67 590 2.77

150% 403 438 469 66 500 2.70

200% 401 439 469 68 510 2.71

Error! Reference source not found. compares the differences in Log10 MF among coal samples treated with different salt concentrations, where a decreasing trend can be clearly seen. The results were in line with the hypothesis that the MF is decreased as the total concentration of inorganic salts used to treat the coal sample was increased. Considering the results obtained from the SEM-EDS analysis, it might be possible that the other inorganic salts worked on the same role as NaCl does. However, a subsequent SEM-EDS analysis did not find the presence of crystalised salt grain on the coal samples treated with process waters with different salt concentrations. It might be due to the fact the in SEM- EDS analysis only a few particles (5 to 10) can be examined, and during the procedure of preparing samples (see section 3.4.5) for the SEM-EDS analysis the selected particles might not be representative enough because they were pressed, which breaks some coal particles. The surfaces observed might be the crushed “fresh” surface of coal particles where the actual treated surface was not facing the microscope therefore cannot be examined.

102 2.86 2.84 2.82

2.8 Milli-Q 2.78 10%

MF MF ddpm 2.76 50% 10 100%

Log 2.74 2.72 150% 200% 2.7 2.68 0 50 100 150 200 250 Dilution ratio (%)

Figure 4.16. Log10 MF of coal samples treated with process waters with different dilution ratios.

4.7 Mechanism of fluidity reduction in coal preparation plant

The Gieseler fluidity test results suggested that the 7-day oxidation regime itself did not affect the fluidity of coal A sample much when no salt was added in the oxidation process. However, the addition of individual inorganic salt solutions and the process water sample reduced the fluidity sharply with the individual effect of each salt being similar.

Interestingly, surface analyses using XPS and SEM-EDS have shown that while no significant adsorption was found, Na and Cl precipitated as NaCl concentrate on the coal surface. Previous studies have investigated the volatilisation behavior of NaCl while it was being co-pyrolysed with coal and found that Na and Cl tend to be volatilised separately following entirely different trends (Quyn et al., 2002, Wu et al., 2002, Pearce and Hill, 1986). According to Pearce and Hill (1986) and Quyn et al. (2002), Cl can be easily removed from the coal structure under either air or N2 atmosphere when heated above 170 °C, usually through bonding with the available H in coal and volatilisation. The Na, on the other hand, has a much higher volatilising point, which is above 800 °C, and has been

103 reported to be retained in the post-pyrolysis char product (Quyn et al., 2002). Further evidence was provided by Patrick and Shaw (1972) who reported that the addition of

Na2CO3 in coal pyrolysis resulted in the production of metallic Na and CO gas and the Na was completely volatilised only during pyrolysis at 1000 °C and above. The presences of other inorganic salts used in the present study have been rarely studied previously. Little is known about their reaction with coal during pyrolysis. However, the inorganic salt solutions used in the present study have in common that the boiling points of the alkali and alkaline earth metals (AAEMs) are all above the re-solidification temperature of the coal A sample (467~473 °C, minimum to maximum as measured).

It is believed that the ageing of the coal in the presence of ions may have been conducive to the oxidation of the coal surface, which affects plastometry. As well, the anions associated with the AAEMs were volatilised at lower temperatures and released from the coal structure during pyrolysis. The remaining AAEMs worked as inert additives during the thermoplastic stage. The non-plastic nature of the AAEMs increased the viscosity of the coal during the thermoplastic stage and was likely bonded with the released free radicals through cross-linking. The cross-linked products were then included in the coke complex after re-solidification (Patrick and Shaw, 1972, Solomon et al., 1990). Furthermore, Lin and Hong (1986) used two plastic coals and pitch to increase the fluidity of a non-plastic base coal. Although their focus was to increase the maximum fluidity of the non-plastic-based coal and to investigate its subsequent effect on coke reactivity, they also showed that adding less- or non-plastic materials in the blend decreases the fluidity of the blend. A similar study was conducted by Kumar et al. (2008b) where a non-plastic coal was mixed with coals with higher fluidities. However, no detailed explanation was given in either study on the chemical interactions between the non-plastic coal and the plastic coal. Despite the proposed mechanism, a more detailed explanation cannot be provided without further investigation of chemical composition of the semi-coke generated from fluidity tests. Future work will focus on the change in surface chemistry of the coal and its corresponding semi-coke product. The comparison will bring more insights on the interaction between the inorganic salts and coal during the pyrolysis process, and possibly establish a relationship between the concentration of added inorganic salts and coal thermoplasticity.

104 4.8 Concluding remarks

The present study investigated the influences of chemical species present in the process water of a coal preparation plant on the behavior of coal thermoplasticity. The results showed that the effects of different inorganic salt solutions were similar in reducing coal fluidity. Interestingly, the negative impact of the salts on coal fluidity was mainly dependent on the total amount of salt rather than the type of salt used in the study. The SEM-EDS analysis showed the dominant percentage of Na and Cl in the gaps between the aluminosilicate particles on the surface of coal sample A following oxidation with the process water, which indicates the occurrence of NaCl precipitation on the surface. On the other hand, there was no evidence from the XPS surface analysis of inorganic ion adsorption on the surface of the coal treated with process water.

The effect of total concentration of inorganic salts on coal fluidity was also discussed. The Gieseler fluidity results agree with the hypothesis that the MF of coal is affected by the total concentration of salts present during the aging of coal rather than a specific type of salt. A possible mechanism for how the inorganic salts interact with coal in the present study was provided in the following process: (1) the dissolved inorganic salts from the process water were absorbed on the coal surface and precipitated after drying, (2) the salt precipitates worked as additives and co-pyrolysed with the coal samples, (3) the anions from the salts were volatilised while the inert AAEMs remained in the coal, and (4) the AAEMs reduced the fluidity of the blend by increasing the viscosity of the fluid and cross- linking with the free radicals. The author believes that the AAEM salts work as fluidity inhibitor of coal fluidity evolution and follow a same mechanism.

105 Chapter 5 Conclusions and recommendations

5.1 Conclusions

The thermoplasticity of metallurgical coal is the most critical factor in the determination of coke quality. Coal with greater thermoplasticity generally produces coke with greater strength, which would benefit the iron blasting process. In addition, coal with greater thermoplasticity normally yields coke with better reactivity, resulting in better reducing ability when working as a reducing agent in the blast furnace. The present study investigated the effect of water chemistry on the fluidity of coal, which is a commonly used indicator of coal thermoplasticity that reflects the thermoplastic behaviours of a coal during pyrolysis. A decrease in the fluidity means that the thermoplasticity of the coal is weakened, which would essentially result in poorer coke strength.

Previous studies reported reductions in coal thermoplastic properties when inorganic salts were co-pyrolysed with coal (Crewe et al., 1975, Barriocanal et al., 1996, Patrick and Shaw, 1972). As more and more coal preparation plants adopt the strategy of reusing the process water for operations, the effect of chemicals accumulated in the reused process water on coal thermoplasticity should be considered. Most previous research investigated process water chemistry and coal thermoplastic properties as separate topics, with little to no attention on the interaction between coal and the inorganics salts in the process water (Wang and Peng, 2014a, Wang and Peng, 2014b, Wang et al., 2013, Bournival et al., 2017). A pronounced negative impact from reused process water would bring drastic financial loss to the metallurgical coal producers because of the poorer product quality. More importantly, there are safety risks associated with using poorer quality coke in iron blasting as coke with poor strength tend to collapse more easily.

The present study explored the interaction between coal and the inorganic substances in the reused process water received from coal preparation plants and examined the effect on coal fluidity. The findings form these studies provide insights on the coal-process water interaction and its consequent effects on coal thermoplasticity, which has hardly been explored previously. The outcomes of this study serve not only as a foundation for future

106 research works in the same area, but also as a reference for the industry when considering possible effects of the chemical composition of process water on coal quality.

The coal and water samples used in the present study included two run-of-mine bituminous coals from Queensland (coal A) and New South Wales (coal B). Both coal samples had similar compositions, with different thermoplastic behaviour. The process water sample used in the study was taken from a coal preparation plant in Queensland. The water contained high concentrations of sodium, chlorine and sulfate. Artificial inorganic salt solutions were also prepared to investigate the effect of each individual salt on coal thermoplastic properties. In addition, the effect of total salt concentration in the process water on coal fluidity was examined by changing the dilution ratio of the total salt content in the process water. The treatments of coal in the present study included soaking, washing and oxidising. Acid leaching using a diluted hydrochloric acid was also conducted to examine the effect of demineralisation on coal ash and fluidity. Surface analyses of the raw and treated coal samples were carried out using XPS analysis and SEM-EDS analysis to better understand the coal-salt interaction during treatments. The thermoplasticity of the coal samples was measured by a Gieseler plastometer, including temperature history and plastic range during the coal’s maximum fluidity. From these empirical investigations, the following conclusions are made.

 As Na2CO3 is known to have a pronounced negative impact on coal thermoplastic properties, and an abundant amount of Na was found in the process water sample,

an experiment was first carried out with a 0.1 M Na2CO3 solution. The results

showed that Na2CO3 solution of 0.1 M, which is close to the concentration of inorganic salts in the process water (0.12 M), reduced the fluidity of the coal sample

by 90%. In addition, solutions of K2SO4, Na2SO4, CaCl, MgCl and CaCO3 with a 0.01 M concentration were found to reduce coal fluidity sharply by 80% to a similar level. It was further found that the plastic range of the coal was reduced by 3-5 °C. While the initial softening temperature and maximum fluidity temperature were unchanged, the shortening of the plastic range was mainly achieved by the early resolidification. The results suggested that the inorganic salts have negative impact on coal fluidity by shifting the resolidification stage to lower temperatures. 107  The reused process water sample, which had a total salt concentration of 0.125 M,

was found to have a similar effect on coal fluidity as the 0.1 M Na2CO3 solution. Treating coal with process water samples of different total concentrations of inorganic salts found a decreasing trend in coal fluidity when the total concentration of inorganic salts in the process water was increased. Meanwhile, early resolidification and a decreased plastic range were observed in the samples treated with increased total salt concentration. Remarkably, the findings from the fluidity investigations of the salt-treated coal strongly suggested that coal fluidity is sensitive to the total concentration of inorganic salts rather than the type.

 The SEM-EDS analysis showed that grains of dried NaCl were retained on the coal surface after treatment with process water. On the other hand, the XPS analysis of the treated coal surface did not find significant changes in spectra of elements. Therefore, the interaction between coal and inorganic salt was believed to be absorption rather than adsorption. The hypothesised mechanism of the interaction is that the salts crystallised on the coal surface during the drying stage.

 Lastly, because all the inorganic salts identified in the process water sample belong to the alkali and alkaline earth metal (AAEM) species, which are generally volatilised at temperatures much higher than the maximum temperature of the coal sample, it is believed that the AAEMs from the inorganic salts worked as inert additives during the thermoplastic stage of the coal samples. A mechanism for the interaction between inorganic salts in process water and the coal surface was proposed: (1) the dissolved inorganic salts from the process water were absorbed on the coal surface and precipitated after drying, (2) the salt precipitates worked as additives and co-pyrolysed with the coal samples, (3) the anions from the salts were volatilised while the inert AAEMs remained in the coal, and (4) the AAEMs reduced the fluidity of the blend by increasing the viscosity of the fluid and cross- linking with the free radicals.

108 5.2 Recommendations

The findings from the present study have laid a foundation for future research with similar interests. The findings have also provided further insights into how the inorganic salts in process water affect the quality of coal and, ultimately, the coke product. However, due to time constraints, further work using other thermoplasticity examination methods could not be conducted. This section describes a number of promising investigations that could be carried out in the future.

 The present study only investigated the effect of inorganic salts in process water on coal fluidity. However, coal fluidity cannot represent thermoplastic properties as a whole. Future studies should focus on investigating other coal thermoplasticity parameters, such as free swelling index and dilatation.

 The current investigation was mainly concerned with the inorganic salts from the process water, which are only one part of the process water constituents. Constituents in process water, especially reused water, can be very complex, and the effect of these constituents on coal thermoplasticity requires extensive work to be fully understood.

 XPS and SEM-EDS techniques were employed in the study to analyse the surface of the coal particles. Each surface analysis technique focuses on a unique aspect of the surface property. Therefore, adopting different techniques such as nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR) may improve understanding of the coal surface interactions.

 The present study only investigated coal from two Australian sources, which might not be representative enough, and the findings from this study may not apply to different coal. Similar studies should be conducted on coal with other ranks or different origins to expand understanding of coal thermoplastic properties and variability.

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124 Appendix

Table A1. Full ICP-OES analysis results.

Analyte Name Symbol Unit Concentration

Aluminium Al mg/l 0.027 Boron B mg/l 0.190 Barium Ba mg/l 0.013 Calcium Ca mg/l 18.084 Cobalt Co mg/l 0.000 Copper Cu mg/l 0.005 Iron Fe mg/l -0.003 Potassium K mg/l 16.336 Lithium Li mg/l 0.270 Magnesium Mg mg/l 28.928 Manganese Mn mg/l 0.000 Sodium Na mg/l 1453.302 Nickel Ni mg/l 0.033 Phosphorus P mg/l 0.047 Lead Pb mg/l 0.001 Sulfur S mg/l 536.344 Silica Si mg/l 1.731 Strontium Sr mg/l 0.507 Titanium Ti mg/l 0.007 Zinc Zn mg/l 0.007 Chlorine Cl mg/kg 638.315 Sulfate SO4 mg/kg 1555.541

125 Table A2. Titration results for process water and replica. HCl addition (mL) pH pH (replica) 0 8.81 8.82 0.2 8.65 8.66 0.4 8.41 8.4 0.6 7.94 7.95 0.8 7.68 7.7 1 7.45 7.46 1.2 7.3 7.31 1.4 7.17 7.19 1.6 7.08 7.1 1.8 7 7.02 2 6.93 6.96 2.2 6.87 6.9 2.4 6.83 6.8 2.6 6.77 6.76 2.8 6.73 6.73 3 6.67 6.67 3.2 6.65 6.65 3.4 6.65 6.6 3.6 6.51 6.57 3.8 6.37 6.52 4 6.33 6.47 4.2 6.24 6.42 4.4 6.19 6.38 4.6 6.14 6.32 4.8 6.08 6.27 5 6.03 6.22 5.2 6.01 6.14 5.4 5.97 6.09 5.6 5.92 6.05 5.8 5.83 5.95 6 5.75 5.8 6.2 5.61 5.7 6.4 5.51 5.56 6.6 5.4 5.44 6.8 5.14 5.14 7 4.56 4.45 7.2 3.74 3.68 7.4 3.39 3.36 7.6 3.19 3.17 7.8 3.06 3.03 126 8 2.96 2.93 8.2 2.88 2.88 8.4 2.8 2.82 8.6 2.74 2.75 8.8 2.69 2.68 9 2.64 2.62 9.2 2.6 2.58 9.4 2.57 2.54 9.6 2.53 2.5 9.8 2.49 2.47 10 2.46 2.44 10.2 2.43 2.41 10.4 2.4 2.38 10.6 2.38 2.35 10.8 2.36 2.33 11 2.34 2.3 11.2 2.31 2.28 11.4 2.28 2.26 11.6 2.26 2.24 11.8 2.24 2.22 12 2.23 2.2

127