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Selection of reagents based on surface chemistry as derived from micro-FTIR mapping of coal surface to… Wei, Wang; Kumar, Amit; Holuszko, M. E.; Mastalerz, Maria D. Apr 30, 2017

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 International Journal of Mineral Processing/Minerals Engineering-special issue after COM 2017     SELECTION OF REAGENTS BASED ON SURFACE CHEMISTRY AS DERIVED FROM MICRO-FTIR MAPPING OF COAL SURFACE TO FACILITATE SELECTIVITY IN COAL FLOTATION Wei Wang1,2, Amit Kumar1, *Maria E. Holuszko1, Maria D. Mastalerz3 1Norman B. Keevil Institute of Mining Engineering, University of British Columbia Vancouver, Canada, V6T 1Z4 (Corresponding author: meh@apsc.ubc.ca) 2China University of Mining and Technology Xuzhou, Jiangsu, China 3Indiana Geological Survey, Indiana University Bloomington, Indiana 47405-2208  ABSTRACT In this study, the micro-FTIR analysis was used to investigate in-situ surface properties of coal and its components (macerals) and these were directly correlated to the coal hydrophobicity as measured by the contact angle before and after reagent addition to understand the suitability of the reagent for flotation. The micro-FTIR technique can provide a semi-quantitative analysis of the chemical functional groups on coal surface, exactly where the contact angle is being measured. Selected functional groups and their ratios that provide information about aromaticity, the contribution of oxygenated groups, and aliphaticity are used to evaluate the reactivity between reagents and various coal components. The observations made in relation to the susceptibility of some reagents towards certain coal components should lead the way to a smarter way of using process design in terms of reagents selection for coal flotation.    KEYWORDS Coal, Flotation, Micro-FTIR, Macerals, Reagents    International Journal of Mineral Processing/Minerals Engineering-special issue after COM 2017   Introduction Coal flotation is used for treating fines of metallurgical coals. Metallurgical coals are used for coke making and then coke is used for steel production. These coals are usually of relatively high rank and high-rank coals are naturally hydrophobic. The changes in the hydrophobicity are attributed to changes in aromatic and aliphatic hydrocarbon coal structures as well as the content of oxygen groups across the coal ranks (Klassen, 1953; Aplan, 1983; Onlin & Aplan, 1984; Arnold & Aplan, 1989; Holuszko & Mastalerz, 2015). The surface properties of the coal are determined by the functional groups of the organic matter, presence and quantities of mineral matter, volumes of pores, and the association between minerals and macerals, where macerals are the smallest microscopically distinguishable organic components in coal. The simplest way to establish the relationship between coal properties and hydrophobicity is by using contact angle measurements on the surface of the coal. The contact angle techniques have been used to characterize the surface hydrophobicity for coal for years (Hanning & Rutter, 1989). A higher contact angle represents a hydrophobic and highly floatable surface while lower contact angle values indicate a less hydrophobic surface with less floatability.  The micro-Fourier Transform Infrared (micro-FTIR) spectroscopic procedures for coal surface analysis had been developed in the 1990’s (Mastalerz & Bustin, 1993). Chen et al. (2013) suggested that a reflectance-based micro-FTIR spectroscopy mapping method could be used to characterize the functional groups in coal macerals and snapping pictures of the measurement area simultaneously can provide high-resolution information about the chemical properties of coal macerals. Liu (2016) developed an in-situ procedure for carrying out micro-FTIR and contact angle measurements of the same areas and attempted to correlate the chemical composition of the coal surface, including macerals, with a corresponding contact angle to investigate the hydrophobicity of low and high-rank coals.  Oily collectors such as kerosene, diesel and pine oil are the most common reagents used as collectors for coal flotation. It has been proven in practice that the addition of these oily collectors increases the hydrophobicity of coal. Selection of reagents for coal flotation is usually accomplished by tedious test work and optimization, without emphasis on the selectivity toward any of the coal components. Studying the adsorption mechanism of various collectors on the coal surface using the micro-FTIR technique is a new approach to study their suitability for coal flotation.  From the previous studies, as deliberated in Holuszko & Mastalerz, (2015), it was demonstrated that floatability of macerals across the various ranks could be stimulated by using chemical reagents that could be selected in such a way that they adsorb specifically on certain macerals. The possibility of the selective flotation of coal to produce concentrates of desired petrographic composition holds tremendous promise for industrial applications, especially for coking coals (Given et al. 1975; Shibaoka & Ueda, 1978; Parkash et al., 1984 &1985). On the other hand, there is a need for finding the suitable flotation reagents for low rank and difficult-to-float coals if they are to be utilized in a sustainable way. Studying the affinity of various flotation collectors to coal surfaces of known chemistry is an important step forward in understanding mechanisms for such affinities.  In this study, the micro-FTIR analysis is used to investigate in-situ surface properties of coal and its components and these are directly correlated to the coal hydrophobicity as measured by contact angle before and after reagent addition to understand the suitability of a reagent for flotation. Micro-FTIR mapping can provide details of the adsorption of reagents on specific areas of coal or macerals and change the coal surface properties, thus affecting the contact angle. The micro-FTIR can also quantify the changes in the surface chemistry of coal after the addition of the reagents and could explain why certain reagents work better for a given coal. The development of the procedures needed to study the above-mentioned effects is described in this paper. Materials and Methodology Materials Medium volatile bituminous coal from a mine in British Columbia was used in this study. This coal is higher rank coal with an average mean vitrinite reflectance Ro of 1.6%, volatile matter content (VMdmmf) of 25.2% and ash content of 17.1%. Two samples were selected and they were cut to form a small block of coal with approximate dimensions of 40 mm × 20 mm × 15 mm. Each block was polished using 200, 600, 800 and 1200 grit papers followed by 6 µm and 1 µm fine diamond suspension and 1 nm silica powder suspension to produce a smooth  International Journal of Mineral Processing/Minerals Engineering-special issue after COM 2017   surface for the test measurements. On each block of coal, several areas (4 mm × 4 mm) were marked off for analysis. Block H1 had two areas while block H2 had four areas. Three reagents, diesel (Shell gas station), pine needle oil (Now Foods) and dodecyl trimethyl ammonium bromide (DTAB, Sigma-Aldrich Chemistry), were used as a collector and to treat the sample surface before measurements. The samples were immersed in an emulsion/solution (in water) of the reagent and the dosage for each reagent was kept constant at 150 g/t with a conditioning time of 3 minutes. The samples were then washed with mild soap and dried at 450C for approximately 30 minutes after the measurement to prevent contamination. Contact Angle Measurement The captive bubble method was adopted to conduct the contact angle measurements using an FTA 1000 Drop Shape Analyzer according to an established procedure (Liu, 2016). The equipment consisted of a manual stage with a specialized illumination source that was situated behind the sample so that the image became a silhouette. In this method, the sample is immersed in the liquid. A small air bubble is produced at the tip of the U-shaped needle (inner diameter 0.152 mm) using the micro-syringe (syringe capacity 100 µl, pimp in and out rate 0.08 µl) and placed in contact at the bottom surface of the sample. A microscope equipped with a camera was used to capture the image. A total of 153 images were captured in approximately 200 seconds and the contact angle was measured for each image. The contact angle values from last 50 seconds were averaged to estimate the final contact angle for that area. The contact angle values were stable after 100 seconds. The measurements were conducted on the marked areas before and after conditioned by the reagents. An illustration of the captive bubble method is shown in Figure 1.  Figure 1: Illustration of the captive bubble contact angle measurement method Micro-FTIR Spectroscopy Measurement A Nicolet 6700 spectrometer paired with a Nicolet Continuum microscope was used to perform the micro-FTIR measurements. The micro-FTIR was operated in reflected light mode with a resolution of 4 cm-1 with a gold plate as a background. The Nicolet Continuum microscope consists of a video camera, a liquid nitrogen cooled Mercury Cadmium Telluride (MCT) detector and a motorized mapping stage. The reflectance micro-FTIR spectra were collected within the 4000-700 cm-1 wavenumber range. The number of sample scans used was 500 and the obtained spectra were subjected to the Kramers-Krönig transformation. Band assignments followed published sources (Painter et al., 1981 & 1985; Wang & Griffiths, 1985; Mastalerz & Bustin, 1996; Chen et al., 2012). The OMNIC software was used for peak area integration. An infrared objective with 15x magnification and aperture size of 100 µm × 100 µm was employed to generate the spectrum of the marked areas of the samples. Sixteen (16) test points, evenly distributed on each marked area, as illustrated in Figure 2 were selected for micro-FTIR measurements. The measurements were performed before and after the conditioning by diesel, pine needle oil, and DTAB. Micro-FTIR mapping was also conducted on selected areas before and after the treatment.  International Journal of Mineral Processing/Minerals Engineering-special issue after COM 2017     Figure 2: Schematic of micro-FTIR test points on H2 area 4 FTIR Measurement A Perkin Elmer Spectrum 100 FTIR Spectrometer was used to perform the FTIR measurements on all marked areas as well as the three reagents. The spectra were obtained between 4000 cm-1 and 350 cm-1 at a resolution of 1 cm-1. Figure 3 shows the FTIR spectra of three reagents.  Figure 3: FTIR spectra of reagents Results and Discussion Contact angle The contact angle results on these high-rank coal samples were found to be very variable depending on the area/spot where the measurements were taken as shown in Table 1. Contact angles were measured on predetermined areas of polished blocks from the high-rank coal. For this research, contact angle values were derived from the captive bubble method and were used for correlating with the chemical composition of the coal surface as derived  International Journal of Mineral Processing/Minerals Engineering-special issue after COM 2017   from the micro-FTIR technique before and after the adsorption of reagents commonly used as collectors in coal flotation. Table 1: Contact angle for different spots on two coal specimens Coal/reagent H1 area 1 H1 area 2 H2 area 1 H2 area 2 H2 area 3 H2 area 4 No reagent 51.75 45.07 46.67 44.21 44.49 43.45 Diesel 58.86 65.87 55.78 54.41 62.17 62.50 Pine needle oil 52.65 51.66 54.28 52.44 55.29 52.23 DTAB 52.23 51.70 50.20 46.48 47.70 50.89  For the original surface (without the reagents) the contact angle values varied between 43.450 to 51.750. All the reagents improved hydrophobicity of the coal since contact angle values rose after using reagents as compared to the original surface. The highest contact angle values were recorded for the coal surface treated with diesel, with the highest contact angle observed on the H1 area2 spot with a value of 65.870.  The lowest increase in contact angle value was observed for the coal surface treated with DTAB, while pine needle oil has shown to cause moderate improvement in the hydrophobicity of coal. Different areas were affected differently depending on their composition. Over 100 measurements of contact angle were taken versus time in each area, hence the standard deviation can represent the stability of the resulting coatings/adsorbed layers, or the homogeneity of the surface. The standard deviation for the contact angle measured on the original coal surface (no reagent) was found between 0.1310 to 1.2220; after using diesel 0.4240 to 2.8310; for pine oil 0.0570 to 0.5580; for DTAB; 0.0750 to 0.7950. The greatest reduction in the variability of measurement was observed for the surface treated with diesel compared to pine needle oil and DTAB, indicating more stability of the surface after treatment with diesel. Generally, the reproducibility of the captive bubble technique was shown to be about ±0.50 with an average standard deviation of ±0.280. Similar standard deviations have been reported by other researchers who have used these techniques (Ofori et al., 2006 & 2010; Liu, 2016).  Even on the same polished block, the average contact angle value is represented by a wide range of values as shown in Table 1. The difference in measured contact angle values could have resulted from (a) the local petrographic and chemical heterogeneity on the chosen spots; (b) the cracks; (c) the roughness and porosity of the coal surface; and (d) the presence of invisibly small amounts of minerals, even though the areas were chosen to minimize these effects.  Semi-quantitative analysis of functional groups  Micro-FTIR spectra were collected in the same areas where contact angles were measured. Each spectrum represents the average FTIR spectrum of all 16 points in the selected areas of the studied coal samples. A semi-quantitative aspect of the FTIR analysis is presented as ratios of the integrated areas of individual functional groups as shown in Table 2 that translate into the following characteristics: a) Aromaticity is defined as C-Haromatic stretch (3000-3100 cm-1)/C-Haliphatic stretch (2800-3000 cm-1). b) Oxygenated is defined as C=O (1650-1800 cm-1)/C=Caromatic (1500-1650 cm-1). c) Aliphaticity is defined as C-Haliphatic stretch (2800-3000 cm-1)/ C=O (1650-1800 cm-1). These ratios have been chosen to represent chemical characteristics that influence coal hydrophobicity followed by results obtained from work by Liu (2016). Table 2: Semi-quantitative ratios on the predetermined areas Area Aromaticity Oxygenated Aliphaticity Contact angle H1 area 1 (no reagent) 0.207 0.042 13.72 51.75 H1 area 1 (pine needle oil) 0.156 0.060 3.61 52.65 H1 area 1 (diesel) 0.074 0.079 12.83 58.86 H1 area 2 (no reagent) 0.179 0.032 19.54 45.07 H1 area 2 (pine needle oil) 0.086 0.051 5.44 51.66 H1 area 2 (diesel) 0.174 0.030 20.73 65.87 H2 area 1 (no reagent) 0.200 0.030 18.19 46.67 H2 area 1 (pine needle oil) 0.224 0.060 4.95 54.28 H2 area 1 (diesel) 0.146 0.074 6.64 55.78  International Journal of Mineral Processing/Minerals Engineering-special issue after COM 2017   H2 area 1 (DTAB) 0.182 0.053 5.27 50.20 H2 area 2 (no reagent) 0.256 0.031 14.84 44.21 H2 area 2 (pine needle oil) 0.145 0.067 3.43 52.44 H2 area 2 (diesel) 0.115 0.071 7.77 54.41 H2 area 2 (DTAB) 0.481 0.070 3.41 46.48 H2 area 3 (no reagent) 0.254 0.053 10.53 44.49 H2 area 3 (pine needle oil) 0.252 0.033 7.07 55.29 H2 area 3 (diesel) 0.172 0.032 18.97 62.17 H2 area 3 (DTAB) 0.095 0.035 7.16 47.70 H2 area 4 (no reagent) 0.289 0.112 5.01 43.45 H2 area 4 (pine needle oil) 0.142 0.055 4.52 52.23 H2 area 4 (diesel) 0.151 0.048 12.63 62.50 H2 area 4 (DTAB) 0.457 0.055 6.12 50.89  The values of Aromaticity (AR) are very low because of the low intensity of the aromatic CHar stretching vibration band (3083-3002 cm-1). Other ratios show a large spread in values within the same rank of coal, reflecting the chemical heterogeneity of the coal surfaces. The averaged values of the semi-quantitative ratio for each of the functional groups is plotted in Figure 4 and Figure 5. It is noticeable that the aromaticity of coal surface is reduced when diesel and pine oil are used, while no reduction in aromaticity is noticed when DTAB is used to treat the coal. On the other hand, the aliphatic character of studied coal surfaces, as represented by aliphaticity (CHaliphatic/CO) is significantly changed after treatment with either DTAB or pine needle oil, but not when diesel is used.  Figure 4: Semi-quantitative ratio for averaged aromaticity (AR) and oxygenated groups on coal before and after reagent treatment   International Journal of Mineral Processing/Minerals Engineering-special issue after COM 2017   Figure 5: Semi-quantitative ratio for averaged aliphaticity (AL) on coal before and after reagent treatment Aromaticity and the contact angle  There is a linear relationship between aromaticity and contact angles, as derived from the captive bubble techniques. In the coal studied, the contact angle increases as the aromaticity decreases and this is consistent with the previous results by Liu (2016). The higher rank coal used in this study is of medium volatile bituminous rank and it is possible that the decrease in hydrophobicity of this coal is due to the increase in aromaticity as suggested by Klassen (1953), which is caused by the increase in aromatic groups while these groups can interact with polar water molecules through the π-electrons, thus resulting in decreasing contact angle values. The trend between AR represented by the ratio of aromatic stretching over the aliphatic stretching region and contact angle shows that an increase in aromaticity causes a decrease in contact angle for the original coal surface as shown in Figure 6. Adding diesel as a reagent leads to the decrease of the overall aromaticity of the coal surface and this, in turn, makes the coal surface more hydrophobic as evidenced by increased contact angle values. However, compared to diesel, both pine needle oil and DTAB failed to reduce the aromaticity which results in a smaller increase in hydrophobicity as evidenced by the contact angle.  Figure 6: Effect of aromaticity on the contact angle with and without reagents Presence of oxygenated groups and their effect on contact angle The amount of oxygen functional groups is one of the major factors that affect the hydrophobicity of a coal surface (Klassen, 1953; Laskowski, 2001). Hydrogen bonding, dipole-dipole, and even electrostatic interactions between water molecules and oxygen functional groups on the coal surface would increase the overall work of adhesion of water to coal.    International Journal of Mineral Processing/Minerals Engineering-special issue after COM 2017   Figure 7: Effect of oxygen functional groups on the contact angle with and without reagents The ratios of C=O/C=Car is used here to demonstrate the relative contents of aliphatic and oxygenated groups, and to assess their effect on contact angle values, as shown in Figure 7. The ratio varies significantly over the tested areas on coal, but there is only a slight effect on the contact angle on the original surface (without reagents), which could be the result of competing effects of aromaticity and oxygen functional groups content on the measured areas representing specific spots. The wide spread of oxygenated groups ratio indicates a wide variation in chemical composition of various spots on coal surface which is due to its petrographic composition. Generally, the addition of reagents improves the hydrophobicity of coal surface with the greatest effect being attributed to the treatment by diesel. Effect of aliphaticity on the contact angle The increase in the abundance of aliphatic groups is significant, as shown in Figure 8, especially for coal surfaces that were affected by the reagents and this effect is emphasized for diesel. Both pine needle oil and DTAB contributed very little to the increase of hydrophobicity for studied coal surfaces. If anything, they reduced the aliphaticity of the coal surface and this did not help to significantly increase contact angle values.   Figure 8: Effect of aliphatic groups on the contact angle with and without reagents FTIR mapping of coal surface before and after the reagents addition For the coal samples tested in this study, aromaticity and aliphaticity of different areas provided evidence that the chemical composition of each of these spots contributed to the hydrophobicity of coal and at the same time general trends were observed on coal hydrophobicity when all the data was plotted together.  The heterogeneity of coal surface arises from the petrographic composition since different coal macerals represent a great diversity in terms of their chemical composition, hence they can also influence the action of the reagents. Micro-FTIR mapping on two areas was performed to find out how the reagents were distributed on the coal surface given its petrographic make-up or how the chemical composition of the surface was changed after the treatment. Since diesel showed the best performance as a reagent, increasing the hydrophobicity for this coal, the mapping was performed on the tracking aromaticity (AR) and aliphaticity (AL) of coal areas before and after the treatment with diesel. Figure 9 and Figure 10 present mapping of H1 area1 while Figure 11 and Figure 12 provide mapping information on H2 area4 in respect to the aliphatic stretching region and aromaticity ratio. Mapping of H1 area1 resulted in the following observations: • The intensity of aliphatic stretching bands follow maceral composition to some extent, being higher in vitrinite than in inertinite. Some semifusinites also show a relatively high intensity (Figure 9b). • Diesel treatment slightly rearranged this distribution of the intensity of aliphatic stretching bands; the zones of highest intensities in the untreated area do not correspond to the zones of highest intensity in the treated area (Figure 9c).  International Journal of Mineral Processing/Minerals Engineering-special issue after COM 2017   • Aromaticity roughly follows maceral composition, with lower aromaticity in vitrinite and higher in inertinite zones in the b) before treatment.  This is also the case c) after diesel addition, although there is a slight shift of the highest aromaticity zones. In addition, vitrinite becomes even less aromatic after diesel treatment, and the inertinite becomes relatively more aromatic, compared to the original aromaticity distribution. This suggests that diesel got adsorbed to vitrinite better than to inertinite (Figure 10c).   Figure 9: Micro-FTIR mapping of aliphatic stretching region on H1 area1; a) image of the mapped area; b) before treatment; c) after treatment with diesel  International Journal of Mineral Processing/Minerals Engineering-special issue after COM 2017    Figure 10: Micro-FTIR mapping of aromaticity ratio at 3000-3100/2800-3000 cm-1 region on H1 area 1; a) image of the mapped area; b) before treatment; c) after treatment with diesel  Figure 11: Micro-FTIR mapping of aliphatic stretching region on H2 area 4; a) image of the mapped area; b) before treatment; c) after treatment with diesel  International Journal of Mineral Processing/Minerals Engineering-special issue after COM 2017    Figure 12: Micro-FTIR mapping of aromaticity ratio at 3000-3100/2800-3000 cm-1 region on H2 area 4; a) image of the mapped area; b) before treatment; c) after treatment with diesel Mapping of H2 area4 resulted in the following observations: • Diesel also rearranged the distribution of the intensity of aliphatic stretching bands.  In this area, dependence on maceral composition is not so obvious. However, this is also an area with the predominance of inertinite type of macerals (semifusinite, fusinite), so relatively similar maceral composition in terms of chemical composition. Generally, aromaticity does not follow maceral arrangement, and diesel causes a shift in maximum aromaticity. However, surprisingly, for H2 area 4, where the effects of diesel were shown to be quite effective in increasing contact angle after treatment (Table 1), diesel action cannot be easily explained. • The effects of diesel addition clearly indicate that while diesel as a chemical compound is highly aliphatic (Figure 3), it was adsorbed onto the areas that were only slightly aromatic (vitrinite) and by doing so, improved the hydrophobicity of this coal surface. It is perceived that the susceptibility of diesel to the vitrinite will result in selectivity towards this maceral when using this reagent for the flotation of this coal. Conclusions In this study, the micro-FTIR semi-quantitative analysis of chemical functional groups on coal surface was performed in combination with mapping of the areas that were subjected to the contact angle measurements, serving as a proxy for evaluation of hydrophobicity of coal, as the hydrophobicity is a prerequisite for successful flotation.  The micro-FTIR analysis and the mapping of several coal surfaces from this high-rank coal provided information on the changes to the surface composition because of the reagent addition as well as the susceptibility of these reagents to specific areas represented by various petrographic composition. Three reagents commonly used in coal flotation such as diesel, pine needle oil, and DTAB were tested and the results showed that diesel made coal surface the most hydrophobic and its adsorption on the coal surface drastically reduced aromaticity. It was shown that for this high-rank coal, an increase in aromaticity led to the reduction of hydrophobicity and an increased presence of aliphatic groups improved it, and this is in an agreement with previous findings (Liu, 2016). Only diesel provided favorable conditions for enhancing hydrophobicity while DTAB increased aromaticity yet at the same time failed to increase aliphaticity, thus improving the hydrophobicity of this coal. On the other hand, pine needle oil decreased aromaticity but did not raise aliphaticity to high enough levels and this resulted in a relatively small increase in contact angle, hence only a slight increase in hydrophobicity. From this analysis of aromatic and aliphatic ratios, one could conclude that some optimal ratio of aromatic and aliphatic groups is required to provide a desired coal surface hydrophobicity for this coal.     International Journal of Mineral Processing/Minerals Engineering-special issue after COM 2017   Micro-FTIR mapping of the two areas that were tested using diesel (the most effective reagent) provided valuable insight on where diesel adsorbed on the coal. It was concluded that it adsorbed onto less aromatic vitrinite, and provided conditions for improved hydrophobicity of coal surface. It is possible that diesel adsorbed preferentially on the vitrinite and this could result in selectivity towards vitrinite if this reagent is used for flotation of this coal.  This study should be considered a work-in-progress in developing a procedure to find suitable collectors for the flotation of coals of different ranks and petrographic compositions. The outcomes of this work will provide a fundamental understanding of why some reagents are better for some coals and then for others and it may lead to finding a way for selectivity in coal flotation. References Aplan, F. F., (1983). 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