Mineralogy and Magnetic Parameters of Materials Resulting from the Mining and Consumption of Coal from the Douro Coalfield, Northwest Portugal

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CHAPTER 18

Mineralogy and Magnetic Parameters of Materials Resulting from the Mining and Consumption of Coal from the Douro Coalfield, Northwest Portugal

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CHAPTER CONTENTS

18.1 Mineralogy and Magnetic Parameters of Materials from the Douro Coalfield, Northwest Portugal  Introduction  Douro Coalfield: Study Objectives  Environmental Magnetic Studies  Sampling and Analytical Methods   Results and Discussion  Conclusions  Acknowledgments   Important Terms  References A polished sample of burnt coal waste from the Midões waste pile, Douro Coalfield, Portugal. The iron-rich sphere with the dendritic structures formed at high combustion temperatures. Thermally affected organic particles also occur. The photomicrograph was taken using an optical microscope and reflected white light. Photo by Joana Ribeiro, 2010.

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Coal and Peat Fires: A Global Perspective Edited by Glenn B. Stracher, Anupma Prakash and Ellina V. Sokol Copyright © 2015 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/B978-0-444-59509-6.00018-1

18.1  Mineralogy and Magnetic Parameters of Materials from the Douro Coalfield, Northwest Portugal Joana Ribeiro, Helena Sant’Ovaia, Celeste Gomes, Colin Ward, Deolinda Flores

Photo by Joana Ribeiro, 2010.

Iron-rich sphere in fly ash from coal combustion in the Douro Coalfield, Portugal.

 Introduction This work describes the mineralogy and magnetic parameters of materials resulting from coal mining in the Douro Coalfield and from consumption of coal in a neighboring thermal power plant. The coal waste and fly ash generated from coal mining cause adverse environmental impact and are a source of particulate matter, which includes a magnetic fraction. The pollutant particles can be released into the atmosphere through various physical and chemical processes. Combustion of the coal is also a potential source of particulate matter to the atmosphere. Once in the atmosphere those particles can adversely affect the environment and human health. X-ray powder diffraction (XRD) was used for mineralogical characterization, and magnetic susceptibility and isothermal remanent magnetization (IRM) determinations were carried to evaluate magnetic parameters of materials. The XRD analysis provided information about the mineral composition of the materials that helped to identify the reactions that occurred during combustion and hence the temperature reached during the combustion process. The use of magnetic susceptibility measurements and IRM determinations allowed identification of the magnetic properties of the materials resulting from both natural coal combustion (in the coal waste piles) and industrial coal combustion (in the thermal power plant). The unburned coal waste material had quartz, illite, mica (mainly muscovite) and in many cases pyrophyllite as the main minerals. Some of the burning/burnt coal waste samples had similar mineralogy to the unburned materials, but some of them also contained mullite, cristobalite, and amorphous material, suggesting that higher temperatures were involved in the in situ combustion process. The fly ash contained amorphous material, mullite, cristobalite, and maghemite, similar to the burnt waste samples. The magnetic parameters showed an increase in magnetic susceptibility and IRM in some of the burning/burnt zones, indicating a magnetic enhancement due to the burning process. The fly ash samples exhibited even higher magnetic susceptibility and IRM values, attributed to the industrial combustion in the thermal power plant. Hematite was not detected by mineralogical analysis in the unburned zones, but magnetic parameters indicated its presence. In the burning/burnt zones, hematite was detected by both XRD analysis and IRM acquisition curves, which points out the increase in hematite concentration during the combustion. The magnetic parameters in the burning/burnt zones also indicate the presence of a magnetite-type structure, which could be maghemite or magnetite. In the fly ash the magnetic parameters and XRD analysis both indicate the presence of maghemite.

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  Douro Coalfield: Study Objectives The Douro Coalfield is the largest outcrop of terrestrial Carboniferous (Upper Pennsylvanian [Lower Stephanian C]) coal-bearing strata in Portugal (Eagar, 1983; Lemos de Sousa and Wagner, 1983; Wagner and Lemos de Sousa, 1983; Fernandes et al., 1997) with a length of 53 km and a width ranging from 30 to 250 m. The geographic limits of the coalfield are São Pedro de Fins (Maia) and Janarde (S. Pedro do Sul) (Pinto de Jesus, 2001). Mining and consumption of anthracite A (ISO 11760, 2005) in the Douro Coalfield have been responsible for several environmental impacts. The most significant are the waste piles that resulted from mining activities in the area and the fly ash that resulted from coal combustion in an associated thermal power plant. Petrographic, mineralogical, and geochemical characterization of these materials has been carried out (Ribeiro et al., 2010a,b, 2011a,b). The coal waste piles (more than 20) in the Douro Coalfield are composed of overburdened material and discards. In addition to the environmental concerns, three of the coal waste piles (S. Pedro da Cova, Midões, and Lomba waste piles) have been burning since 2005, after ignition by forest fires (Ribeiro et al., 2010a), contributing to atmospheric pollution and changing the nature of the materials. Figure 18.1.1 shows a general view of S. Pedro da Cova and Lomba waste piles. The coal waste and fly ash disposal are a potential source of particulate matter that can be released in the atmosphere through physical and chemical interactions. Coal combustion is responsible for contributing particulate matter to the atmosphere (Evans and Heller, 2003; Suárez-Ruiz and Crelling, 2008). Once in the atmosphere, the particulate matter that includes a magnetic fraction may suffer different fates, such as deposition on vegetation, buildings, soils, and so on. This forms the basis of many environmental studies of particulate pollution (Evans and Heller, 2003).

(a)

(b) Figure 18.1.1.  General view of the S. Pedro da Cova (a) and Lomba (b) waste piles. Note the red colored material, dead vegetation, and gaseous exhalations. The height of the slope in (a) is about 80 m and the width of the stairs in (b) is about 1 m. Photos by Joana Ribeiro, 2009.

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Besides the natural occurrence of magnetic minerals in the environment, anthropogenic sources of magnetic minerals have been investigated in environmental magnetism studies (Evans and Heller, 2003). The combustion of fossil fuels has been described in literature as one of the most significant anthropogenic sources of magnetic particles released into the atmosphere (Dekkers, 1997; Maher and Thompson, 1999 and references therein; Evans and Heller, 2003; McIntosh et al., 2007). In the Douro Coalfield the oxidation of waste material has resulted in the formation of iron oxides. Combustion in the thermal power plant and the burning of coal waste piles has also generated iron oxides, which could later be released to the atmosphere. During combustion pyrite and siderite break down to form iron oxide minerals such as hematite, maghemite, and magnetite (Evans and Heller, 2003; Suárez-Ruiz and Crelling, 2008 and references therein). Therefore the materials resulting from coal mining and coal consumption may affect the environment as well as human health. Finkelman and Stracher (2011) report skin, lung, and heart diseases related to coal fires. Respiratory and cardiovascular illnesses are often related to ambient particulate matter (Chapman et al., 1997 and references therein), especially if the particles are small enough to be inhaled (2.5–10 μm) (Maher and Thompson, 1999). Few environmental magnetism studies have been described based on the natural burning of coal or coal waste materials. Hooper (1987) and De Boer et al. (2001) report that the thermally affected materials as a result of burning coal seem to give good geomagnetic field records due to the magnetic enhancement caused by heating. A recent study provides information on the in situ measurement of magnetic parameters in burning coal seams and states that coal fires are accompanied by the acquisition of a significant magnetization, which reflects the mineralogical conversion (Sternberg, 2011). The objectives of this study were (1) to characterize the mineral composition and magnetic parameters of the coal and other main lithologies in the Douro Coalfield, the materials from non-burning and burning coal waste piles (natural burning), and fly ash that resulted from coal combustion in a thermal power plant (industrial combustion); (2) to compare the mineral composition and magnetic properties of materials to establish and characterize the magnetic particles; (3) to assess the changes in mineral phases that take place during natural and industrial combustion processes and the temperature attained by materials in the natural burning processes; and (4) to assess any potential environmental impacts associated with the materials.

  Environmental Magnetic Studies In recent years increasing consideration has been given to the use of magnetic parameter measurements for environmental applications (Verosub and Roberts, 1995; Dekkers, 1997; Maher and Thompson, 1999; Evans and Heller, 2003). In soils the magnetic susceptibility and IRM values are the sum of the contributions of all soil-forming minerals and vary with concentration and composition of those minerals (which can be diamagnetic, paramagnetic, or ferromagnetic s.l. species). The main minerals found in sediments/soils, such as carbonates, feldspars, and quartz, are diamagnetic, whereas clay minerals, biotite, olivine, and pyroxene are paramagnetic. Among the Fecontaining minerals, siderite and pyrite are paramagnetic, whereas magnetite, maghemite, and hematite are ferromagnetic. The magnetic parameters recorded in geological materials depend on their ferromagnetic content, especially magnetite, maghemite, and hematite (Mullins, 1977). For a magnetic study the most important minerals are the Fe-containing species. These may have originated by desegregation of parent rocks during pedogenesis, which is mediated by bacterial activity, by lithogenic processes, and by anthropogenic activities (Dekkers, 1997). The anthropogenic contribution to magnetic parameters can be a significant factor in soils. Anthropogenic particles originating during high-temperature combustion of fossil fuels, dusts, and fly ashes from various industries and from vehicle emissions may create a significant enhancement of soil magnetic susceptibility (Flanders, 1994). Major sources for the formation of ferromagnetic minerals are hightemperature processes like fire. In the burning process, hematite and goethite can be reduced to magnetite. Upon heating to approximately 250 °C lepidocrocite converts to maghemite. This conversion, for example during forest and bush burning, has been considered a major process of maghemite formation in soils (Schwertmann, 1988). Magnetic parameters, such as magnetic susceptibility, IRM and S-ratio, are important in characterizing these materials, especially those that are ferromagnetic s.l. Magnetic susceptibility is a measure of the magnetic response of a material to a “weak” external magnetic field. The volume susceptibility, k, measured in dimensionless units, is defined as the ratio of the material magnetization J (per unit volume) to the weak external magnetic field H: J = kH.

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Alternatively, the specific or mass susceptibility, χ, measured in units of m3/kg, is defined as the ratio of the material magnetization J (per unit mass) to the weak external magnetic field H: J = χH. The remanence acquired by exposure of a sample to a “strong” magnetic field at ambient temperature is called the IRM. The IRM acquisition curves are important in obtaining signatures of the coercivity of ferromagnetic structures. The main purpose of the so-called S-ratio is to provide a measure of the relative amounts of high-coercivity (hard) remanence to low-coercivity (soft) remanence. In many cases, this provides an estimation of the relative importance of magnetically “hard” minerals such as hematite versus magnetically “soft” minerals such as magnetite. Usually the S−300 ratio (IRM−300/IRM1 T) is calculated and represents the remanent magnetization acquired at a field of 300 mT and 1 T.

  Sampling and Analytical Methods Numerous samples of coal and non-coal waste were collected and analyzed for this chapter. The samples and analytical procedures are discussed below. Samples Samples of coal and the main non-coal lithologies (lithic arenites and carbonaceous shales) from the Douro Coalfield, samples from an unburned coal waste pile (Serrinha) and from burning coal waste piles (S. Pedro da Cova, Lomba and Midões), as well as the fly ash that resulted from coal combustion were studied. As the coal waste material is in fact a mixture of coal and the associated lithologies of the Douro Coalfield, samples of coal (DC Coal 1 and DC Coal 2), samples of lithic arenite (LA—DC1 and DC2), and carbonaceous shale (CS—DC3) were analyzed. The mineralogical composition of these samples, already published in previous work (Ribeiro et al., 2010b), gives some information about the mineralogical background of the waste material. In the Serrinha coal waste pile six samples (S4, S13, S16, S18, S20, and S21) of waste material, representing unburned material, were analyzed for determination of magnetic parameters. The mineralogical composition of the Serrinha waste material has been already published (Ribeiro et al., 2010b). Samples from burning coal waste piles were collected from both burning/burnt zones and unburned zones. In the S. Pedro da Cova waste pile three samples were collected from unburned zones (SP10, SP37, and SP38) and three from burning/burnt zones (SP30, SP33, and SP34). In Lomba waste pile two samples were collected from unburned zones (L71 and L74) and three from burning/burnt zones (L68, L70, and L73). In Midões waste pile three samples were collected from burning/burnt zones (M3, M11, and M68). Selected samples were subjected to mineralogical analysis, whereas magnetic measurements were performed on all samples. The fly ash that resulted from anthracite combustion (at 1300–1400 °C) was disposed in three landfills adjacent to the thermal power plant. A total of six fly ash samples were collected from two landfills (landfill 1: FA3, FA5, and FA6; landfill 2: FA13, FA15, and FA16). Each sample was collected, using an auger, at a depth of about 50 cm, except sample FA13, which was collected at a depth of about 1 m. Analytical Methods The methods applied were XRD for determination of the mineralogical composition and magnetic susceptibility and IRM measurements for determination of magnetic parameters. The powdered samples were analyzed by XRD using a Philips PW-1830 diffractometer with Cu K-alpha radiation. Quantitative analyses of mineral phases in each sample were made from the X-ray diffractograms using Siroquant™, a commercial quantitative XRD program (Taylor, 1991) based on the Rietveld XRD analysis technique. The proportions of amorphous material in the samples were determined using data from a poorly crystallized siliceous phase, identified in the Siroquant database as “tridymite” as described by Ward and French (2006). Magnetic susceptibility of the powdered samples was measured using a KLY-4S Kappabridge (Agico) susceptibility meter equipped with the Sumean software. The IRM values were measured using a Molspin Minispin spinner magnetometer and fields were imparted with a Molspin magnetizer. Measurements were performed on samples to obtain the IRM values, the IRM and the −IRM acquisition curves. Samples were magnetized firstly in the same direction from 12.5 mT up to 1 T and second in the opposite direction also from 12.5 mT up to 1 T. Some S-ratios

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were calculated, and S300 (IRM300/IRM1 T) and S−300 (IRM−300/IRM1 T) values were analyzed to determine the ferromagnetic mineralogy of samples and to compare it with the results obtained by XRD.

  Results and Discussion The mineral composition of coal and non-coal waste, the composition of fly ash, magnetic minerals, and magnet parameters of the samples studies are next discussed. Mineralogy Coal Waste and Coal  Table 18.1.1 presents information on the mineralogical composition of the coal waste materials (unburned and burning/burnt) and the fly ash. Data about the mineralogical composition of the (unburned) Serrinha waste pile materials, as well as the coal and main non-coal lithologies of the Douro Coalfield, are also presented in Table 18.1.1. The mineralogical compositions of the coal and associated lithologies are included in this work to establish the mineralogical background of the coal waste materials. The low-temperature ash of the coal contains illite + mica as the predominant mineral phase, with quartz, kaolinite, and siderite in lesser proportions. Traces of pyrite, jarosite, and anatase are also present. The mineralogical composition of the lithic arenite from the Douro Coalfield includes muscovite, quartz, chlorite, and rutile, while the carbonaceous shale comprises quartz, muscovite, illite, kaolinite, pyrophyllite, and traces of rutile and anatase. The Serrinha waste material has illite as the most abundant mineral, followed by quartz, muscovite, chlorite, and pyrophyllite. Traces of other minerals such as rutile, anatase, and jarosite are also present. Table 18.1.1 Mineralogical composition (wt. %) of coal, lithic arenites, and carbonaceous shale from the Douro Coalfield; the Serrinha, S. Pedro da cova, Lomba, and Midões coal waste piles; and fly ash from a coal-power plant.*

Sample

DC Coal†

Amorphous Anatase Chlorite (Fe rich) Cristobalite Hematite Illite Illite + mica Jarosite Kaolinite Maghemite Mullite Muscovite Pyrite Pyrophyllite Quartz Rutile Siderite

– 0.4 – – – – 78.9 1.9 5.7 – – – 0.8 – 6.8 – 5.6

DC Lithologies Unburned Waste Material

Burning/Burnt Waste Material

LA

CS

Serrinha

SP37

L71

SP34

L68

L70

M11

FA‡

– – 17.3 – – – – – – – – 56.7 – – 24.5 1.4 –

– 0.6 – – – 39.0 – – 6.1 – – 13.0 – 11.5 29.0 0.8 –

– 0.7 8.0 – – 45.8 – 0.9 – – – 11.0 – 8.7 24.3 0.8 –

– – – – – 64.5 – – 8.1 – – 4.7 – 8.3 13.7 0.5 –

– 0.3 1.8 – – 60.8 – – 4.5 – – 6.2 – 4.9 21.5 0.1 –

– – – – 0.5 55.1 – 1.3 8.9 – – 6.5 – 8.0 18.7 0.5 –

25.1 0.6 – – – 34.3 – – 0.3 – – 8.1 – 5.9 25.4 0.4 –

38.8 – – 2.0 2.8 11.2 – – – – 23.8 – – – 21.3 – –

48.6 – – 0.2 0.4 25.5 – 3.3 – – 9.3 – – – 12.4 – –

64.6 – – – – 0.9 – – 2.9 0.7 16.9 – – – 15.9 – –

*DC Coal—coal from the Douro Coalfield, DC Lithologies—lithologies from the Douro Coalfield (LA—lithic arenite, CS—carbonaceous shale); SP—S. Pedro da Cova waste pile; L—Lomba waste pile; M—Midões waste pile; FA—fly ash from coal combustion in a thermal power plant. One sample was analyzed for DC Coal, DC Lithologies LA and CS, and for SP37, L71, SP34, L68, L70, and M11; two samples for Serrinha; six samples for FA. †Ribeiro et al., 2010b. ‡Ribeiro et al., 2011a.

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Samples SP37 and L71 were collected from unburned zones of the S. Pedro da Cova and Lomba waste piles, respectively. Mineralogical analysis shows that illite and quartz are the main constituents of these samples, with muscovite, kaolinite, and pyrophyllite also being significant components. Minor proportions of chlorite, rutile, and anatase are also present. The samples from Serrinha and samples SP37 and L71 both represent unburned coal waste material. As indicated in Table 18.1.1 these samples have similar mineralogical composition. Table 18.1.1 also shows that the composition of the unburned coal waste material includes contributions from both the coal and the associated lithologies of the Douro Coalfield. Samples SP34, L68, L70, and M11 represent burning or already burnt material from the S. Pedro da Cova, Lomba, and Midões waste piles. The results in Table 18.1.1 show that the minerals found in samples from the unburned zones are also present in the burning/burnt samples. However, new mineral constituents, mullite, cristobalite, and hematite, as well as a significant abundance of amorphous material are also very evident, especially in samples L70 and M11. Previous studies have demonstrated that these samples show evidence (petrographic and geochemical) of a much more intense and complete combustion (Ribeiro et al., 2010a). Some studies about the coal fires focus on the new minerals arising from the thermal effects, where similar mineralogic phases were identified (Clark et al., 1992; Masalehdani et al., 2007; Sokol and Volkova, 2007). The major thermal decomposition reactions of minerals in coal and associated rocks, and also the melting temperatures of the products, have been studied by many authors, including Raask (1985), Reifenstein et al. (1999), Saxby (2000), French et al. (2001), Suárez-Ruiz and Crelling (2008), and references therein. The quartz content of the samples from burning zones in this study suggests that little if any of the quartz in the unburned waste is decomposed, and hence the abundance of amorphous material in the burned waste is probably mainly due to the decomposition of the illite from around 500 °C. The thermal decomposition of kaolinite from 500 °C and amorphous metakaolinite probably results in mullite formation around 1100 °C. The small amount of cristobalite may possibly result from the thermal decomposition of quartz, starting at about 900 °C, but may also be derived, along with the mullite, from kaolinite and amorphous metakaolinite decomposition. The thermal decomposition of pyrite and siderite starts at about 500 °C to form successive iron oxides, although the formation of hematite and magnetite requires temperatures that are much higher. A thermomechanical analysis by French et al. (2001), combined with dynamic high-temperature X-ray diffraction data, shows the relative abundance of the different mineral and amorphous phases develop as coal mineral matter is heated progressively in an oxidizing atmosphere to more than 1500 °C. Between 900 °C and 1000 °C maghemite, cristobalite, and mullite begin to form, accompanied by a decrease in the amount of amorphous material that results from the thermal decomposition of kaolinite at about 550 °C. At approximately 1500 °C, the proportions of quartz, cristobalite, and maghemite start to decrease, and these phases are replaced by a second generation of amorphous material, the proportion of which rises sharply. The proportion of mullite also starts to increase at about 1500 °C. In the light of both this information and the results obtained from XRD analysis it appears that during the combustion process the waste piles could have reached temperatures of at least 1000 °C and may have reached temperatures of 1500 °C or even higher. However, it is possible that such temperatures were only reached in some areas of the coal waste piles. Fly Ash  XRD analysis of the fly ash samples (Table 18.1.1) reveals that amorphous material (or glass) is the main constituent, followed by mullite, quartz, and traces of maghemite and clay minerals. The mineralogy of the ash is thus similar to that of the samples from the burning/burned zones of the waste piles. Based on suggestions by Ward and French (2006), the crystalline iron oxide phase (maghemite) in the fly ash of this study probably represents a breakdown product of the pyrite and any jarosite (a common oxidation product of pyrite) occurring in the feed coal material. The quartz in the fly ash probably represents particles of quartz in the coal that did not react significantly during the combustion process. Magnetic Minerals The XRD analysis of the samples detected the presence of hematite in the burning/burnt coal waste materials and maghemite in the fly ash samples. The quantification of these magnetic minerals is expressed in Table 18.1.2.

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Table 18.1.2 Magnetic mineralogy, magnetic susceptibility, isothermal remanent magnetization and s ratios (300, -300) of the samples studies.*

Samples DC Coal DC Lithologies Serrinha waste pile

Sample No. DC Coal 1 LA LA CS

S. Pedro da Cova waste pile

Lomba waste pile

Midões waste pile Fly ash

DC Coal 2 DC1 DC2 DC3 S4 S13 S16 S18 S20 S21 SP10 UB SP37 SP38 SP30 B SP33 SP34 L71 UB L74 L68 B L70 L73 M3 B M11 M68 FA3 FA5 FA6 FA13 FA15 FA16

Maghemite Hematite MS × 10−8  wt.% wt.% m3/kg

IRM × 10−3 Am2/kg S300

S−300

– – – – – – – – – – – – – – – – – – – – – – – – – 0.8 0.7 0.9 0.3 0.8 0.7

0.3 1.0 0.05 0.04 2.8 2.9 3.2 4.2 4.9 7.8 6.7 2.3 3.4 2.2 2.2 1.8 69.5 2.1 5.2 3.5 133 2.0 47.8 134 22.7 232 157 234 119 213 134

0.89 0.99 0.90 0.93 0.98 0.91 0.95 0.89 0.93 0.93 0.95 0.92 0.89 0.95 0.90 0.93 0.95 0.96 0.76 0.98 0.53 0.31 0.83 0.89 0.94 0.94 0.95 0.93 0.90 0.94 0.93

– – – – – – – – – – – – – – – – 0.5 – – – 2.8 – – 0.4 – – – – – – –

2.9 8.0 2.1 15.4 16.4 23.9 31.9 34.4 39.0 59.7 53.8 19.1 24.8 19.3 18.7 19.7 320 20.5 33.4 33.0 158 9.3 151 403 110 967 844 1103 792 1029 726

0.91 0.95 0.96 0.96 0.91 0.95 0.93 0.91 0.95 0.95 0.95 0.96 0.92 0.93 0.92 0.92 0.98 0.94 0.80 0.92 0.81 0.65 0.94 0.94 0.96 0.96 0.97 0.97 0.95 0.97 0.95

*DC Coal—coal from the Douro Coalfield; DC Lithologies—lithologies from the Douro Coalfield (LA—lithic arenite, CS—carbonaceous shale); UB—unburned coal waste; B—burning/burnt coal waste; MS—magnetic susceptibility; IRM—isothermal remanent magnetization; S300 ratio = IRM300 mT/IRM1 T; S−300 ratio = IRM−300 mT/IRM1 T. mT—millitesla, 1 T—1 Tesla.

Coal Waste  The magnetic mineralogy of the coal waste material is represented by hematite in samples SP34, L70, and M11, in which the thermal changes are more evident. The presence of hematite in these samples is attributed to the burning of the coal waste material, which is recognized in coal combustion processes (Suárez-Ruiz and Crelling, 2008) and in studies about coal fires (Hooper, 1987; Clark et al., 1992; Masalehdani et al., 2007; Sokol and Volkova, 2007). Previous studies of the coal waste material have dealt with the optical petrography and scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDX) (Ribeiro et al., 2010a) and with nanominerals and nanoparticles (Ribeiro et al., 2010c). Although the XRD analysis detected only hematite in three samples, the petrographic and SEM observations demonstrated the presence of variable amounts of iron oxides in the coal waste materials. The study of nanominerals and nanoparticles reported the occurrence of hematite, magnetite, and maghemite. These iron oxides could be part of the samples’ original composition, they could be produced by oxidation, or they could be produced by the high temperatures in the burning coal waste piles (Hooper, 1987). Samples L70 and M11 have the highest proportions of iron oxides and were the only ones in which iron spheres were observed. The EDS analysis provides evidence of their iron composition. Figure 18.1.2 illustrates iron oxides observed by optical petrography and SEM in selected burning/burnt samples and provides an overview of the particle size and structure.

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(e)

(b)

(f)

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(g)

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Figure 18.1.2.  Photomicrographs taken under the optical microscope (a–d) and SEM images (e–h) of burning/ burnt coal waste material in polished blocks. (a, b, e, and f) show iron particles. (c, d, g, and h) show iron-rich spheres. Photos by Joana Ribeiro, 2010. Figures 18.1.2(a, b, e, and f) represent iron oxides, possibly hematite. Figures 18.1.2(c, d, g, and h) show iron oxides with spherical forms, possibly magnetite or maghemite, the formation of which is commonly associated with coal combustion (Suárez-Ruiz and Crelling, 2008). The observed particle sizes vary widely from about 20 μm to more than 400 μm. Fly Ash  The magnetic mineralogy of the fly ash is represented by maghemite in all samples (Table 18.1.2). The formation of maghemite is often attributed to coal combustion (French et al., 2001; Suárez-Ruiz and Crelling, 2008). Optical petrography and SEM-EDX analysis of the fly ash have been carried out in previous studies (Ribeiro et al., 2011a), and the observations demonstrate the presence of iron oxides with essentially spherical forms (Figure 18.1.3). The predominance of iron-rich spheres is in accordance with the XRD analysis that detected maghemite in the fly ash

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(a)

(e)

(b)

(f)

(c)

(g)

(d)

(h)

Figure 18.1.3.  Photomicrographs taken under the optical microscope (a–d) and SEM images (e–h) of fly ash samples, showing iron-rich spheres. Photos by Joana Ribeiro, 2010. samples. Figures 18.1.3(a–d) are photomicrographs taken under the optical microscope that show iron-rich spheres with variable dimensions and internal structures. Figure 18.1.3(e) gives an overview of the fly ash constituents, including iron-rich spheres with variable particle sizes. Figure 18.1.3(f) is of a freshly collected fly ash sample that shows dendritic forms. The SEM images represented in Figures 18.1.3(g and h) illustrate the internal structure of the iron-rich spheres. The images provide some information about particle sizes, which vary roughly between 20 and 100 μm. Magnetic Parameters Magnetic susceptibility and IRM measurements of the studied samples are presented in Table 18.1.2, together with the calculated S300 and S−300 ratios. Figure 18.1.4 represents the magnetic susceptibility of the studied unburned material, specifically, the coal, lithic arenite, and carbonaceous shale from Douro Coalfield and waste piles material (Figure 18.1.4(a)) and from the burning/burnt waste piles material and fly ash (Figure 18.1.4(b)) for comparison.

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Figure 18.1.4.  Histograms of mean magnetic susceptibility measured in the studied samples. (a) Mean magnetic susceptibility of coal (DC Coal) and lithologies (DC Lithologies) from the Douro Coalfield, waste material from the Serrinha waste pile (S), unburned waste material from the S. Pedro da Cova (SP UB) and Lomba (L UB) waste piles. (b) Mean magnetic susceptibility of burning/burnt material from the S. Pedro da Cova (SP B), Lomba (L B), and Midões (M B) waste piles and from the fly ash. Figures by Joana Ribeiro, 2010.

Coal Waste Material  In the coal samples the magnetic susceptibility ranges between 2.9 × 10−8 and 8.0 × 10−8 m3/ kg and the IRM ranges between 0.3 × 10−3 and 1.0 × 10−3 Am2/kg. As expected (van Krevelen, 1993), the lowest values of magnetic susceptibility are in coal. The magnetic susceptibility of the main lithologies of the Douro Coalfield varies between 2.1 × 10−8 and 15.4 × 10−8 m3/kg in lithic arenites and is 16.4 × 10−8 m3/kg in carbonaceous shales. The IRM varies between 0.04 × 10−3 and 0.05 × 10−3 Am2/kg in lithic arenites and is 2.8 × 10−3 Am2/kg in carbonaceous shales. Once again the characteristics of the coal and the main lithologies of the Douro Coalfield allow the establishment of the background levels for the coal waste material. In the Serrinha waste pile, without combustion, the magnetic susceptibility has values between 23.9 × 10−8 and 59.7 × 10−8 m3/kg and the values of IRM range from 2.9 × 10−3 to 7.8 × 10−3 Am2/kg. These values are higher than those reported above for the coal, lithic arenites, and carbonaceous shales, as can be observed in Figure 18.1.4(a). The magnetic susceptibility values for samples from the burning/burnt zones were compared with samples from material that was not affected by combustion, that is from the unburned zones. In the S. Pedro da Cova and Lomba waste material samples the measured values of magnetic susceptibility are very similar, ranging between 9.3 × 10−8 and 33.4 × 10−8 m3/kg, except for samples SP34 and L70, which have very high values (320 × 10−8 and 158 × 10−8 m3/kg, respectively). The burning/burnt material from the Midões waste pile has values closer to the magnetic susceptibility reported for samples SP34 and L70, ranging between 110 × 10−8 and 403 × 10−8 m3/kg.

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The results demonstrate that magnetic susceptibility varies significantly in burning/burnt samples, which is attributed to the variable magnetic mineralogy present in the studied samples. This mineralogy depends on many factors such as temperature and availability of oxygen (Hooper, 1987). The magnetic enhancement of some burning/burnt samples when compared with the unburned material from those coal waste piles is illustrated in Figure 18.1.4 and is attributed to the combustion process (Hooper, 1987; Flanders, 1994; De Boer et al., 2001; Sternberg, 2011). The IRM measurements show the same trend as that noted for magnetic susceptibility. The samples from S. Pedro and Lomba present values ranging between 1.8 × 10−3 and 5.2 × 10−3 Am2/kg, with the exception of samples SP34 and L70 with values of 69.5 × 10−3 and 133 × 10−3 Am2/kg, respectively. The burning/burnt samples from Midões present values ranging between 22.7 × 10−3 and 134 × 10−3 Am2/kg. Generally, the Serrinha waste material presents higher values of magnetic susceptibility and IRM than the coal and associated lithologies from the Douro Coalfield and the unburned coal waste samples and some burning/burnt coal waste samples. This slight magnetic enhancement (Figure 18.1.4(a)) may be related to oxidation processes in the Serrinha waste pile, promoted by restoration carried out to address environmental problems (Ribeiro et al., 2010b). Samples SP34, L70, and M11 present the highest values of magnetic susceptibility and IRM, which is in accordance with the XRD analyses that show the presence of hematite in these samples. The formation of hematite is attributed to the combustion in the coal waste piles. The results for the magnetic parameters also demonstrate that the burning/burnt samples were thermally affected in different ways, which has already been reported based on mineralogy for samples SP34, L70, and M11. Figure 18.1.5 represents the characteristic IRM acquisition curves of the coal waste material and of fly ash. As the IRM acquisition curves from the unburned coal waste material samples are all very similar, the IRM acquisition curve from sample S16 was selected to represent the unburned waste materials. Similarly, the IRM acquisition curves from samples SP30 and L70 were selected to characterize the burning/burnt materials. The IRM acquisition curves from the fly ash are also very similar and the curve from sample FA5 was selected. Figure 18.1.5 allows comparison between the unburned material and the material related with natural combustion and industrial combustion.

Figure 18.1.5.  Characteristic IRM and −IRM acquisition curves for unburned coal waste material, burning/burnt coal waste material, and fly ash. Figure by Joana Ribeiro, 2010.

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The shape of the IRM acquisition curves gives information about the magnetic composition. The saturation in IRM acquisition curves is mainly influenced by the concentration of magnetite-type and hematite-type minerals. Magnetite and maghemite are magnetite-type minerals and have low coercivity, which means that they are magnetically soft and reach saturation in fields up to 300 mT, resulting in a flat curve at higher fields. In contrast, hematite has high coercivity, which means that it is magnetically hard and normally requires a field higher than 1000 mT to reach saturation. The characteristic IRM acquisition curve for the unburned coal waste material is represented by sample S16, where the saturation is achieved at an applied field of about 1000 mT, much later than 300 mT. This suggests the occurrence of hematite-type structures, although some magnetite-type structures can also be present. None of these minerals have been detected by XRD, indicating that they are present only in small amounts. Generally, the IRM acquisition curves for the burning/burnt samples presented two different trends. These samples required higher fields than the unburned material to achieve saturation, which demonstrates greater abundance of hematite-type structures. Sample SP30 represents the burning/burnt samples that were less affected by combustion; these magnetic properties are similar to those reported for unburned material. Saturation is achieved much later than an applied field of 300 mT, which indicates an important hematite-type mineral contribution. Sample L70 represents burning/burnt material with different magnetic parameters. The characteristic IRM acquisition curves reach saturation at higher fields, which indicates that these samples have a more significant contribution from hematite-type structures. This is in accordance with the XRD analysis that detected hematite in the most thermally affected samples. Hematite was not detected by mineralogical analysis in the unburned zones and some of the burning/burnt zones, but the IRM acquisition curves indicated its presence. However, hematite was detected by both XRD and IRM acquisition curves in the burning/burnt zones, which points out the increase in hematite concentration during combustion. The IRM−300 mT is mainly influenced by the low coercivity (magnetite-type) structures. Therefore S−300 ratio was calculated for each sample to estimate the relative contribution of high coercivity and low coercivity minerals (Evans and Heller, 2003). When S−300 is close to 1 the magnetic mineralogy is composed of low coercivity minerals (magnetite type). The lower the S−300 the higher is the content of high coercivity (hematite-type) minerals. Almost all samples have a low coercivity (magnetite-type) component shown by S−300 values higher than 0.90. Samples L70, L73, L74, M3, and M11 have the lowest S−300 values (0.53, 0.31, 0.76, 0.83, and 0.89, respectively), showing the occurrence of a high coercivity (hematite-type) component, especially in samples L70 and L73. This is in accordance with the detection of hematite in samples L70 and M11 and also with the IRM acquisition curves that indicate the presence of magnetite-type and hematite-type structures in the studied samples. Fly Ash  The magnetic susceptibility of the fly ash samples ranges between 726 × 10−8 and 1103 × 10−8 m3/kg. The IRM presents values from 119 × 10−3 to 234 × 10−3 Am2/kg. The comparison between the magnetic parameters of the coal and the fly ash samples indicates a magnetic enhancement (Figure 18.1.4(b)) that is attributed to the coal combustion (French et al., 2001; Suárez-Ruiz and Crelling, 2008). Maghemite was detected by XRD analysis in the fly ash samples. In addition, the highest values of magnetic susceptibility provide supporting evidence that strongly magnetic minerals are present. The characteristic IRM acquisition curve of the fly ash samples (Figure 18.1.5) shows that saturation is achieved with lower applied fields than for the coal waste samples, which demonstrates the considerable presence of magnetite-type minerals, such as the maghemite detected in the XRD analyses. The S−300 ratio for the fly ash samples is higher than 0.90, indicating the presence of low coercivity (magnetite-type) structures. These magnetic features are compatible with the existence of maghemite. Ferrimagnetic iron oxide particles in fly ashes, mainly magnetite and maghemite originating during high-temperature combustion of fossil fuels, are potentially the most significant source of anthropogenic ferrimagnetics (Flanders, 1994).

 Conclusions The materials resulting from mining and consumption of anthracite A in Portugal represent an area of concern because of the potential impact on the environment and human health. To complement studies already done on these materials, which include coal waste and fly ash, mineralogical analyses and magnetic parameters determination were performed.

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Disposal of the coal waste and fly ash represents a potential source of particulate matter into the atmosphere, which can be released through the interaction with physical and chemical processes, such as weathering agents. Although no longer active in the thermal power plant, combustion in the coal waste piles may also be responsible for direct generation and delivery of particulate matter into the atmosphere. The pollutant particles may affect the environment, depositing on vegetation, soils, and so on, and they may also affect human health, through inhalation of small particles. The results of this study indicate the formation of amorphous material, along with cristobalite, hematite, and mullite, due to the burning process in the coal waste piles, suggesting that temperatures of 1000 °C to possibly 1500 °C were involved in the combustion process. Similar assemblages of amorphous material, mullite, and maghemite in the fly ash are attributed to the combustion in the thermal power plant. Considering the magnetic mineralogy identified by XRD, hematite was detected in the most affected burning/burnt samples and maghemite was detected in the fly ash samples. Magnetic parameter measurements show an increase in magnetic susceptibility and IRM in some burning/burnt zones, which indicates a magnetic enhancement due to the burning process. The fly ash samples exhibit even higher magnetic susceptibility and IRM values, attributed to combustion in the thermal power plant. Hematite was not detected by mineralogical analysis in the unburned zones, but magnetic parameters indicated its presence. In the burning/burnt zones hematite was detected by both XRD and IRM acquisition curves, which point out the increase on hematite concentration during the combustion. Magnetic parameters in the burning/burnt zones also indicate the presence of a magnetite-type structure, which could be maghemite or magnetite. Magnetic parameters and XRD analysis both indicate the presence of maghemite in the fly ashes. Maghemite is probably present in both the fly ashes and the burning/burnt zones, but only in small proportions in waste material. The difference in maghemite concentration is probably due to the temperature levels that were reached and oxygen access in the burning/burnt zones. In the coal waste piles the material was affected differently by the burning process; this induced an increase in magnetic parameters, probably due to the formation of hematite. In the thermal power plant the combustion process induced the formation of maghemite, which in turn caused the very high values of magnetic susceptibility and IRM. The ferrimagnetic iron oxide particles in the coal waste piles and fly ash landfills are potentially a source of pollution to the environment and human health because they can be released to the atmosphere, especially through weathering agents and burning processes.

 Acknowledgments This work was financially supported by “Centro de Geologia da UP,” FCT, POCI 2010. The first author’s research was supported by a Ph.D. scholarship financed by FCT—Fundação para a Ciência e Tecnologia, Portugal, Ref: SFRH/BD/31740/2006. The authors are grateful to Robert Sternberg (Franklin & Marshall College, Lancaster, Pennsylvania, USA), David French (CSIRO Energy Technology, Bangor, New South Wales, Australia), and Anupma Prakash, (University of Alaska, Fairbanks, USA) for their review of this chapter.

  Important Terms anthracite A (ISO 11760, 2005) burning/burnt coal waste coal waste coercivity environmental magnetism fly ash hematite

magnetic mineralogy magnetic parameters magnetic susceptibility magnetite magnetite-type structure mineralogy particulate matter

Mineralogy and Magnetic Parameters of Materials Resulting from the Mining

hematite-type minerals hematite-type structures isothermal remanent magnetization maghemite

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S-ratio unburned coal waste X-ray powder diffraction

 References Chapman, R.S., Watkinson, W.P., Dreher, K.L., Costa, D.L., 1997. Ambient particulate matter and respiratory and cardiovascular illness in adults: particle-borne transition metals and the heart–lung axis. Environmental Toxicology and Pharmacology 4, 331–338. Clark, B.H., Peacor, D.R., 1992. Pyrometamorphism and partial melting of shales during combustion metamorphism: mineralogical, textural, and chemical effects. Contributions to Mineralogy and Petrology 112, 558–568. De Boer, C.B., Dekkers, M.J., van Hoof, T.A.M., 2001. Rock-magnetic properties of TRM carrying baked and molten rocks straddling burnt coal seams. Physics of the Earth and Planetary Interiors 126, 93–108. Dekkers, M.J., 1997. Environmental magnetism. Geologie en Mijnbouw 76, 163–182. Eagar, R.M.C., 1983. The non-marine bivalve fauna of the Stephanian C of North Portugal. In: Lemos de Sousa, M.J., Oliveira, J.T. (Eds.), The Carboniferous of Portugal. Memórias dos Serviços Geológicos de Portugal, vol. 29, pp. 179–185. Evans, M.E., Heller, F., 2003. Environmental Magnetism. Principles and Applications of Enviromagnetics. Academic Press, Elsevier. 299 p. Fernandes, J.P., Pinto de Jesus, A., Teixeira, F., Sousa, M.J.L., 1997. Primeiros resultados palinológicos na Bacia Carbonífera do Douro (NO de Portugal). In: Grandal d’Angale, A., Gutiérrez-Marco, J.C., Santos Fidalgo, L. (Eds.), XIII Jornadas de Paleontologia “Fósiles de Galicia” y V Reunión Internacional Proyecto 351 PICG “Paleozoico inferior del Noroeste de Gondowana,” A Coruna, 1997, Libro de Resúmenes y Excursiones. Sociedade Espanola de Paleontologia, pp. 176–179. Finkelman, R.B., Stracher, G.B., 2011. Environmental and health impacts of coal fires. In: Stracher, G.B., Prakash, A., Sokol, E.V. (Eds.), Coal and Peat Fires: A Global Perspective, vol. 1. Elsevier, pp. 115–125. Flanders, P.J., 1994. Collection, measurements and analysis of airborne magnetic particulates from pollution in the environment. Journal of Applied Physics 75, 5931–5936. French, D., Dale, L., Matulis, C., Saxby, J., Chatfiel, P., Hurst, H.J., 2001. Characterization of mineral transformations in pulverized fuel combustion by dynamic high temperature X-ray diffraction analyzer. In: Proceedings of 18th Pittsburg International Coal Conference, Newcastle, Australia. Hooper, R.L., 1987. Factors affecting the magnetic susceptibility of baked rocks above a burned coal seam. International Journal of Coal Geology 9, 157–169. ISO 11760, 2005. Classification of Coals, first ed. International Organization for Standardization, Geneva, Switzerland. 9 p. Lemos de Sousa, M.J., Wagner, R.H., 1983. General description of the terrestrial carboniferous basins in Portugal and history of investigations. In: Lemos de Sousa, M.J., Oliveira, J.T. (Eds.), The Carboniferous of Portugal. Memórias dos Serviços Geológicos de Portugal, vol. 29, pp. 117–126. Maher, B.A., Thompson, R., 1999. Quaternary, Climates, Environments and Magnetism. Cambridge University Press. 390 p. Masalehdani, M.N.-N., Black, P.M., Kobe, H.W., 2007. Mineralogy and petrography of iron-rich slags and paralavas formed by spontaneous coal combustion, Rotowaro coalfield, North Island, New Zealand. In: Stracher, G.B. (Ed.), Geology of Coal Fires: Case Studies from Around the World. Geological Society of America, Boulder, pp. 117–131. McIntosh, G., Gómez-Paccard, M., Osete, M.L., 2007. The magnetic properties of particles deposited on Platanus x hispanica leaves in Madrid, Spain, and their temporal and spatial variations. Science of the Total Environment 382, 135–146. Mullins, C.E., 1977. Magnetic susceptibility of the soil and its significance in soil science. A review. Journal of Soil Science 28, 223–246. Pinto de Jesus, A., 2001. Génese e evolução da Bacia Carbonífera do Douro (Estefaniano C inferior, NW de Portugal): Um Modelo. Ph.D. Thesis, University of Porto, Portugal. Vol. Text: 232 pp.; vol. Atlas: 71 p. Raask, E., 1985. Mineral Impurities in Coal Combustion: Behaviour Problems and Remedial Measures. Hemisphere Publishing Corporation, New York. 484 p.

508

Ribeiro et al.

Reifenstein, A.P., Kahraman, H., Coin, C.D.A., Calos, N.J., Miller, G., Uwins, P., 1999. Behaviour of selected minerals in an improved ash fusion test: quartz, potassium feldspar, sodium feldspar, kaolinite, illite, calcite, dolomite, siderite, pyrite and apatite. Fuel 78, 1449–1461. Ribeiro, J., Ferreira da Silva, E., Flores, D., 2010a. Burning of coal waste piles from Douro Coalfield (Portugal): Petrological, geochemical and mineralogical characterization. International Journal of Coal Geology 81, 359–372. Ribeiro, J., Ferreira da Silva, E., Li, Z., Ward, C., Flores, D., 2010b. Petrographic, mineralogical and geochemical characterization of the Serrinha coal waste pile (Douro Coalfield, Portugal) and the potential environmental impacts on soil, sediments and surface waters. International Journal of Coal Geology 83, 456–466. Ribeiro, J., Ferreira da Silva, E., Pinto de Jesus, A., Flores, D., 2011b. Petrographic and geochemical characterization of coal waste piles from Douro Coalfield. International Journal of Coal Geology 86, 216–226. Ribeiro, J., Flores, D., Ward, C., Silva, L.F.O., 2010c. Identification of nanominerals and nanoparticles in burning coal waste piles from Portugal. Science of the Total Environment 408, 6032–6041. Ribeiro, J., Valentim, B., Ward, C., Flores, D., 2011a. Comprehensive characterization of anthracite fly ash from a thermo-electric power plant and its potential environmental impact. International Journal of Coal Geology 86, 204–212. Saxby, J.D., 2000. Minerals in coal. In: Glikson, M., Mastalerz, M. (Eds.), Organic Matter and Mineralisation. Kluwer Academic Publishers, pp. 314–326. Schwertmann, U., 1988. Occurrence and formation of iron oxides in various pedoenvironments. In: Stucki, J.W., Goodman, B.A., Schwertmann, U. (Eds.), Iron in Soils and Clay Minerals. Dordrecht Reidel Publishing, pp. 267–308. Sokol, E.V., Volkova, N.I., 2007. Combustion metamorphic events resulting from natural coal fires. In: Stracher, G.B. (Ed.), Geology of Coal Fires: Case Studies from Around the World. Geological Society of America, Boulder, pp. 97–115. Sternberg, R.S., 2011. Magnetic signatures of rocks and soils affected by burning coal seams. In: Stracher, G.B., Prakash, A., Sokol, E.V. (Eds.), Coal and Peat Fires: A Global Perspective, vol. 1. Elsevier, pp. 173–208. Suárez-Ruiz, I., Crelling, J.C. (Eds.), 2008. Applied Coal Petrology. The Role of Petrology in Coal Utilization. Elsevier, p. 388. Taylor, J.C., 1991. Computer Programs for Standardless Quantitative analysis of minerals using the full powder diffraction profile. Powder Diffraction 6, 2–9. van Krevelen, D.W., 1993. Coal: Typology–Physics–Chemistry–Constitution. Elsevier. 979 p. Verosub, K.L., Roberts, A.P., 1995. Environmental magnetism: Past, present and future. Journal of Geophysical Research 100, 2175–2192. Wagner, R.H., Lemos de Sousa, M.J., 1983. The Carboniferous megafloras of Portugal—a revision of identifications and discussion of stratigraphic ages. In: Lemos de Sousa, M.J., Oliveira, J.T. (Eds.), The Carboniferous of Portugal. Memórias dos Serviços Geológicos de Portugal, vol. 29, pp. 127–152. Ward, C.R., French, D., 2006. Determination of glass content and estimation of glass composition in fly ash using quantitative X-ray diffractometry. Fuel 85, 2268–2277.