Characteristics of ferrospheres in fly ashes derived from Bokaro and Jharia (Jharkand, India) coals

International Journal of Coal Geology 153 (2016) 52–74

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Characteristics of ferrospheres in fly ashes derived from Bokaro and Jharia (Jharkand, India) coals Bruno Valentim a,⁎, Neha Shreya b, Biswajit Paul b, Celeste Santos Gomes c, Helena Sant'Ovaia a, Alexandra Guedes a, Joana Ribeiro a, Deolinda Flores a, Sílvia Pinho d, Isabel Suárez-Ruiz e, Colin R. Ward f a

Instituto de Ciências da Terra (Pólo da Faculdade de Ciências U.P.), Departamento de Geociências, Ambiente e Ordenamento do Território, Faculdade de Ciências, Universidade do Porto, Portugal Indian School of Mines, Department of Environmental Science and Engineering, Dhanbad 826004, Jharkhand, India Departamento de Ciências da Terra, Centro de Geofísica da Universidade de Coimbra, Faculdade de Ciências e Tecnologia da Universidade de Coimbra, Portugal d Faculdade de Engenharia, Universidade do Porto, Portugal e Instituto Nacional del Carbón (INCAR-CSIC), Francisco Pintado Fe 26, 33011 Oviedo, Spain f School of Biological, Earth and Environmental Sciences, University of New South Wales, Australia b c

a r t i c l e

i n f o

Article history: Received 28 October 2015 Received in revised form 25 November 2015 Accepted 26 November 2015 Available online 26 November 2015 Keywords: Fly ash Raman spectrometry Remanent magnetism Magnetite Hematite India

a b s t r a c t Coal burning power plants in the state of Jharkand, India, are facing the problem of fly ash landfilling and their economic and environmental impact. However, fly ash may be used in civil engineering constructions, including as geoliners for municipal wastes landfilling, however in this case the groundwater contamination should be taken in consideration. In this work a combination of analytical techniques is used to study the nature, composition and potential environmental impact of Fe-bearing morphotypes (ferrospheres) in fly ash from thermal power plants fed with coals from Bokaro and Jharia coalfields (Jharkand, India). The results show that the feed coals are sulfur-poor and ash-rich, dominated by quartz, clays and minor portions of Fe-bearing carbonates, such as siderite. Pyrite was not identified. Although iron is present in the fly ashes in significant proportions (from 2.7 wt.% to 4.5 wt.%), equivalent to an Fe2O3 content ranging from 3.5 wt.% to 5.8 wt.%, mineral phases such as magnetite and hematite are only present in minor proportions, or below detection limits of the XRD analyses. Iron in the ferrospheres occurs as massive or dendritic crystals, or as finely dispersed crystals trapped inside a glassy aluminosilicate matrix resulting from the release of iron oxide plumes into the aluminosilicate melt. In addition to these phases, iron also occurs as a component of the glass that makes up most of the fly ash materials. Finally, the contaminant potential of groundwater by the fly ash iron is negligible. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Iron is a common element in coal and one of the main inorganic constituents of fly ash phases. It is generally associated with pyrite and/or siderite in the mineral matter of coal, and its evolution during heating in coal power plants is well understood, especially with respect to pyritic iron evolution (Raask, 1985). In low-rank coals, i.e. lignites and sub-bituminous coals, some of the iron may be present in the form of organometallic complexes (Francis, 1961; Li et al., 2007, 2010; Raask, 1985). However, iron occurs mainly in the mineral-fraction of bituminous coals as sulfides, clays, carbonates, sulfates, and oxides and hydrated oxides (Raask, 1985): • Iron disulfide (pyrite) is the most important of the iron bearing minerals in coal. However, other iron sulfides such as marcasite (FeS2), chalcopyrite (CuFeS), melnikovite (FeS2 + (As,Fes,H2O)), mispickel (FeS2·FeAs2), and pyrrhotite (Fe(1 − x)S) may also occur in coals ⁎ Corresponding author.

http://dx.doi.org/10.1016/j.coal.2015.11.013 0166-5162/© 2015 Elsevier B.V. All rights reserved.

(Raask, 1985; Ward, 2002). Minerals as ferroselite (FeSe2), and eskebornite (CuFeSe2) may also occur in coal but were inferred from scanning electron microscopy with X-ray microanalysis (SEM/EDS), and not detected by X-ray diffraction (XRD) (Dai et al., 2015). The occurrence of such phases is mainly influenced by the relative abundance of sulfur and the fixation of sulfur by anaerobic bacteria. The sulfur may have an organic origin or be brought in by marine waters in the form of sulfates. Silicates weathering typically provides the iron for the formation of syngenetic pyrite (Neavel, 1966; Chou, 2012, and references therein); • Iron-bearing sulfates include a number of hydrated ferrous and ferric sulfates occurring in weathered coals, such as coquimbite (Fe2(SO4)3· 9H2O), jarosite (K2SO4·xFe2(SO4)3), natrojarosite (NaFe3(SO4)2(OH)6), melanterite (FeSO4·7H2O), rozenite (FeSO4·4H2O), szomolnokite (FeSO4·H2O), roemerite [FeSO4·Fe2(SO4)3·14H2O], and halotrichite [FeAl2 (SO 4 )4 ·22H 2O] (Kossenberg and Cook, 1961; Ehlers and Stiles, 1965; Gluskoter and Simon, 1968; Gruner and Hood, 1971; Rao and Gluskoter, 1973; Gluskoter, 1977; Ward, 2002; Chou, 2012);

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• Clay minerals, such as illite (K1.5Al4(Si6.5Al1.5)O20(OH)4 with Fe) and chlorite ((MgFeAl)6(AlSi)4O10(OH)8), namely chamosite and clinochlore (Dai and Chou, 2007), are the most common iron-bearing silicates in coals. Where present, illite is commonly a clastic mineral brought in by water to the peat swamp (Dixon et al., 1970; O'Gorman and Walker, 1972; Raask, 1985; Rao and Gluskoter, 1973); however, illite and chlorite may also be formed by diagenetic processes in some coal seams (e.g. Permana et al., 2013); • Carbonates such as siderite (FeCO3) and ankerite (CaCO3·FeCO3) are the most common iron-bearing carbonates. Siderite is chiefly a syngenetic carbonate while ankerite is mostly epigenetic, i.e. a product formed of later-stage mineralization process (Stach et al., 1982; Permana et al., 2013); • Iron oxides and hydrated Fe-oxides in coals may include magnetite (Fe3O4), hematite (Fe2O3) and limonite (Fe2O3·H2O) (Raask, 1985). During the burning of coal these iron-bearing phases may undergo transformations, forming crystalline phases such as magnetite and hematite (Anshits et al., 1998, 2000, 2001; Raask, 1985; Vassilev and Vassileva, 1996a, 1996b) or interacting with other components such as clay mineral residues to form an amorphous, iron-bearing aluminosilicate glass (Matjie et al., 2011; Sokol et al., 2000, 2002; Ward and French, 2006). Ferrospheres (also known as magnetic microspheres or magnetite globules), as initially proposed by Lauf (1982) and Lauf et al. (1982), are highly reflective spherical particles seen in cross sections under reflected white light microscopy, which are characterized by their iron content, density and magnetic properties as being composed of magnetite and hematite (Bibby, 1977; Hansen et al., 1981). Magnetite is a spinel (nominally Fe3O4) derived from glass particles containing fine magnetite (Fe3O4) precipitates (Lauf et al., 1982), which originated after crystallization from melts derived from illite and pyrite (Raask, 1985), or just pyrite, corresponding to the eutectic (T = 1070 °C) of wüstite–fayalite (FeO–Fe2SiO4), with a total content of up to 80 wt.% FeO (Anshits, 1998, 2000, 2001; Sokol et al., 2000, 2002). Ferrospheres have been classified on the basis of the iron oxidation state, iron content and morphology based on their three dimensional surface topography seen under secondary electron detection mode on scanning electron microscopy (SEM): • According to the type of iron (Fe2+ or Fe3+) ferrospheres may be divided into: (i) Ferrospheres, essentially composed of magnetite (ferrous–ferric oxide); (ii) Ferrispheres, essentially composed of hematite and limonite (Vassilev and Vassileva, 1996b); On the basis of the iron content Zhao et al. (2006) divided the ferrospheres into ferrooxides (Fe ≥ 75%), aluminosilicate-bearing ferrooxides (75% N Fe ≥ 50%), high-ferriferous aluminosilicates (50% N Fe ≥ 25%), and ferroaluminosilicates (Fe b 25%). • Based on their appearance under the SEM, Anshits et al. (2011) divided the ferrospheres into five main morphologic types: porous (foamlike); glass-like; dendritic; skeleton-dendritic; and block-like. Apart from the size and form of the Fe-crystals this classification also reflects the changes in the aluminosilicate matrix from the foam-like to the block-like morphology. • Also based on SEM studies, Zhao et al. (2006) classified the ferrospheres into seven groups according to their microstructure: sheet ferrospheres, dendritic ferrospheres, granular ferrospheres, smooth ferrospheres, ferroplerospheres, porous ferrospheres, and molten drop ferrospheres. • Among the most common carbonate minerals occurring in coal (siderite, ankerite, dolomite and calcite), siderite is the first to

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dissociate during combustion, forming wüstite (FeCO3 → FeO (wüstite) + CO2) (Raask, 1985) under reducing conditions, which then oxidizes to magnetite (Fe3O4) or hematite (Fe2O3) (Powell, 1965; McLennan et al., 2000). However, as a result of the rapid CO evolution, siderite particles in contact with aluminosilicates may disintegrate into 0.1 μm to 1.0 μm fragments; the resulting iron oxide is highly reactive and readily combine with the aluminosilicate melt forming Fe spinels or enriching the aluminosilicate glass with Fe (Raask, 1985; McLennan et al., 2000; Creelman et al., 2013). Ferrospheres and other Fe-bearing morphotypes may also occur in fly ash from pyrite-poor coals. However, they are mostly ignored in such cases, probably because of their low-proportions, and the consequent lack of relevance for technological applications and hence their low economic value, which tend to discourage more detailed study. However, the Indian fly ashes covered by this work are being studied for use as a geoliner material (Shreya and Paul, 2015; Shreya et al., 2014, 2015), and such newly formed Fe-bearing minerals may contribute to the environmental impact of ash use. Therefore, the genesis and characteristics of the ferrospheres may have some relevance to the use of the ashes in this way. The properties of coal fly ash are a function of several variables such as the coal source, the degree of coal pulverization, the design of the boiler unit, the loading and firing conditions, and the handling and storage methods (Mandal and Sengupta, 2002; Baba, 2003; Department of Forests et al., 2007; Valentim and Hower, 2010). An overall understanding of fly ash mineralogy, geochemistry and leaching behavior should be obtained to complement the understanding about ferrosphere genesis, and their potential impact, for example in geotechnical applications. For that purpose a set of techniques are commonly used to study the fly ash as a whole or its components: • The nature and proportion of both the crystalline (mineral) and noncrystalline or amorphous (glass) components can be obtained by Xray diffraction (XRD) analysis (Ward and French, 2006). This technique is particularly useful to detect minor crystalline phases such as magnetite and hematite (Vassilev and Vassileva, 2005); • Identification and characterization of the different phases in fly ash and their mode of occurrence at a particle-by-particle scale may be accomplished using scanning electron microscopy in conjunction with X-ray microanalysis (SEM/EDS) (Vassilev and Vassileva, 2005). The combination of SEM/EDS with petrographic techniques (polished blocks and observed under reflected white light microscopy) is a powerful method to obtain information on the individual components of the fly ash (Hower and Mastalerz, 2001; Hower et al., 2005; Suárez-Ruiz and Valentim, 2007; Suárez-Ruiz and Ward, 2008; Suárez-Ruiz et al., 2008a, 2008b, 2015; Lester et al., 2010; Hower, 2012); • The major element concentrations can be determined by X-ray fluorescence (XRF), and the combination with XRD results allows to infer the glass composition (Ward and French, 2006). • Leaching and pH studies are important in predicting the environmental impacts associated with ash disposal (Liu et al., 2009; Praharaj et al., 2002; Ward et al., 2009), especially the impact on water quality (EPA, 2000), and to follow the leaching behavior of a particular element in the fly ash over time using techniques such as XRF and inductivelycoupled plasma mass spectrometry (ICP-MS) (Izquierdo and Querol, 2012; Kim and Hesbach, 2009; Liu et al., 2009; Ward et al., 2009; Xie et al., 2007). In the case of the Fe content, these studies may determine the removal, or not, of the iron-morphotypes, e.g. by magnetic methods. In addition to these techniques Raman microspectroscopy (MRS), magnetic susceptibility (χ), isothermal remanent magnetization (IRM), and Mössbauer spectroscopy (MS) are very useful for the ferrospheres in fly ashes studies. MRS offers the possibility of performing 1 μm2 analysis areas of the fly ash morphotypes to obtain information concerning the mineral phase present, and may be used to

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provide further information about the nature of the iron oxide phases in the ferrospheres. The magnetic susceptibility of fly ash reflects the total mineralogical composition of the material with a focus on the contribution from ferromagnetic minerals, which have much higher magnetic susceptibility values than paramagnetic and diamagnetic minerals, such as clay or quartz (e.g., Dekkers, 1997; Evans and Heller, 2003; Maher and Thompson, 1999; Verosub and Roberts, 1995). The remanence acquired by a sample exposed to a direct magnetic field at ambient temperature is called the isothermal remanent magnetization (IRM). IRM acquisition curves are important for estimating the characteristic coercivity of ferromagnetic structures. The trend of the IRM acquisition curves depends on the relative concentrations of lowcoercivity, magnetite-type, minerals versus high-coercivity, hematitetype, minerals (e.g., Thompson and Oldfield, 1986). Mössbauer spectroscopy (not used in this study) provides qualitative and quantitative information on occurrences of iron minerals and amorphous phases in fly ash (Vassilev and Vassileva, 2005). A combination of analytical techniques was used in this work to provide the most effective basis for studying the nature and composition of the ferrospheres present in fly ash from combustion of Indian coals, the iron distribution and its potential contaminant influence if the fly ash is used, for example, as a geoliner. These include proximate and ultimate analysis, X-ray fluorescence (XRF), reflected light microscopy, scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), X-ray diffraction (XRD), magnetic susceptibility (χ), isothermal remanent magnetization (IRM), Raman microspectroscopy (RMS), and analysis of leachate compositions by inductively-coupled plasma mass spectrometry (ICP-MS). 2. Materials and methods 2.1. Samples The three fly ash samples are identified in this paper as Bokaro, Chandrapura and Jharia, based on the location of the power stations

from which they were derived. The Bokaro and Chandrapura samples were collected directly from the hopper of the No. 4 electrostatic precipitators (ESP) of the respective pulverized coal power plants, and the Jharia sample from the ESP hopper of the relevant fluidized bed combustion (FBC) power plant, in 25th and 26th of June 2013, respectively. The Bokaro pulverized fuel (p.f.) thermal power plant, with an installed capacity of 630 MW and is located at Bokaro in Jharkhand state, (Fig. 1). The Chandrapura p.f. thermal power plant, has an installed capacity of 890 MW, is located in the same district on the banks of Damodar River (Fig. 1). Both these thermal power plants burn coal supplied from the Bokaro Coalfield (Fig. 1), the maximum temperature inside the furnace zone is usually between 1150 °C and 1200 °C. The Jharia fluidized bed combustion (FBC) power plant is located on the northern bank of the Damodar River, with an installed capacity of 10 MW (Fig. 1). The temperature inside the boiler is 1000 °C, and the feed coal is supplied from the Jharia Coalfields (Fig. 1). The Bokaro and Chandrapura feed coals are in the upper limit of the medium rank C (bituminous C) range, while the Jharia feed coal is composed of a blend of a low-rank C (lignite C) coal and a medium rank C (bituminous C). However, the Bokaro feed coal is an high-ash coal, while the Chandrapura and Jharia fall in the carbonaceous rocks category (ISO 11760, 2005; Shreya et al., 2015). The ash deformation temperature of the feed coals used in all three power plants is 1100 °C.

2.2. Proximate and ultimate analysis Proximate analysis and calorific value determinations of the feed coal samples were carried out at the Instituto Nacional del Carbón (INCAR-CSIC) in Oviedo, Spain, following ASTM standards (ASTM D3175-11 (2011); ASTM D3174-12 (2012); ASTM D5865-13 (2013), and ASTM D3302/D3302M-15 (2015)), and the ultimate analysis (C, H, N, and total S) was determined using LECO elemental analysis methods.

Fig. 1. Geographical location of Bokaro coalfield, and Bokaro and, Chandrapura and Jharia Thermal Power Plants (Jharkhand State, India; adapted from Saikia and Sarkar, 2013).

B. Valentim et al. / International Journal of Coal Geology 153 (2016) 52–74

2.3. Petrography Polished blocks of the coal samples were prepared for petrographic analysis according to ISO 7404-2 (2009). The petrography of each coal was evaluated following the nomenclature and procedures used by the International Committee for Coal and Organic Petrology (International Committee for Coal and Organic Petrology (ICCP), 1998, 2001; ISO 7404-2, 2009; ISO 7404-3, 2009; ISO 7404-5, 2009; Taylor et al., 1998). In the case of Jharia sample, however, the reflectance measurements were made on both huminite (Sýkorova et al., 2005) and vitrinite since the coal sample is a blend. Each fly ash sample was prepared according to ISO 7404-2 (2009). However, Sudan Black B was added to the mounting medium for the fly ash samples to increase the contrast during observation, following the method proposed by Hower et al. (1995). After hardening and demolding, each block was sliced in two halves perpendicular to the top and bottom surfaces, to prevent the influence of any vertical segregation that may have occurred due to sink–float properties. The new surfaces of these sub-blocks were then polished following ISO 7404-2 (2009) instructions. The nomenclature used in petrographic analysis of the fly ashes was that proposed by Hower and Mastalerz (2001), Hower et al. (2005) and Hower (2012), which include high reflecting iron oxides under the term “spinels”. However, these were combined for the present study under the heading “iron minerals”, which includes ferrospheres and other Fe-hosting morphotypes. Point count methods were used to quantify the fly ash morphotypes, with 500 points being counted for each sample. The particles were counted only when the cross-wire was superimposed on a particle, i.e. ignoring voids. 2.4. Scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS) The set of samples subjected to SEM/EDS analyses comprised: (i) three bulk fly ash samples; (ii) three polished blocks of coals, used also for petrographic analysis and (iii) three polished blocks of fly ash, used also for petrographic analysis. The SEM/EDS analyses were made at the CEMUP (Materials Centre of the University of Porto, Portugal) using an FEI Quanta 400 FEG-ESEM/EDAX Genesis X4M instrument. The SEM was operated at 15 kV in high-vacuum mode, manual aperture, 4.5 spot size of the bean, and to improve the analysis quality under the high-vacuum conditions all the samples were previously covered by sputtering a thin Au and Pt film coating, and then adhered to the mounting block by double sided adhesive carbon tape. The backscattered electron (BSE) detector mode was preferentially used, due to its capability for discrimination based on atomic weight. However, the secondary electron detector mode was also used for topographic characterization of the particle surfaces. Identification of the fly ash morphotypes was carried out following the nomenclature and terminology described by Valentim et al. (2009).

Table 1 Coal proximate and ultimate analyses, and chemical analysis of coal high temperature ash by X-ray fluorescence, and LOI. Coal sample

Moisture Ash Volatile matter C H N St

Bokaro

Chandrapuraa

Jhariaa

3.13 40.51 15.66 82.11 4.62 1.97 0.30

1.72 51.16 14.98 70.40 4.22 1.75 0.25

1.08 60.86 17.21 49.10 3.66 1.23 0.25

(%, a.r.) (wt.%, db) (wt.%, db) (wt.%, daf)

(wt.%, db)

Coal high temperature ash

Al2O3 SiO2 CaO K2O TiO2 Fe2O3 MgO Na2O Mn3O4 P2O5 SO3 LOI Total

Bokaro

Chandrapura

Jharia

26.94 61.54 1.64 1.78 2.03 3.21 0.56 0.12 0.05 0.44 0.00 1.45 99.74

24.57 57.86 1.91 1.99 1.64 7.44 1.30 0.15 0.09 0.77 0.48 1.51 99.70

23.56 62.21 0.48 1.28 1.55 9.48 0.84 0.11 0.11 0.29 0.03 0.07 100.00

(wt.%)

a.r.: air dry; db: dry basis; daf: dry ash free. a According to the ISO Norm 11760 these are not classified as coals because the ash content is N50 wt.%.

The isothermal remanent magnetization (IRM) was measured using a Minispin fluxgate magnetometer (Molspin Ltd) after magnetization in a pulse magnetizer (Molspin Ltd) at the Departamento de Ciências da Terra, Universidade de Coimbra (Portugal). In the case of the prevalence of low-coercivity ferrimagnetic minerals the IRM acquired in the magnetic field of one Tesla (T) can be defined as Saturation Isothermal Remanent Magnetisation (SIRM). The IRM was imparted at fields of 0/12.5/ 25/50/75/100/150/200/250/300/500/700/900 mT and up to 1000 mT, and backfield was imparted up to −1000 mT. The S−300 ratio parameter was used to determine the relative contribution of ferromagnetic

Table 2 Petrographic analysis results of coal and fly ash samples from Chandrapura, Bokaro and Jharia power plants. Coal a

Rr(%) Stdb Vitrinite Liptinite Inertinite MMc

2.5. Magnetic susceptibility analysis and isothermal remanent magnetization (IRM) To investigate the concentration and type of natural magnetic carriers in the fly ash samples, two magnetic parameters were measured and analyzed, namely magnetic susceptibility and isothermal remanent magnetization. The specific or mass susceptibility, χ (measured in m3/kg), is defined as the ratio of the material magnetization, J (per mass unit) to the weak external magnetic field, H, according to the equation J = χH. Magnetic susceptibility was measured using a KLY-4S Kappabridge instrument in the Instituto de Ciências da Terra, Universidade do Porto (Portugal). At least three susceptibility measurements of each sample were taken and the average value was used for the study.

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Char

a

Fly ash Glass Glassy rimsd Quartz Iron mineralse Other minerals Isotropic Anisotropic Inertinitic Total char

(vol.%)

(vol.%)

Bokaro

Chandrapura

0.99 0.084 5 0 24 71

0.99 0.113 6 2 16 76

0.33 and 0.69 0.026 and 0.072 2f 1 10 87

65 15 10 3 1

76 1 2 1 17 1 1 1 3

32 30 9 1 10 2 15 1 18

1 5 6

Jharia

Rr: vitrinite random reflectance. Std: Standard deviation. MM: mineral matter. d Partially baked rock fragment with clay and/or carbon at core of particle with glassy rim. e Including ferrospheres with magnetite and hematite, and partially fused siderite at Jharia sample. f Vitrinite and huminite since it is a coal blend of lignite and bituminous coal. b c

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Fig. 2. Optical microscopy images of minerals occurring in coal from the thermal power stations (reflected white light): A) quartz (Qz; Chandrapura); B) clay (Chandrapura); C) siderite (Chandrapura); D) siderite (Sid; Bokaro); E) and F) siderite (Chandrapura; color camera).

minerals by dividing the IRM value at −300 mT with the corresponding value from the SIRM (S−300 = IRM −300 mT/SIRM) (Evans and Heller, 2003; Maher and Thompson, 1999; Thompson and Oldfield, 1986). An S−300 ratio close to unity indicates that the remanence is dominated

by magnetite-like structures (Bloemendal et al., 1988). The S−25 ratio obtained by dividing the IRM value at −25 mT with the corresponding value from the SIRM (S−25 = IRM−25 mT/SIRM) was also calculated, because it gives information about the size of the magnetic particle.

Table 3 Main phases (wt.%) determined by XRD on coal low temperature ash and fly ash samples. Phase

Composition

Quartz Kaolinite Illite Mixed layer illite–smectite Mullite Hematite Magnetite Dolomite (?) Siderite Calcite Apatite Anatase Rutile Anhydrite Sylvite Amorphous (as metakaolin)

SiO2 Al2Si2O5(OH)4 K1.5Al4(Si6.5Al1.5)O20(OH)4

a b

a

Al6Si2O13 Fe2O3 Fe3O4 CaMg(CO3)2 FeCO3 CaCO3 Ca5F(PO4)3 TiO2 TiO2 CaSO4 KCl Al2O3·2SiO2

Coal low temperature ash (LTA)

Fly ash samples

Bokaro

Chandrapura

Jharia

Bokaro

Chandrapura

Jharia

21.6 34.9 13.5 25.3

25.1 34.1 8.4 26.1

31.7 40.7 7.6 14.1

27.3

9.4

25.6

0.2 1

1.6 1.6

1.3 1.5

2.3 0.8

8.2 26.8 0.1

b

31.1

b

b

1.2 0.6 b

4.6

b

1.2 0.3

0.4

45.5

59.1

0.7 0.5 0.6 0.3

0.7

Smectite composition: Na0.33(Al1.67Mg0.33)Si4O10(OH)2. Minerals below the detection limit of the XRD/Siroquant system that were tested in the study.

62.2

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In order to determine frequency dependence of magnetic susceptibility, a Bartington magnetic susceptibility meter with a dual frequency sensor was used. This allows for the determination of the parameter χfd which is the difference in magnetic susceptibility obtained at two frequencies (470 Hz and 4700 Hz). 2.6. Raman microspectroscopy (RMS) Raman spectra were recorded at room temperature on a Jobin–Yvon LabRaman spectrometer equipped with a CCD camera using a He–Ne laser at an excitation wavelength of 632.8 nm and a power of 20 mW. An Olympus optical microscope with a × 100 objective lens was used to focus the laser beam on the sample and to collect the scattered

57

radiation. The laser power was reduced 50% with a neutral density filter to avoid thermal decomposition of the particles. Scans from 150 cm−1 to 1000 cm−1 were performed on the particles, and whenever possible in selected areas with different reflectance of the interior and exterior of each particle. The time of acquisition and the number of accumulations varied in order to obtain an optimized spectrum for each analyzed particle at spectral resolutions near 1 cm−1. 2.7. X-ray diffraction (XRD) and chemical analysis A combination of facilities at the University of New South Wales (Australia) provided mineralogical and chemical characterizations of the coal and fly ash samples. A representative portion of each coal was

Fig. 3. Optical microscopy images of Chandrapura fly ash (reflected white light). Ferrospheres: A) to F) Al–Si glassy matrix embedding dendritic iron oxide crystals (probably magnetite); G) and H) massive crystals, probably of magnetite and hematite forming dense ferrospheres.

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subjected to low-temperature oxygen-plasma ashing using an IPC fourchamber asher, and the percentage of low-temperature ash (LTA) was determined in each case. A representative portion of each LTA was analyzed by X-ray powder diffraction (XRD) using a Phillips PW1830 diffractometer with CuKα radiation. The minerals were identified by reference to the ICDD Powder Diffraction File. Quantitative analyses of the mineral phases in the LTA sample were made using Siroquant™, commercial interpretation software (Taylor, 1991), based on the profile fitting techniques described by Rietveld (1969). A representative portion of each fly ash was also analyzed by powder XRD and Siroquant. The diffraction pattern of a poorly-crystalline mineral phase (described in the Siroquant database as “metakaolin”) was

used to represent the amorphous material in the ashes, following the technique described by Ward and French (2006). Representative portions of each coal were ashed at 815 °C (high temperature ash, HTA). The HTA and the fly ashes were then calcined at 1050 °C, fused with lithium metaborate and cast into discs, following the method of Norrish and Hutton (1969). Each disc was analyzed by XRF using a Philips PW 2400 spectrometer and SuperQ software. The results were expressed as percentages of the major element oxides in each coal ash and fly ash. Two replicates of each fly ash sample, crushed to b75 μm, were analyzed at the Acme Analytical Laboratories (Canada) for total carbon and total sulfur determination using LECO elemental analysis methods, and trace elements with ICP-MS techniques following digestion using a

Fig. 4. Optical microscopy images of Bokaro fly ash (reflected white light). Ferrospheres: A) and B) ferrospheres under formation; C) to H) ferrospheres with dominant Al–Si glassy matrix embedding dendritic iron oxide crystals (probably magnetite).

B. Valentim et al. / International Journal of Coal Geology 153 (2016) 52–74

combination of HCl (hydrochloric acid), HNO3 (nitric acid), HF (hydrofluoric acid) and HClO4 (perchloric acid).

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determined for the ash leachates, and the eluates were analyzed by ICP-MS at the ACME Analytical Laboratories in Canada. For more details concerning the geoliner effluent characterization see also Shreya and Paul (2015) and Shreya et al. (2014, 2015).

2.8. Leaching tests on fly ashes Leaching tests were performed on the fly ash samples at the Departamento de Engenharia Metalúrgica e de Materiais (DEMM), University Porto (Portugal), according to BS EN 12457-2 (2002). The ashes were tested using deionized water (1000 ml water/100 g solid; pH 5.7) in high-density polyethylene bottles and shaken on a rotary shaker for 24 h at room temperature. The eluate was then filtered at 0.45 μm and acidified with nitric acid (HNO3 1% v/v) to less than pH 2. The pH was

3. Results 3.1. Coal samples Data from proximate and ultimate analyses and major element oxide analysis of the high-temperature ash (HTA) for the coal samples are shown in Table 1. All coal samples are ash-rich (ISO 11760, 2005),

Fig. 5. Optical microscopy images of Bokaro fly ash (reflected white light). Ferrospheres made of Al–Si glass and dendritic iron oxide crystals (probably magnetite) with different degree of density and size: A, B and C) supermicrometric hollow ferrospheres; D, F, G and H) dense ferrospheres.

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and sulfur-poor (~0.3 wt.%, db) (Table 1), which is a relatively rare occurrence on a worldwide basis (Raask, 1985). Petrographically, all of the coal samples have very high mineral matter content (Table 2), with the visible minerals represented by clay, quartz, siderite, and other carbonates (Fig. 2). The coal ashes are of the high-silica type (N 55%; Raask, 1985), with 3 wt.% to 9 wt.% Fe2O3 (Table 1). The ashes of the Chandrapura and Jharia coal samples have higher proportions of Fe2O3 than the ash of the Bokaro coal sample. X-ray diffraction data (Table 3) show that the LTAs of the coal samples are dominated by quartz, kaolinite, illite and mixed-layer illite/ smectite (I/S). Minor proportions of siderite are present in the LTA of all three coals, especially the LTA of the Jharia coal sample. Apatite also

occurs in the LTA of the three coals, and traces of possible dolomite, anatase, and in one case possibly sylvite, are noted in the Bokaro and Chandrapura LTA residues. Although siderite is present, pyrite was not identified in the mineral matter of any of the three coal samples.

3.2. Fly ash samples 3.2.1. Petrography The volume of iron-bearing morphotypes in the fly ashes is up to 3% (Table 2). These are mostly represented by 50 μm to 75 μm spheres made of a glassy aluminosilicate matrix with dispersed iron oxide crystalloids, and by smaller ferrospheres (b 50 μm) (Figs. 3 to 6).

Fig. 6. Optical microscopy images of Bokaro fly ash (reflected white light). Ferrospheres with iron oxide crystals other than dendritic: A) and B) ferrospheres with octahedral Fe oxide crystallization; C) to H) probably hematitic type.

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Fig. 7. SEM/EDS micrographs of Bokaro ferrospheres: A) fly ash general view a bright ferrosphere is inside the dashed-square area (Ch: char;×200, BSE); B) magnification of ferrosphere in “A” (×1500; SE); C) the same as in “B” under BSE mode (×1500); D) magnification of dashed square area in “C” (×4000; BSE), and EDS areas Z1 (aluminosilicate glassy matrix) and Z2 (Fe-rich area); E) and F) EDS spectra of Z1 and Z2 areas in “D”.

Cross-sections of these iron-bearing morphotypes show different internal structures: (i) ferrospheres with different proportions of iron-phases: (1) the iron-phase is composed by submicrometric Fe-crystals concentrated at the particle surface, which are almost unnoticed under the analytical conditions used (Fig. 3A); (2) clearly visible dendritic (magnetite) iron-crystals embedded in different amounts of vacuolated aluminosilicate matrix (Fig. 3B to F, and Fig. 4C to H); (ii) transitional forms, probably with partially disintegrated siderite (Fig. 4A and B);

(iii) vesicular spheres (50 − 75 μm in diameter), which are the most common iron-bearing morphotype, having one or several gas bubbles and dendritic iron-crystals dispersed in the glassy aluminosilicate matrix (Fig. 5). (iv) massive and small-size (b50 μm) ferrospheres of coarse-grained iron and blocky crystals (Figs. 3G, H and 6).

3.2.2. Scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS) The iron-rich morphotypes have a high brightness under BSE mode due to the high density of the Fe, which makes them easily recognizable even with the low proportions found in the studied fly ash samples. The

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Fig. 8. SEM/EDS (BSE mode) micrographs of Chandrapura ferrospheres: A) bulk fly ash general view with arrows pointing at bright ferrospheres (×1000); B) magnification of a micrometric ferrosphere (×20 000), and EDS spectrum; C) “pseudo-plerosphere ferrosphere” with quartz (Qz) relics (×2000; cross-section), and Z1 area EDS spectrum. Fe-rich areas (brightest areas) are soaked by the aluminosilicate glassy matrix; D) magnification of the area inside the dashed square in “A”, showing that micrometric spheres fill the “pseudoplerosphere” through openings in the ferrosphere (×15,000; cross-section); E and F) spectra of Z2 and Z3 areas in “C”: Z2 is the aluminosilicate glassy matrix with Fe, K, Ca and Ti, i.e., the composition of illite; Z3 quartz (Si–O) relic.

most common iron-rich morphotypes found in the Bokaro and Chandrapura fly ash samples are relatively small in size (ca. 25 μm to 75 μm), mostly well rounded, with pores at surface, and containing a considerable proportion of glassy aluminosilicate matrix (Figs. 7 and 8). Cross-sections show different internal structures: dense or vacuolated, variable amounts of glassy aluminosilicate matrix, variable crystal development (Figs. 9 and 10), and infillings, i.e., hollow ferrospheres filled by micrometric-sized fly ash (Fig. 8C and D). The latter forms should be named pseudoplero-ferrospheres, since hollow glassy-

spheres that are filled by micrometric-sized fly ash are referred to as pseudo-plerospheres (Goodarzi and Sanei, 2009). Two situations may contribute to these occurrences: infilling of large open and porous particles by smaller ones during the fly ash collection, and infilling during handling and preparation of the samples for analysis (Shibaoka and Paulson, 1986). True plerospheres, and encapsulations during sphere formation, are not common (from 5% to 10% of the ferrospheres (Anshits et al., 1998, 2011)), and pseudo-plerospheres may be confused with plerospheres.

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Fig. 9. SEM/EDS micrographs of Bokaro fly ash (BSE mode; cross-sections): A) ferro-morphotype composed iron-spherules (×1500); B) magnification of dashed square in “A” (×5000), and EDS spectrum; C) dense ferrosphere with large octahedral Fe crystals (probably hematite and magnetite; Z15 EDS spectrum). The aluminosilicate glassy matrix is the minor fraction of the ferrosphere; D) Z16 EDS spectrum of the aluminosilicate glassy matrix in “E”.

However, among the iron-rich morphotypes identified in this study many cannot classified as ferrospheres. These include: (i) siderite and possible pyrite relics, or partially baked or disintegrated forms of these minerals, occurring in association with the glassy matrix (Fig. 11A to D) or as discrete particles (Fig. 11E and F). These are the most common iron-rich morphotypes occurring in the Jharia fly ash (Figs. 10B, C and 11); (ii) some of the morphotypes observed are composed of an Fe-rich aluminosilicate matrix (Fig. 12A and B). However, no visible (bright) distinctive iron-phases are noticeable, and such particles could be classified as glass-like ferrospheres according to Anshits et al. (2011). (iii) morphotypes with visible (bright) iron-phases that are irregular in shape (Fig. 12C and D), and are better referred to as “ferrofragments”.

3.2.3. XRD and XRF analyses The proportion of each mineral, and also the amorphous phase, identified from the X-ray diffractograms (Shreya et al., 2015) of the fly ash samples is given in Table 3. These results show that the concentrations of magnetite and hematite in the Chandrapura fly ash and magnetite in the Bokaro fly ash are below the detection limit of the XRD/Siroquant system. Hematite is present in low proportions in the Bokaro fly ash, but is relatively abundant in the Jharia fly ash, along with small proportions of magnetite.

3.2.4. Raman spectroscopy The Raman spectra obtained from the different particles studied in the fly ash samples reveal the presence of iron oxides (Figs. 13 and 14): magnetite, with the Raman phonon modes, A1g (672 cm− 1), T2g (450 cm− 1), and Eg (309 cm− 1) (Figs. 13A, C2, D, E2 and 14 A, B, C, E, F, G) and hematite, with shifts at 225 cm − 1 [Fe+ 3 displacements (A1g)], 245 cm− 1 and 292 cm− 1 [Fe+ 3 displacements (Eg)], 410 cm − 1 [O = displacements (p 1 − Eg)], 497 cm − 1 [O = displacements (a − A1g)] and 611 cm− 1 [O = displacements (a − Eg)] (Figs. 13B, C1, F and 14) (Guedes et al., 2008). In the hematite spectra an additional band occurs at 672 cm− 1, corresponding to the A1g mode of magnetite (Figs. 13B, C1, F and, 14. D). With the exception of one particle where both magnetite and hematite were found (Fig. 13C), occurring randomly in the particle and easily detected by the differences in reflectance, most of the analyzed particles contain only magnetite or hematite. All the analyzed magnetite particles show similar crystallinity, with mean values of Full Width at Half Maximum (FWHM) of 60 cm−1, with the exception of one magnetite particle from Chandrapura sample that shows a FWHM of 84 cm−1 (Fig. 14F).

3.2.5. Magnetic susceptibility analysis and isothermal remanent magnetization (IRM) The χ, SIRM, S300 ratio, and SIRM/χ for the different samples of each section are presented in Table 4 and Fig. 15. The values for magnetic susceptibility, IRM and the S− 300 ratio indicate a predominance of

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ferromagnetic particles in the Bokaro and Jharia fly ash samples. The fact that the S−300 value is close to unity indicates that the remanence is dominated by magnetite-like structures.

3.2.6. Chemical composition of fly ashes and ash leachates The relative proportions of major element oxides in the fly ash samples are presented in Table 5, together with data from a range of European fly ashes (Moreno et al., 2005). The proportion of Fe2O3 ranges between 3.53 wt.% and 5.83 wt.%, which is within the range of

chemical compositions for European fly ashes, even being samples from India. The percentage of Fe determined for the same ashes by ICP-MS ranged between 2.7 wt.% and 4.6 wt.% (Table 5). When converted to Fe2O3, the values are 6.4%, 3.9% and 5.0% for the Bokaro, Chandrapura and Jharia ashes, respectively, which are close to those determined by XRF analysis. In both cases the highest value was found in the Bokaro fly ash and the lowest in the Chandrapura fly ash sample. The concentration of Fe in the leachates from the ashes is relatively low, especially for the Bokaro and Jharia samples. When compared to

Fig. 10. SEM/EDS micrographs of Jharia fly ash (BSE mode): A) ferrosphere with a major aluminosilicate glassy matrix composition (×900; bulk sample), and EDS spectrum; B) partially baked iron-rich mineral phase (×7500; cross-section); C) partially baked Ca-carbonate with small Fe-rich nodules (×5000; cross-section); D, E and F) EDS spectra in “C”.

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the results obtained from similar leachates published by Moreno et al. (2005), it is noticed that the Fe concentration in the Chandrapura fly ash leachate is in the lower part of the range of concentrations identified for European fly ashes, and that the Fe in the Bokaro and Jharia fly ash leachates is well below the European range. 4. Discussion Comparison of the XRD results (Table 3) to the chemical analysis data (Table 1) indicates that the proportion of Fe2O3 in the coal high-

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temperature ashes, especially for the Chandrapura and Jharia coals, cannot be accounted for by the concentration of siderite observed in the mineral matter of the respective coal samples. While some Fe may also occur in the mixed-layer illite/smectite, significant proportions of Fe may also occur in a non-mineral form, either dissolved in the pore water or associated in some way with the organic matter (Ward, 2002). Such occurrences are common in lower-rank coals (e.g. Li et al., 2010), and may be particularly significant in the Jharia coal sample. If occurring in such a form most of the iron could be reactive during combustion (Creelman et al., 2013), and be readily incorporated into

Fig. 11. SEM/EDS micrographs of partially baked F-bearing phases in fly ash (BSE mode; polished block): A) Chandrapura fly ash (×2000); B) Z3 EDS spectrum in “A”; C) Jharia (×625); D) magnification of the area inside the dashed square in “C” (×3000); E) Jharia (×850); F) example of EDS spectrum obtained both in “C” and “E” particles in the dashed square areas.

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the Al–Si glassy phase of the respective fly ashes. The siderite and other discrete Fe-mineral particles, on the other hand, would be expected to be less reactive and hence to form small iron-rich spheres. This is corroborated by the results of the optical microscopy and the SEM analysis since the iron-morphotypes proportion is low, and their size is relatively small. Sarkar et al. (2005, 2006) also found similar results for Bokaro fly ash. These results also reflect the possible absence of pyrite since ferrospheres attaining sizes up to several hundreds of micrometers are mentioned to occur from pyrite-rich coals (e.g. Anshits et al., 2011), while coals with less than 0.2% pyrite framboids generate low proportions of iron-morphotypes (Lauf et al., 1982). Partially baked minerals in fly ash provided clues concerning the origin of elements, i.e. in which mineral phases they were hosted in the coals, and the formation steps for the inorganic morphotypes in the fly ash. Siderite (FeCO3) and the mixed-layer illite/smectite are the obvious parental mineral for some of the ferro-morphotypes found in the fly ashes. However, some SEM/EDS analyses (Fig. 9) indicate that some of the Fe may also be hosted in carbonates of the dolomite-ferroan dolomite–ankerite series (CaMg(CO3)2 − (Fe,Ca,Mg)CO3) since the EDS spectra show that the partially baked iron-bearing mineral phases in Jharia fly ash are composed of carbonate relics with distinct Ca concentrations and a diversity of other elements. The petrographic properties (shape and brightness) and the SEM/ EDS results indicate the presence of dendritic magnetite crystals.

These are commonly classified as spinels by coal and coal ash petrographers (Hower and Mastalerz, 2001). Such particles are mostly formed from the FeO–SiO2 –Al2 O 3 melt (Anshits et al., 2011), and their properties and characteristics are determined by this system (Sharonova et al., 2013). The dominant occurrence of magnetite, in some cases with associated hematite, and the lack of evidence for the formation of other iron-bearing phases with Ca is clear from the EDS spectra, where Ca is not detected in the iron-phases but only in the aluminosilicate matrix. The Ca is detected in partially baked minerals but occurs separately from Fe in the aluminosilicate melts. Apparently, Fe from siderite and Ca from Ca-bearing minerals in the coals did not interact during combustion, otherwise a calcium ferrite phase could have been formed (Bibby, 1977; Huffman et al., 1981; Raask, 1985; Reifenstein et al., 1999; Dai et al., 2014). The nature of the iron-bearing minerals, namely magnetite and hematite, was confirmed in the fly ashes by XRD and Raman microspectroscopy techniques. As a bulk analysis technique the XRD did not detect these phases in the Chandrapura fly ash, and did not identify magnetite in the Bokaro fly ash. However, Raman microspectroscopy (RMS), as a point-specific technique, provided confirmation that magnetite is present in all the ferrospheres in the three ash samples. In some cases it was found that hematite coexists with magnetite, as a second crystal phase in the same ferrospheres. The reason for this is not clear from this study, however it is known that bivalent iron makes magnetite easily prone to

Fig. 12. SEM/EDS micrograph of a Bokaro fly ash cross-section (BSE mode): A) aluminosilicate glassy sphere with Fe, probably of clay origin (×450); B) Z4 EDS spectrum of “A”; C) inside dashed-square area, iron-rich fragment (×1000, BSE), and Z7 EDS spectrum of the aluminosilicate glassy matrix; D) Z8 spectrum in “C”. The “bright” areas of the fragment are Fe- and Al-rich.

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oxidation (Feitknecht and Gallagher, 1970) with temperature playing a dominant role in this process (Shebanova and Lazor, 2003).

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The observation of the fly ashes by microscopic methods and Raman spectroscopy did not provide accurate information concerning the size and extension of growth of the magnetite and hematite crystals. That

Fig. 13. Raman micro-spectroscopy spectra of Bokaro (A to E) and Jharia (F) ferrospheres. (A) A1 and A2 Raman spectrum of magnetite; (B) B1 and B2 Raman spectrum of hematite; (C) C1 (high reflectance) and C2 (low reflectance) Raman spectrum of hematite and magnetite, respectively; (D) D1 and D2 Raman spectrum of magnetite; (E) E1 and E2 Raman spectrum of magnetite; F) Jharia ferrosphere and hematite spectra.

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Fig. 14. Raman micro − spectroscopy spectra of Chandrapura ferrospheres. (A, B and C) Raman spectrum of magnetite; (D) D1 (high reflectance) and D2 (low reflectance) Raman spectrum of hematite; (C) C1 (high reflectance) and C2 (low reflectance) Raman spectrum of hematite; (E, F and G) Raman spectrum of magnetite.

B. Valentim et al. / International Journal of Coal Geology 153 (2016) 52–74 Table 4 Magnetic susceptibility, χ, SIRM, S−300 and S−25 ratios, and SIRM/χ in the fly ash samples from the three thermal power stations. Samples

χ(×10–8 m3/kg)

Jharia 1497.906 Bokaro 1210.902 Chandrapura 443.120

S-300 S-25 SIRM (×10–3 Am2/kg) 228.660 251.200 107.230

0.970 0.968 0.787

Table 5 Results of the fly ash analysis by XRF, and of the fly ash and respective leachates by ICP‐MS LOI (including data from European fly ash from Moreno et al., 2005).

SIRM/χ(kA/m)

0.221 15.27 0.342 20.74 0.343 24.20

information was obtained by remanent magnetism, magnetic susceptibility and complemented the data obtained using XRD, optical microscopy, SEM/EDS, and RMS techniques: (i) the IRM acquisition curves show saturation in fields between 200 and 300 mT, confirming the presence of magnetite-like structures. However, in the Chandrapura ash sample, the IRM acquisition curves show a small increase in intensity in increasing fields, suggesting that these samples may contain both multi-domain magnetite structures and antiferromagnetic minerals such as hematite (Fig. 16); (ii) the SIRM/χ ratio depends on the composition and grain size of the magnetic particles. When the magnetic mineralogy is homogeneous, this ratio indicates changes in the grain size assemblage of the ferrimagnetic minerals (e.g. Thompson and Oldfield, 1986; Moreno et al., 2003). As the magnetic mineralogy indicated by the S-300 ratio is homogeneous in all of the ash samples, the SIRM/χ ratio can be used to evaluate the grain size of the magnetic particles. Thompson and Oldfield (1986) considered that a mean SIRM/χ value close to 10 kAm− 1 indicates a magnetite grain size of 5 μm. However, values of SIRM/χ higher than 20 kAm−1 may indicate a smaller grain size for the ferrimagnetic particles in the Bokaro and Chandrapura samples. The values of S−25, when the magnetic structures are magnetitetype, may indicate the size of the ferrimagnetic particles. Using the AF demagnetization curves of Dunlop and Özdemir (1997) for magnetites with different grain sizes (Fig. 17), the projection of S− 25 ranging from 0.221 to 0.343 for the different samples shows that the particle size ranges from 3 μm in the Bokaro and Chandrapura ashes to 9 μm in Jharia fly ash samples; (iii) measurements made at two frequencies are used to detect the presence of ultrafine (b 0.03 μm) superparamagnetic minerals occurring as crystals. Samples comprising ultrafine minerals will show lower values when measured at high frequencies. In this study the frequency dependence of magnetic susceptibility is low (b5%), which indicate the absence of ultrafine magnetic grains.

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Bokaro Fly ash XRF Al2O3 SiO2 CaO K2O TiO2 Fe2O3 MgO Na2O Mn3O4 P2O5 SO3 LOI

wt.%

Eluate pH After leaching test After acidification for ICP-MS analysis Fly ash ICP‐MS Fe Cr Fly ash leachates ICP‐MS Fe Cr

wt.% ppm

ppm

21.91 58.26 0.35 1.02 1.59 5.83 0.47 0.11 0.072 0.2 0.019 10.17a

Chandrapura

Jharia

27.42 47.89 0.69 1.28 2.01 3.53 0.44 0.21 0.043 0.43 0.029 16.05a

21.93 53.65 1.82 1.71 1.49 4.97 1.08 0.14 0.061 0.85 0.083 12.22a

7.9

7.98

1.47

1.4

1.47

4.46 129

2.7 172

3.51 116

0.012 4

10.6

0.124 1.2

0.019 21.5

European FA 17.6−35.6 28.5−59.6 0.5−27.3 0.4−4.0 0.5−2.6 2.6−16.0 0.6−3.8 0.1−1.2 0.03−0.1 0.1−1.7 0.1−8.6 1.1−8.1

n.a. n.a.

n.a. 47−281 b0.1−9.8 17−9264

n.a. — data not available. a Only a small sample was available, LOI (loss‐on‐ignition) was determined by difference.

Taking into account the IMR considerations, SEM/EDS micrographs, seen for the first time, provide new insights on how iron-bearing mineral phases are formed in the ferrospheres. The cross-section of a hollow ferrosphere (Fig. 18A and B) shows walls that are apparently composed of dendritic crystals, most probably a magnetite-phase, embedded in the aluminosilicate matrix. However, magnifications of these show that some phases are not dendritic but are still in an intermediate stage, as shown by its roundness, and the iron-phases are distributed near the ferrosphere external surface (Fig. 18C and D). Nevertheless, this distribution ends ca. 5 μm to 10 μm below the surface. The crystallite distribution is also affected by the vacuoles, since the Fe-dendrites also grow around the internal surface of those features (Fig. 18C), but no multilevel core–shell (Zyryanov et al., 2010) structure is evident since in the middle of these surfaces the dendrites do not show any specific orientation and are formed by four branches

Fig. 15. Plot of the relation between SIRM and magnetic susceptibility, χ, in the fly ash samples from the three thermal power stations.

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Fig. 16. IRM acquisition curves.

diverging at 90° from each other, crossing other dendrite branches also growing in the aluminosilicate melt (Fig. 18B). Further magnification of the ferrosphere internal structures, from ×15,000 up to ×75,000, shows that nanometric iron-plumes are being released from the melted iron bubbles surface (Fig. 19C dashed-square 1). They then undergo a necking-down at their base near the motherbubble (Fig. 19C dashed-square 2), and are released into the aluminosilicate matrix (Fig. 19B and C dashed-square 3) where they lose the round

shape and start their dendritic crystallographic organization (Fig. 19B dashed-square 3). These observations do not support a process (Anshits et al., 1998; Sokol et al., 2002) in which the magnetite precipitates from aluminosilicate melts and the glass is formed after that and is located between the dendrite crystals. At this size level, the flow direction of the iron-plumes observed seems to be mainly influenced by forces related to the ejection process, i.e. perpendicular to the iron-bubble surface. These plumes are still on

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Fig. 17. Projection of S − 25 ranging from 0.221 to 0.343 in the AF demagnetization curves from Dunlop and Özdemir (1997).

Fig. 18. SEM/EDS micrographs of Bokaro (BSE mode; cross-sections): A) hollow ferrosphere (×1752), and Z8 EDS spectrum in “B”; B) magnification of dashed square in “A” (×5000). Formation of Fe dendritic crystals (probably magnetite; Z9 EDS spectrum) in the aluminosilicate matrix (Z8 EDS spectrum in “A”); C) magnification of dashed-square in “A”. The Fe-plumes migrate through the aluminosilicate matrix to the particle surface forming a sheet of submicrometric Fe-rich particle (×15,000; Z13 EDS spectrum); D) Z14 EDS spectrum in “C”.

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Fig. 19. SEM/EDS micrographs of Bokaro (BSE mode; polished block). Formation of Fe-crystals in ferrospheres: A) melted Fe-bearing coal minerals (×15,000); B) magnification of “A”. Release of Fe-plumes to the aluminosilicate matrix (×30,000); C) magnification of “B”: (1) plumes, (2) plumes necking-down, (3) new Fe-protocrystals (×75,000). EDS spectrum Z10, Z11 and Z12; D) Z10 EDS spectrum of the Fe-bearing phase; E) Z11 EDS spectrum of the Fe-plume flowing in the aluminosilicate glassy matrix; F) Z12 EDS spectrum of the aluminosilicate glassy matrix.

an amorphous state, since remanent magnetism analysis indicates the absence of ultrafine magnetic grains. Therefore, it is probably that the formation of iron-crystallites, with more than 3 μm, occurs only after the coalescence process of ultra-fine the plumes. 5. Potential environmental impacts One of the factors included in the study was the contamination potential associated with the fly ash, since material with potential for release of contaminants cannot be used as bottom liner material in landfills or geoliners. The Fe-morphotypes may cause corrosion of

cement in concrete by setting up small electrochemical cells (Sarkar et al., 2006), but perhaps more importantly the leaching of iron-bearing phases in the fly ashes may contribute to elevated levels of flocculated iron (EPA, 2003) and contaminants (e.g. Cr from spinel (Metalic elements,Fe,Zn,Mn)(Al,Cr,Fe)2O4), and chromospinel (CrFe2O4); Zhao et al., 2006) in underlying groundwater percolating through the bottom liner. Therefore, the concentration of Fe, as well as the phases in which it occurs, and the Cr concentration were studied using a combination of techniques, and it was found that iron is relatively abundant in the ashes. However, leaching tests and the eluates analysis by ICP-MS shows that the eluate pH is not acidic (Table 5), and the Fe neither Cr are not

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likely to be significant contaminants. Therefore, the removal of the ironmorphotypes, e.g. by magnetic methods, would appear to be unnecessary to reduce the potential for iron contamination, or the Cr from possible spinel and chromospinel. Samples of fly ash from Bokaro, Jharia, and Chandrapura produced very low iron concentrations (12 ppb, 19 ppb and 124 ppb, respectively; Table 5), when the ashes were subjected to leaching. The leaching of the Fe was very low since the pH after leaching test was near neutral for the Bokaro and the Chandrapura samples and alkaline for Jharia sample (Table 5), as Fe is not easily removed from the spinel structures. 6. Conclusions Chemical analyses indicate that iron is present in the fly ashes in significant proportions, with Fe making up from 2.7 wt.% to 4.5 wt.% of the ash material; this is equivalent to an Fe2O3 content ranging from 3.5 wt.% to 5.8 wt.% (Table 5). The main iron-bearing phases are magnetite and hematite. However, XRD reveals that these are present only in minor proportions, and are below detection limits in some cases. These crystalline iron-bearing phases occur as massive or dendritic crystals forming iron-rich spheres, or as finely dispersed crystals trapped inside a glassy aluminosilicate matrix. The trapping appears to have resulted from the breakdown of iron-rich minerals in the feed coal to release small particles of iron oxide into the aluminosilicate melt. In addition to these phases, the Fe occurs as a component of the glass that makes up most of the fly ash materials. The formation of dendritic iron in the ferrospheres is a multi-stage process starting with the disintegration and/or melting of the iron in association with an aluminosilicate matrix. The melted iron forms rounded particles and the melted aluminosilicate matrix acts as a flowing environment for release of nanometric Fe-plumes. However, these are in an amorphous state, and magnetite and hematite crystals will form with sizes above 3 μm. Fe is not likely to be a significant contaminant, neither Cr. Therefore, the removal of the iron-morphotypes appears unnecessary to the use of these fly ashes as a geoliner. Acknowledgments The author Neha Shreya benefited from a PhD scholarship (UID INDI1202711) at the University of Porto financed by the Erasmus Mundus Programme. References Anshits, A.G., Kondratenko, E.V., Fomenko, E.V., Kovalev, A.M., Anshits, N.N., Bajukov, O.A., Sokol, E.V., Salanov, A.N., 2001. Novel glass crystal catalysts for the processes of methane oxidation. Catal. Today 64, 59–67. Anshits, A.G., Kondratenko, E.V., Fomenko, E.V., Kovalev, A.M., Bajukov, O.A., Anshits, N.N., Sokol, E.V., Kochubey, D.I., Boronin, A.I., Salanov, A.N., Koshcheev, S.V., 2000. Physicochemical and catalytic properties of glass crystal catalysts for the oxidation of methane. J. Mol. Catal. A Chem. 158, 209–214. Anshits, A.G., Sharonova, O.M., Anshits, N.N., Vereshchagin, S.N., Rabchevskii, E.V., Solovjev, A.V., 2011. Ferrospheres from fly ashes: composition and catalytic properties in high-temperature oxidation of methane. Abstracts of the World of Coal Ash (WOCA) Conference. May 9–12, 2011, Denver, CO, USA (http://flyash.info/). Anshits, A.G., Voskresenskaya, E.N., Kondratenko, E.V., Fomenko, E.V., Sokol, E.V., 1998. The study of composition of novel high temperature catalysts for oxidative conversion of methane. Catal. Today 42, 197–203. ASTM D3174-12, 2012. Standard Test Method for Ash in the Analysis Sample of Coal and Coke from Coal. ASTM International, West Conshohocken, PA (www.astm.org). ASTM D3175-11, 2011. Standard Test Method for Volatile Matter in the Analysis Sample of Coal and Coke. ASTM International, West Conshohocken, PA (www.astm.org). ASTM D3302/D3302M-15, 2015. Standard Test Method for Total Moisture in Coal. ASTM International, West Conshohocken, PA (www.astm.org). ASTM D5865-13, 2013. Standard Test Method for Gross Calorific Value of Coal and Coke. ASTM International, West Conshohocken, PA (www.astm.org). Baba, A., 2003. Geochemical assessment of environmental effects of ash from Yatagan (Mugla-Turkey) thermal power plant. Water Air Soil Pollut. 144, 3–18. Bibby, D.M., 1977. Composition and variation of pulverized fuel ash obtained from the combustion of sub-bituminous coals, New Zealand. Fuel 56, 427–431.

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