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Das, Subir Kumar, 2016, Characteristics and Composition of Magnetic Concentrates of Bottom Ash Sample from Two Types of Thermal Power Plants in Odisha, India. Coal Combustion and Gasification Products 8, 30-43, doi: 10.4177/CCGP-D-14-00010.1

Characteristics and Composition of Magnetic Concentrates of Bottom Ash Samples fromTwo Types of Thermal Power Plants in Odisha, India

Subir Kumar Das*Chief Scientist (Retired), CSIR-IMMT, Bhubaneswar-751 013, India

A B S T R A C T

Aluminosilicate bottom ash samples were examined from a thermal power plant in Talcher (sample NT-BA) and in Sambalpur(sample HR-BA), Odisha, India, to determine magnetic particle morphology, internal structure, mineralogy, mineral chemistry,ash fusion temperature, chemical composition, and leaching characteristics. The Talcher plant has adopted pulverized coalcombustion, whereas the Sambalpur plant uses circulating fluidized bed combustion boiler technology. For NT-BA, mostmagnetic particles were ideal solid spheres (ferrospheres) with diverse morphological features (smooth, granular, dendritic andskeletal, porous, spotted, polygonal, and hollow [magnetite cenospheres]). For HR-BA, magnetic particles were subspherical,oval, and angular and showed widely varying surface morphologies, internal structures, and textures. Magnetite crystals occuras fine lacy, vermicular, granular, and anastomizing veinlets; agglomerated masses; and patches intimately admixed with anamorphous silicate mass/phase. Magnetite is considerably altered to hematite. Magnetite-rich magnetic concentrates (NT-BA)have higher ash fusion temperatures compared with hematite + magnetite mixed magnetic concentrates (HR-BA). Energydispersive X-ray analysis identified magnetite, low- and high-Fe aluminosilicate glass, native Fe, and complex oxides andsilicates. In the magnetite crystal structure, small amounts of Al, Mg, Mn, Ti, and Ca occur by isomorphous substitution offerrous and ferric ions. The magnetic and nonmagnetic samples contained predominantly SiO2, Al2O3, and iron oxides derivedfrom mineral matter in feed coal. The coarse dense ferrosphere and ferro-fragments containing large amounts of iron oxideminerals were probably derived from decomposition and oxidation of pyrite, siderite, and ankerite in feed coal. Iron oxideparticles containing both iron oxides and glass are possibly derived from simultaneous melting of iron oxide minerals and clayminerals in feed coal. Sequential leaching experiments evaluated the elemental (Cr, Co, Ni, Mn, Cu, Zn, Pb, Na, K, Ca, and Mg)mobility from magnetic and nonmagnetic concentrates of bottom ash samples. These experiments provide information on thecrystallization of magnetite from Fe-bearing melts in different coal combustion technologies and help evaluate environmentalissues related to ash disposal and use.

– 2016 The University of Kentucky Center for Applied Energy Research and the American Coal Ash AssociationAll rights reserved.

A R T I C L E I N F O

Article history: Received 5 November 2014; Received in revised form 17 April 2016; Accepted 22 April 2016

Keywords: ferrosphere; pulverized coal combustion; circulating fluidized bed combustion; Talcher; Sambalpur; Odisha

1.1. IntroductionIntroduction

Combustion of coal for energy production in thermal powerplants generates industrial wastes referred to as coal combustionproducts (fly ash and bottom ash). Because of environmental anddisposal problems, the mineralogy, geochemistry, leaching, and

use of these materials have been extensively studied. In contrast,the products can also be considered as value-added materials(Kumar et al., 2007). Fly ash has been used in construction; asa low-cost absorbent for the removal of organic compounds;as a catalyst for methane oxidation; in treatment of flue gases(nitrogen and sulfur oxides) and mercury in air; as a lightweightmine back fill; in concrete; and for silica extraction, censosphereseparation, and low Si/Al zeolite synthesis (Ahmaruzzaman, 2010).* Corresponding author. Tel.: +91-674-235-0632. E-mail: subir@immt.res.in

I SSN 1946 -0198

jou r na l homepage : www.coa l cgp - j o u rna l . o rg

doi: 10.4177/CCGP-D-14-00010.1– 2016 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association. All rights reserved.

The magnetic concentrates separated from fly ash are being consid-ered for potential use as heavy media in coal beneficiation plants, indense concrete production (Vassilev et al., 2004; Sarkar et al., 2011),as advanced magnetic materials for shielding radioactive materialsand catalytic agents, and in iron metallurgy (Yang et al., 2014a). Itis also reported, however, that magnetic particles in coal ash maybe enriched in toxic elements such as Cr, Co, Ni, Cu, Pb, and Znand may therefore impart an environmental pollution risk (Vassilevet al., 2004).Several studies have documented considerable variation in the

morphology, chemistry, texture, and origin of magnetic particlesin fly ash (Sokol et al., 2002; Sharonova et al., 2003, 2013; Vassilevet al., 2004; Kutchko and Kim, 2006; Zhao et al., 2006; Xue and Lu,2008; Sarkar et al., 2011; Yang et al., 2014a; Valentim et al., 2016).These variations depend on the Fe-bearing mineral phases present inthe feed coal, the oxidizing or reducing conditions of the combus-tion chamber, the retention time, the viscosity of the ferriferoussilicate melt, the temperature conditions, and the effects of post-combustion cooling. The magnetic particles in coal combustion prod‐ucts typically consist of ferrite spinel, magnetite, magnesioferrite,and hematite. Zhao et al. (2006) classified the ferrospheres in flyashes into seven groups according to their microstructure: sheet,dendritic, granular, smooth, ferroplerospheres, porous, and moltendrop. Zhao et al. (2006) indicated that the formation mechanismsfor the different types of ferrospheres inside the combustion cham-ber were also different because of the complex eutectics of Fe-bearing minerals and clays at very high temperatures during coalcombustion. Sharonova et al. (2013) observed that, with an increasein Fe content and a decrease in viscosity of the melt, the morpholo-gies of ferrospheres changed from porous (foam-like) to glass-like,fine grained (dendritic and point), skeletal-dendritic, and coarsegrained (block-like). Ferro-oxides and aluminosilicate-bearingferro-oxides are important sources of the initiation layer that occursin deposits formed in coal-burning systems (Creelman et al., 2013).Fe occurrences in coal are classified into two parts: (1) discrete min-eral matter and (2) organically associated (in pore water and organicmatter) (Zhao et al., 2006; Valentim et al., 2016). Magnetic particlesin coal combustion products are products of decomposition and oxi-dation of discrete Fe-bearing minerals, most commonly pyrite,

siderite, ferroan dolomite, and ankerite, whereas organically associ-ated Fe is strongly partitioned into the aluminosilicate glassy matrix(Valentim et al., 2016).In India, coal-based thermal power plants are the major sources

for power generation. The generation of coal combustion productsis ,170 million tonnes/yr. The present rate of use of fly ash is,50%, mainly in civil construction and building materials, butwith a small contribution in agriculture, and the bulk is disposedin ash ponds and as landfills. In the state of Odisha in India,coal resources are located in two adjacent basins: Talcher(20u509–21u159N, 84u099–85u339E) and Ib River (21u319–22u149N,83u329–84u109E) (Figure 1); the coal reserves are 50.0 and 23.8 Gt,respectively (Indian Minerals Yearbook, 2013). The coal deposits ofthese two basins are non-coking, sub-bituminous to bituminouscoals with high ash yields (up to 50%), and they are mainly usedin thermal power plants. Recently, Odisha has been witnessing rapidgrowth of industrialization, and there is a huge demand for coal bythermal power plants.Several studies have investigated the petrography and geo‐

chemistry of coals in the Talcher and Ib River basins (Pareek,1963; Mishra and Mohanty, 2005; Senapaty and Behera, 2012,2015; Singh et al., 2013). The chemistry and mineralogy of coalfrom both deposits have been reported to vary widely within andbetween the seams. The general characteristics for the Talchercoal deposit are 2–10% moisture, 18–47% ash, 22–32% volatilematter, 24–44% fixed C, 8–45% mineral matter, ,1% S (mineralmatter–free [mmf] basis), and 3258–6000 kcal/kg calorific value.The general characteristics for the Ib River coal deposit are 4–9%moisture, 5–45% ash, 21–39% volatite matter, 26–59% fixed C,6–38% mineral matter, ,1% S (mmf basis), and 3280–6660 kcal/kg calorific value. The mineral matter, occurring as excluded andincluded forms in coal samples, mostly consisted of quartz, kaolin‐ite, and illite, with minor to trace amounts of pyrite, siderite, cal-cite, dolomite, montmorillonite, feldspar, magnetite goethite,limonite, and hematite.The mineralogy, leaching characteristics, and pozzolanic proper-

ties of bulk fly ash from the Talcher area are well known (Kanungoand Mohapatra, 2000; Praharaj et al., 2002; Das and Yudhbir,2006; Mishra and Das, 2010; Nayak and Panda, 2010). Prakashet al. (2001) recovered magnetic particles from the fly ash ofTalcher thermal power plants by beneficiation using a magneticcoating technique. Bottom ash constitutes 15–35% of the coal com-bustion products from the thermal power plants. A literature searchrevealed a lack of studies on the morphology, internal texture andstructure, fusion temperature determination, mineral chemistry,and leaching characteristics of the magnetic concentrates in thebottom ash of the thermal power plants in Talcher and Sambalpur(Nayak and Panda, 2010; Nayak, 2015). The purpose of this investi-gation was to use optical microscopy, X-ray diffraction, scanningelectron microscopy (SEM), energy dispersive X-ray spectrometry(EDS), heating microscopy, chemical analysis, and leaching studiesfor comprehensive physicochemical and mineralogical characteriza-tion of the magnetic particles of bottom ashes from two thermalpower plants adopting different coal-burning technologies: pulver-ized coal combustion (PCC) boilers and circulating fluidized bedcombustion (CFBC) boilers. The results may be helpful in interpret-ing the potential uses and also the possible environmental risks ofthe coal ashes.

Fig. 1.Fig. 1. Coal-bearing Gondwana formations in Talchir basin and Ib River basin(Manjrekar et al., 2006).

Das / Coal Combustion and Gasification Products 8 (2016) 31

2.2. Materials and MethodsMaterials and Methods

2.1. Sampling and size analysis

The bottom ash samples were collected from two thermal powerplants: a plant in Talcher (NTPC plant; sample NT-BA) and a plantin Sambalpur (Hindalco captive power plant; HR-BA) (Figure 1).The NTPC plant adopts PCC technology and the Hindalco powerplant uses CFBC technology for power generation. The bottom ashin the NTPC plant is collected in water-impounded hoppers andsluiced into a storage lagoon. The Hindalco power plant uses a sys-tem of dry ash disposal to storage silos. Approximately 20 kg of bot-tom ash was collected from each power plant. The samples werethoroughly mixed following the coning and quartering methodand wet sieved down to ,25 mm in size. The magnetic particleswere separated from each size fraction by a hand-held permanentbar magnet.

2.2. SEM and EDS

The micromorphology of the magnetic particles was studiedusing a scanning electron microscope (EVO 50-06, Carl Zeiss,Jena, Germany). The samples were sprinkled on double-sided adhe-sive carbon tape fixed on an aluminum metal stud, carbon coated ina vacuum sputter coater, and viewed under the scanning electronmicroscope operating at 25 kV, with a beam current of 11.0 mA.Elemental analysis of mineral phases was performed using SEM in

spot mode, with attached liquid nitrogen–free XFlashH silicon driftdetectors (Bruker AXS Microanalysis, Berlin, Germany), combinedwith a pulse processor for optimal performance. Chemical analyseson the spots were done using a standardless quantification method,based on peak-to-background ZAF evaluation.

2.3. Optical microscopy

The magnetic particles were thoroughly mixed with polyesterresin and hardener and poured into 30-mm-diameter molds.The resulting blocks were polished by adopting standard polishedsection preparation techniques. Final polishing of the blocks wasdone using 1-mm diamond paste. The sections were studied using anincident light polarizing microscope (DM2500P, Leica Microsystems,Wetzlar, Germany). The coarse magnetic particles were observedunder a binocular microscope (Leica Microsystems).

2.4. X-ray diffraction studies

X-ray powder diffraction patterns were recorded for magneticand nonmagnetic fractions of two bottom ash samples by usingan XRD unit (PW 3710, Philips, Almelo, The Netherlands) withCuKa radiation operating at a voltage of 40 kV and a current of20 mA.

2.5. Ash fusion temperature determination

The magnetic and nonmagnetic concentrates of the two sampleswere taken for ash fusion temperature estimation by using a heatingmicroscope (Hesse Instruments, Osterode am Harz, Germany). Thesamples were heated in a furnace at,850uC for 5 hours, cold pressedto cylinders (,4 mm in length and 2 mm in diameter), and mountedon the ceramic plate in the furnace chamber. The samples were thenheated at heating rate of 25uC/min up to 1569uC under an air

atmosphere to measure the deformation, softening, hemispherical,and fluid or flow temperatures.

2.6. Chemical analysis and leaching experiments

Approximately 0.30 g of each sample was taken in a Teflonbeaker, and 10 ml of an HF–H2SO4 mixture was added. The beakerswere heated on a hot plate and evaporated to dryness. The residuewas dissolved by gently heating in 25 ml of an acid mixture of con-centrated HCl and distilled water in a 1:1 proportion, and then thesolution was transferred to a volumetric flask and the volume wasmade up to 100 ml. The sample solutions were used for analysesof major (Al2O3), minor (Ca, Mg, Na, K, and Mn), and trace (Cr, Co,Ni, Cu, Pb, and Zn) metals by an atomic absorption spectrophotom-eter (AA6300, Shimadzu, Kyoto, Japan). The concentration of Fe inthe solutions was estimated by a titrimetric method. Silica in thesamples was determined by a gravimetric method.Sequential leaching experiments were carried out on the samples to

establish partitioning of the metals among the water-extractable frac-tion (leaching by distilled water), exchangeable fraction (leaching by1 N ammonium acetate + acetic acid, pH 5.0), and carbonate– andsurface-oxide–bound fractions (leaching by 2 N HCl) (Palmer et al.,1995). The sample-to-solution ratio in each case was 1:20, and thesamples were shaken for 24 hours. Leachates were collected, filtered,and acidified with HNO3. The concentrations of minor and trace ele-ments in each solution were determined using an AA6300 atomicabsorption spectrophotometer.

3.3. Results and DiscussionResults and Discussion

3.1. Size analysis and magnetic concentrate

The distributions (wt%) of magnetic and nonmagnetic fractionsin each size fraction of the two bottom ash samples are given inTable 1. The proportion of magnetic particles in sample NT-BA ishigh (25.7%) compared with that in HR-BA (4.03%). Table 1 alsoindicates that the NT-BA sample contained 34.20%, 11.22%, and3.04% magnetic particles compared with 49.49%, 36.26%, and7.87% in HR-BA in the size fractions 0.125+0.063, 0.063+0.025,and −0.025 mm, respectively, indicating that the magnetic particlesare finer in HR-BA.

3.2. Morphology

3.2.1. NT-BAThe morphological features of the magnetic particles in NT-BA

are shown in Figures 2a–o. The coarse magnetic particles (.1.0 mm)are irregular, angular, and subrounded to rounded (Figures 2aand 2b). The finer magnetic particles (,0.50 mm) are predominantlyspherical (ferrospheres) (Figures 2c–e). Backscattered electronimages of the ferrospheres indicate widely varying morphologicalfeatures on the surface (Figures 2f–o).The ferrospheres can be divided into the following types:

(1) smooth, (2) granular, (3) dendritic and skeletal, (4) porous,(5) spotted, (6) polygonal (cubo-octahedral and polygonal grains),and (7) hollow (magnetite cenospheres). Smooth ferrospheres havesmooth surfaces with tightly packed ultrafine crystallites of magne-tite intermixed with a large proportion of glass components (Figure 2f).Smooth ferrospheres are likely to be formed via mixing of a largevolume of aluminosilicate and small Fe-mineral melts in thecombustion zone. Granular ferrospheres have rough surfaces with

32 Das / Coal Combustion and Gasification Products 8 (2016)

tightly packed fine granular magnetite grains (Figure 2g). This tex-ture is the result of magnetite crystal deposition on the ferrospheresurface when the temperature decreases, and it is derived from thefusion of inherent minerals in coal (Zhao et al., 2006). The dendriticand skeletal patterns of magnetite crystals wetted by variableamounts of glassy matrix are shown in Figures 2h and 2i, respec-tively. Dendritic ferrospheres consist of fine magnetite crystallitesoccurring as laths, lamellae, and streaks. Conglutination after theoxidation of iron oxide minerals has been identified as the maincause for the formation of dendritic and granular ferrospheres(Zhao et al., 2006). Dendritic magnetite crystals are mostly formedfrom the FeO-SiO2-Al2O3 melt, and their characteristics are deter-mined by this system (Sharonova et al., 2013).Minute-to-coarse pores were observed on the surfaces of the fer-

rospheres, indicating escape of gas during solidification of the melt.Coarse cubo-octahedral and flower-like magnetite grains within theglass matrix exhibit blocky surface textures, as shown in Figures 2jand 2k, respectively. Star-like magnetite grains are found within theglass matrix, showing a spotted texture (Figure 2l) that seems tohave formed from minute needle-like magnetite crystals assemblingtogether (Sarkar et al., 2011).Typical hollow ferrospheres (magnetite censopsheres) are shown

in Figures 2m and 2n. The magnetite cenospheres are often broken,with radiating cooling cracks (Figure 2m). Hollow ferrospheres havethin shells with tightly packed octahedral magnetite grains; coarsevoids are also at times distinct (Figure 2n). Thin-shelled magnetitecenospheres are formed owing to surface deposition of magnetiteon gas bubbles (Sokol et al., 2002). The vesicular walls of the ceno-spheres suggest that the gas and melt generation have a commonfocus of activity (Hubbard and McGill, 1984). Frequently, ferro-spheres are also partially enveloped by glassy matrix and glassspheroids (Figure 2o).

3.2.2. HR-BAThe morphology of the magnetic particles in HR-BA is shown in

binocular microscopy images (Figures 3a and 3b) and SEM-backscattered electron micrographs (Figures 3c–l). The coarse mag-netic particles are predominantly elongated, angular, and irregular(Figures 3a and 3b), whereas the finer particles (,0.5 mm) aresubspherical, oval, subhedral, and angular (Figures 3c and 3d). InFigure 3e, the ferrospheres exhibit surface coating by a massivemass of magnetite with minute pores; the magnetite crystallitesoccur as tightly packed grains without revealing crystal outlines.Fine laths of magnetite crystals form triangular patterns with tinyintergranular pores in subrounded magnetic particles (Figure 3f).

Figure 3g shows the molten drop ferro-fragments with abundantstout cubes and fine granules on the surface. These molten drop fer-rospheres/ferro-fragments form due to transformation of Fe-bearingphases at higher temperatures (.1000uC) under reducing conditions(Zhao et al., 2006; Xue and Lu, 2008). Coagulation or segregation ofmagnetite grains is observed in globular to angular magnetite parti-cles (Figure 3h). The broken subspherical and angular particles havethin shells of magnetite, with the inside being composed of quartz,amorphous glassy masses, and dispersed speckled magnetite parti-cles with large amounts of voids (Figures 3i and 3j). Solid ferro-spheres have a thin crust of magnetite on the surface, whereas theinner materials contain intimately admixed glassy phase and mag-netite crystallites (Figure 3k). The magnetite grains are globular,stout platy, and squarish shaped, and they are tightly packed toform a massive crust. A few ferrospheres contain thinly dispersedmagnetite grains within glassy masses; the iron oxides are globular,platy, lacy, and irregular types (Figure 3l).

3.3. Polished section studies by optical microscopy and SEM

3.3.1. NT-BAFrom the SEM images showing the external surface morphology

of the sprinkled particles, it is difficult to ascertain whether the par-ticles are hollow or solid, and it is also difficult to identify the inter-nal intergrowth patterns of the mineral phases. General views ofpolished cross sections of the magnetic particles indicate that theferrospheres are predominantly solid spheres (Figure 4a). The coarseirregular particles (ferro-fragments; Valentim et al., 2016) consist ofdensely packed fine magnetite crystallites forming massive bodieswith radiating cracks, and coarse and fine voids, intimately admixedwith a glass matrix (Figure 4b). This type of discrete magnetic parti-cle possibly represents siderite or pyrite relics or partially baked ordisintegrated forms of these minerals (Valentim et al., 2016). Theagglomerated masses in the bottom ash consist of ferrospheres, glassparticles (irregular, angular, and rounded), magnetite veinlets, andresidual quartz grains that are loosely packed with high proportionsof interstitial voids (Figures 4c and 4d). The agglomerated particleshave probably been formed owing to interparticle contact or rapidcooling (Kutchko and Kim, 2006). Thick- and thin-shelled magnetitecenospheres mostly have undulating inner walls and contain tightlypacked magnetite euhedra with interstitial glass masses (Figure 4e).The ferrospheres also contain diversely oriented submicrometer-

sized magnetite laths and streaks with small amounts of aluminosil‐icate glassy matrix (Figure 4f). Skeletal and dendritic texturesshown by magnetite crystallites within the solid ferrospheres aredemonstrated in Figure 4g. These textures are indicative of

Table 1Table 1Size distribution and magnetite contents in different size fractions of bottom ash samples

NT-BA HR-BA

Size (mm) wt% Magnetite % Magnetite wt% Magnetite % Magnetite

+2 2.93−2+1 3.23 1.31 40.60 9.16 0.13 1.42−1+0.5 6.10 1.44 23.58 19.61 0.31 1.58−0.5+0.25 25.04 5.33 21.29 47.96 0.73 1.52−0.25+0.125 36.13 11.24 31.10 13.91 1.29 9.27−0.125+0.063 17.40 5.95 34.22 1.96 0.97 49.99−0.063+0.025 6.84 0.77 11.22 0.91 0.33 36.26−0.025 5.26 0.16 3.04 3.56 0.28 7.87Total 100 25.7 100 4.03

Das / Coal Combustion and Gasification Products 8 (2016) 33

magnetite crystallization under supercooling conditions (Howeret al., 1999; Vassilev et al., 2004, 2005). Vassilev and Vassileva(1996) concluded that the dense and vesicular spheres originatefrom fully and partly degassified melts of various minerals. The fer-roplerospheres contain inclusions of solid glass spheres, vesicularferrospheres, and small residual quartz grains (Figure 4h). Martitiza-tion of magnetite is occasionally observed in the ferro-fragmentsand in the ferrospheres (Figure 4i), in which hematite also occursas plates and crusts. Hematite is formed by oxidation of magnetiteduring cooling under oxidation conditions (Vassilev and Vassileva,1996). The subrounded vacuolated magnetic particles contain coarseand fine voids, densely packed cubic magnetite grains with smallamounts of interstitial glass, and thin glass rims (Figure 4j). Coarse

detrital quartz grains are often encapsulated in the ferrospheres, asdepicted in Figure 4k. In NT-BA, native Fe is rare and occurs as tab-ular grains and as inclusions within the porous ferrospheres (Figures4l and 4m). The tabular native Fe contains inclusions of magnetite,glass particles, and is partially mantled by magnetite. Underextremely reducing condition in the combustion chamber of theboiler, small amounts of native iron may form from the high iron–bearing melt at a temperature of .1200uC. The paucity of nativeiron in the sample NT-BA indicates formation of the ferrosphereunder conditions of high partial oxygen pressure (Sokol et al.,2002). Figure 4n depicts a mosaic texture due to intergrowth ofrounded to subrounded magnetite grains and interstitial glassmasses, possibly indicating simultaneous crystallization. A few

Fig. 2.Fig. 2. Morphotypes of magnetic particles of NT-BA. Stereomicroscope (a and b) and scanning electron microscope (backscattered scanning electron microscope; c–o)images are shown. (a) Subrounded and angular magnetic particles, +1 mm. (b) Subspherical magnetite grains adhering to each other, +0.5 mm. (c) Ferrospheres with voids,250+100 mm. (d and e) Ferrospheres, 100+53 mm, 53+25 mm. (f) Smooth ferrosphere. (g) Granular ferrosphere. (h) Ferrosphere showing skeletal magnetite. (i) Ferrosphereshowing dendrites of magnetite. (j) Ferrosphere with octahedral and polygonal magnetite grains. (k) Flower-like magnetite crystals in ferrosphere. 1 and 2 = energydispersive X-ray spectrometry (EDS) points. (l) Star-like magnetite grains within glass matrix. (m) Thin-walled broken magnetite cenosphere with cooling cracks. (n) Thin-walled magnetite cenosphere. (o) Ferrosphere showing cubo-octahedral magnetite grains coated with glass spheres and irregular glass masses. 3 and 4 = EDS points.

34 Das / Coal Combustion and Gasification Products 8 (2016)

rounded grains show composite intergrowth between euhedral andskeletal-dendritic crystallites of magnetite with a small amount ofinterstitial glass (Figure 4o). Ferrospheres of different sizes areshown in Figure 4p; some of these forms are pseudomorphs afterpyrite crystal aggregates and framboids observed in coal (Lauf,1982). Vesicular subrounded magnetic particles (ferro-fragments)and ferrospheres are shown in Figures 4q and 4r, respectively. Theferrospheres contain agglomerated octahedral magnetite grainsnear the core surrounded by submicrometer-sized magnetite needlesdispersed within aluminosilicate glass. The morphotype shown inFigure 4r is probably under formation (transitional form) from par-tially disintegrated siderites (Valentim et al., 2016).

3.3.2. HR-BAThe internal structure and texture of the magnetic particles of in

HR-BA are shown in Figures 5a–r. Polished sections of the particlesunder optical microscopy and SEM reveal that the particles are pre-dominantly solid; coarse particles are irregular (ferro-fragments);and finer particles are subrounded, subhedral, and angular (Figures5a–c). Reflected light microscopy studies show that the magnetiteis variously altered to hematite. A few vacuolated massive magnetitefragments contain tightly packed fine magnetite crystals having

transverse fractures (Figure 5d). Mostly the magnetic particles con-tain varying proportions of an amorphous glass phase intermixedwith magnetite (Figures 5a, b, e, f, i, l, m, and o–q). In the dense par-ticles, magnetite occurs as massive bodies containing tightly packedfine lamellae, laths, cubes, and granules of magnetite (Figures 5g,5h, and 5j). Fine to coarse quartz grains are frequently observedwithin the magnetic grains (Figures 5f and 5i). In some rounded par-ticles, successive rims of magnetite deposition are observed, separat‐ed by thin amorphous masses or glass zones (Figures 5h and 5k).Magnetite occurs as thin rims around angular Fe-rich glass

particles that also contain inclusions of coarse-to-fine detritalquartz grains (Figure 5i). Figure 5l depicts a rounded grain havingagglomerated and dispersed magnetite grains, with voids near thecenter surrounded by a thick zone of quartz. In rounded particles,magnetite grains occur as fine lacy, vermicular, anhedral grainsand patches (Figure 5m). In some particles, hematite grains occuras anastomizing veinlets and as massive masses/patches exhibitinga honeycomb texture (Figure 5n). Morphotypes shown in Figures5m and 5n are rather common in the magnetic concentrate of thissample. In the rounded magnetic particles (Figure 5o), patchy orgraphic intergrowth grades to coarse massive masses occur nearthe core and mantle, whereas the grain is rimmed by thin magnetite

Fig. 3.Fig. 3. Morphotypes of magnetic particles of HR-BA. Stereomicroscope (a and b) and scanning electron microscope (backscattered scanning electron microscope; c–l)images are shown. (a and b) Irregular and angular magnetite particles, +1 mm, +0.5 mm. (c and d) Subspherical, oval, angular, and irregular magnetite particles, 250+125mm, 125+63 mm. (e) Massive magnetite coating on the surface of ferrospheres. (f) Fine hematite plates forming triangular network. (g) Molten drop ferro-fragment. (h) Clottexture due agglomeration of fine magnetite crystals. (i) Thin magnetite rim around the porous spheroids. (j) Angular particle showing thin magnetite laths and intergrowthof porous glass and magnetite grains. (k) Solid ferrosphere with surface coating of octahedral magnetite. (l) Dispersed lacy and vermicular magnetite in glass matrix.

Das / Coal Combustion and Gasification Products 8 (2016) 35

Fig. 4.Fig. 4. Internal texture of the magnetite grains of NT-BA. Scanning electron microscope (backscattered scanning electron microscope; a, c, d, h, and j–o) andphotomicrographs by reflected light microscope (b, e–g, i, and p–r) are shown. (a) General view of polished section showing solid magnetic particles. (b) Ferro-fragment(partially baked siderite or pyrite). (c) Agglomerated fragments consisting of vacuolated ferrospheres, magnetite veinlets, glass sphere, irregular glass mass, and quartzgrains. 6 = energy dispersive X-ray spectrometry (EDS) point. (d) Enlarged part of the box in (c). Q = quartz; G = glass. 5 = EDS point. (e) Thin-walled magnetite censopherewith undulating inner rim. (f) Ferrosphere showing closely spaced and diversely oriented magnetite crystallites. (g) Ferrosphere with dendritic and skeletal magnetitecrystallites. (h) Ferroplerosphere showing inclusions of quartz, solid ferrosphere, and glass mass. (i) Ferrosphere showing tightly packed euhedral magnetite (gray) andhematite (white) grains. (j) Ferrosphere showing interlocking euhedral magnetite grains wetted by aluminosilicate glass. 7–10 = EDS points. (k) Detrital quartz (dark gray) inferrosphere. 11 = EDS point. (l) Tabular iron metal contains inclusions and partially rimmed by magnetite. 12–15 = EDS points. (m) Solid ferrosphere showing (a) inclusionsof Fe metal (left side grain) and (b) dendrites of magnetite with interstitial glass mass. 16–18 = EDS points. (n) Mosaic texture owing to magnetite and glass intergrowth.19–20 = EDS points. (o) Porous ferrosphere with euhedral and skeletal magnetite crystallites. 21 = EDS points. (p) Ferrospheres of different sizes. (q) Porous ferro-fragments.(r) Ferrosphere, probably transitional form of partially disintegrated siderite.

36 Das / Coal Combustion and Gasification Products 8 (2016)

Fig. 5.Fig. 5. Internal texture of the magnetite particles of sample HR-BA. Scanning electron microscope (backscattered scanning electron microscope; a, c, f–l, and o) andphotomicrographs by reflected light microscopy (b, d, e, m, n, and p–r) are shown. (a) Irregular and subrounded magnetite fragments, +0.5 mm. (b) Subrounded particleshowing intimately admixed hematite and porous glass mass, +250 mm. (c) Subrounded and angular magnetic particles, 250+125 mm. (d) Fractured subrounded magnetitefragment. (e and f) Ferro-fragments showing intergrowth of fine hematite grains and massive mass with glass matrix. Note quartz grains in (f). (g) Magnetic particleshowing massive patches of magnetite wetted by aluminosilicate glass. (h) Thin amorphous glass rim separates the magnetite zones. 22 = energy dispersive X-rayspectrometry (EDS) point. (i) Thin ribbon of hematite around the angular glass particle. 23 and 24 = EDS points. (j) Magnetite ribbon around the rim and as large occlusionswithin the grain. 25 and 26 = EDS points; Qz./Si-gl. = quartz/silicon glass. (k) Alternate zones containing densely packed and disseminated magnetite grains. Qt. = quartz;27 and 28 = EDS points. (l) Magnetite clusters near the core surrounded by thick zone of Qz./Si-gl. 29 and 30 = EDS points. (m) Vermicular and irregular magnetite grainsintermixed with amorphous silicates. (n) Porous rounded particle showing interconnecting hematite masses and veinlets. (o) Fine graphic intergrowth between magnetiteand glass grading to massive mass. (p and q) Magnetite possibly representing partially baked or disintegrated form of siderite or pyrite. (r) Ferrrosphere with magnetiteeuhedra.

Das / Coal Combustion and Gasification Products 8 (2016) 37

ribbons. Figures 5p and 5q depict the textures of ferro-fragmentsthat possibly represent partially baked or disintegrated forms of sid‐erite or pyrite (Valentim et al., 2016). Ferrospheres of typical formshown in Figure 5r are rare in this sample; it contains tightly packedmagnetite euhedra and possibly a pseudomoph of framboidal ormassive pyrite in coal. Thick- and thin-walled magnetite cenospheregrains are occasionally noted; however, dendritic and skeletalmagnetite grains and native Fe were not noticed in the magneticparticles.

3.4. X-ray diffraction studies

X-ray diffraction data for of the magnetic and nonmagnetic frac-tions of the bottom ash samples are shown in Figures 6a–d. Theresults indicate the following: (1) the magnetic fraction of NT-BAis dominantly constituted of magnetite with minor amounts ofquartz (Figure 6c); (2) the nonmagnetic fraction of NT-BA containsmullite, quartz, and a trace amount of cristobalite (?) (Figure 6d);(3) the magnetic concentrate of HR-BA contains significantamounts of hematite and magnetite and minor amounts of quartzand cristobalite (Figure 6b); and (4) the nonmagnetic fraction ofHR-BA contains quartz as the major mineral phase and small tomoderate amounts of hematite, mullite, and cristobalite (Figure 6a).Magnetite is the principal iron oxide mineral in the magnetic con-

centrates of both NT-BA and HR-BA, although HR-BA also containsa significant amount of secondary hematite. Mullite and cristobaliteare noted in the nonmagnetic fractions of both bottom ash samples;

however, cristobalite is more distinct in HR-BA (CFBC plant). Glass,magnetite, cristobalite, and mullite are the common mineral phasesin ashes from PCC plants burning low-Ca bituminous coal, wherethe boiler temperature mostly exceeds 1200uC (Vassilev et al.,2005; Xue and Lu, 2008; Valentim et al., 2016). For CFBC ashsamples, where the optimum boiler temperature is ,950uC, theseminerals are either absent or occur in small to moderate amounts(Demir et al., 2001; Li et al., 2006; Silva et al., 2014; Yang et al.,2014b; Valentim et al., 2016).The presence of mullite and cristobalite in the CFBC bottom ash

HR-BA indicates that flame temperature on the particle surfacewithin the combustion chamber of the plant was more than1000uC. Mollah et al. (1999) observed that cristobalite can formfrom quartz, glass, and mullite at ,950uC.

3.5. Ash fusion temperature determination

In the ash fusion temperature studies, the shapes of the cylindersof magnetic and nonmagnetic particles prepared from NT-BAand HR-BA at different temperatures are shown in Figures 7a and7b and 7c and 7d, respectively. Magnetic NT-BA (dominantly com-posed of magnetite) has higher softening (1300uC), deformation(1429uC), spherical (1463uC), and melt ($1548uC) temperaturescompared with the respective temperatures (1200uC, 1377uC,1414uC and 1436uC) for HR-BA (mainly constituted of magnetiteand hematite) (Figures 7a and 7c). These temperatures reflect differ-ent phases of the ash melting process. The nonmagnetic fractions ofNT-BA and HR-BA have deformation temperature of 1474uC and1556uC, respectively, and both of the samples have a very highfusion temperature (.1569uC) (Figures 7b and 7d). Mishra andMohanty (2005) and Chakravarty et al. (2015) recorded widely vary-ing flow temperatures from 1380uC to 1500uC for Talcher and from1510uC to .1600uC for Ib Valley coal ashes. The differences in theparameters in the ash fusion experiments are explained by the min-eral compositions of the studied magnetic and nonmagnetic ashsamples. Liu et al. (2013) have reported that the three major chemi-cal components that affect the ash fusion temperature are CaO,Fe2O3 content, and SiO2/Al2O3 ratio (S/A). Ash fusion temperaturedecreases with an increase of CaO and Fe2O3 and an increase inthe S/A ratio up to 1.5. Both of the magnetic concentrates studiedcontain very high percentages of Fe2O3 (magnetite and hematite)and higher proportions of CaO compared with the nonmagneticfractions, resulting in lower ash fusion temperatures.

3.6. SEM-EDS studies

Semiquantitative chemical compositions of the mineral phases inthe ashes were determined by SEM-EDS (Table 2). Typical EDS pat-terns are shown in Figures 8a–l. The ranges of chemical compo‐nents for the mineral phases in the magnetic concentrates NT-BAand HR-BA are given in Table 3. The chemical data indicate thatthe magnetite grains of both of the samples are enriched in FeO(t)(NT-BA, 90.03–97.60%; HR-BA, 76.41–92.73%) and contain lowto moderate concentrations of Mg (MgO, 0–6.24% and 0–7.19%,respectively) and Al (Al2O3: 0–3.44% and 1.38–16.62%, respective-ly) and small amounts of Mn (MnO, 0–2.97% and 0–1.06%, respec-tively), Si (SiO2, 0–1.64% and 0.94–3.12%, respectively), Ti (TiO2,0–1.05% and 0–1.58%, respectively), and Ca (CaO, 0–2.07% and0–1.09%, respectively). During coal combustion, a high ferrousmelt originates from melting of ferrous carbonates and Fe silicates.

Fig. 6.Fig. 6. X-ray diffractograms of magnetic and nonmagnetic fractions: HR-BAnonmagnetic (a), HR-BA magnetic (b), NT-BA magnetic (c), and NT-BAnonmagnetic (d). Q 5 quartz; M 5 mullite; Mt 5 magnetite; H 5 hematite; Cb5 cristobalite.

38 Das / Coal Combustion and Gasification Products 8 (2016)

At high temperature, Mg2+and Mn2+ substitute for Fe2+ over a widetemperature range, and at lower temperature (Fe, Mg, Mn)O Fe2O3

preserves its homogeneity. Ca can also isomorphically substitutefor ferrous Fe in the magnetite structure, but to a limited extentand at high temperature (Sokol et al., 2002). Al3+ isomorphicallyreplaces Fe3+ in the crystal structure. High proportions of Mg (spotno. 25) and Al (spot no. 30) in some of the analyzed magnetitegrains indicate the presence of magnesioferrite and hercynite com-ponents, respectively. Calcite, dolomite, and siderite (Mn-bearing)minerals in the feed coal are the principal source of Ca, Mg, andMn in the magnetite grains in the ash samples. The EDS spectra ofthe magnetite grains show strong Fe and O and weak Mn, Al, andSi peaks (Figure 8a). The elemental spectra of Al-rich magnetiteand Mg-rich magnetite are shown in Figure 8b and 8c, respectively,in which small Ca and Mn peaks are also distinct.The glass phase, associated with the magnetic particles of both

samples, exhibits wide variations in chemical composition, mostlywith respect to Fe, Si, and Al. Small amounts of K, Ca, Ti, Mg, andMn are also detected in the glass phases. The glass phases can beclassified into two types: low-Fe and high-Fe aluminosilicate glass.The range of compositional variation for the glass phases is given inTable 3. In NT-BA and HR-BA, the maximum values of FeO in thelow-Fe glass phases are 7.18 and 4.14% and in the high-Fe glassphases are 68.67% and 45.23%, respectively (Table 3). The low-Feglass contains higher proportions of Ti, compared to the high-Feglass of both of the samples. K is not detected in glasses of sampleHR-BA, but it is significant in the low-Fe glass of sample NT-BA.K is mostly contributed by the illite in the feed coal, and it is

incorporated into the glass structure during crystallization of themelt. Mn and Ca are also detected by EDS analyses in the glassphases. Valentim et al. (2016) observed that Ca preferentially entersinto the glass structure of the fly ash. Mg is recorded in one glassanalysis (spot no. 27). Booher et al. (1994), Vassilev and Vassileva(1996), and Sokol et al. (2002) also noted wide variation in composi-tions of the glass particles in fly ash samples. Energy dispersivespectra of the glasses in the present study revealed strong siliconX-ray peaks and strong-to-small Fe and Al peaks (Figures 8d–f).Small peaks of Mn, Ca, K, and Ti are also noted in these analyzedspots. Elemental spectra of the Ti-rich (TiO2, 7.41%; spot no. 29)and Mg-rich (MgO, 21.82%; spot no. 26) aluminosilicate glassphases are shown in Figures 8g and 8h, respectively. Analysis no.9, in which the EDS spectrum shows strong peaks of Fe, O, and Si(Figure 8i), is indicative of an Fe-rich silica glass or Fe-rich quartzparticle with an FeO content of 10.76%. The heterogeneity in chem-ical composition of the glass phases in the particles is due to rapidchanges in crystallization of the chemically inhomogeneous dropsformed by melting of different mineral mixtures in the feed coal(Booher et al. 1994; Sokol et al., 2002). Trace amounts of Si (SiO2,0.20%; spot no. 16) were detected in the Fe-metal encapsulatedwithin the ferrospheres (Figure 8j). The elemental spectra of Fe-Mnsilicate (spot no. 14) and complex Fe-Ti-Mn oxide (spot no. 13) aregiven in Figures 8k and 8l, respectively.

3.7. Chemical analysis and leaching studies

Major (Fe%, SiO2%, Al2O3%), minor, and trace element data for themagnetic and nonmagnetic fractions of NT-BA and HR-BA are given

Fig. 7.Fig. 7. Ash fusion experiment showing shapes of the cylinder in magnetic and nonmagnetic concentrates at different temperatures: (a) NT-BA-M (magnetic), (b) NT-BA-NM(nonmagnetic), (c) HR-BA-M (magnetic), and (d) HR-BA-NM (nonmagnetic).

Das / Coal Combustion and Gasification Products 8 (2016) 39

in Table 4. The magnetic concentrates are strongly enriched in Fe (NT-BA, ,10 times; HR-BA, ,23 times) compared with nonmagneticfractions, indicating dominant Fe partitioning to iron oxide minerals(magnetite and hematite) and a small amount to the associated Fe-rich glass phase. The magnetic fractions of NT-BA and HR-BA contain15.65% and 14.74% SiO2 and 7.67% and 13.38% Al2O3, respectively.The analyzed magnetic and nonmagnetic samples are mainly com-posed of SiO2, Al2O3, and iron oxides (Table 4). In the feed coal, Aloccurs mainly in the clay minerals (kaolinite), whereas silica isalso found in clay minerals and quartz. The relatively low levels ofK (maximum, 1933 ppm [NT-BA-NM]; Table 4) and high percen-tages of Al2O3 in the samples suggest that the dominant parentclay mineral was kaolinite rather than illite. The trace element distri-bution patterns in the magnetic and nonmagnetic fractions of thestudied samples show distinctly different results. The magnetic con-centrate of HR-BA has distinct enrichment of Mn, Cu, Cr, and Zn;almost equal concentration of Co and Na; and depletion of Pb, K,and Ni, compared with the respective concentrations in the non-magnetic fraction. The magnetic concentrate of NT-BA has strongand moderate enrichment of Mn and Co, respectively, and distinctdepletion of Cr, Ni, Cu, Zn, Pb, and K. Ca is strongly and Mg is mod-erately enriched in the magnetic fractions of both samples. High Mn,Ca, and Mg concentrations in the magnetic concentrates suggeststhat Fe2+-Mn2+, Fe2+-Ca2+, and Fe2+-Mg2+ diadochy are prominentin the magnetite crystal structure; Cr3+ replaces Fe3+ in the magne-tite, as well as Al3+ in the glass phase. Hower et al. (1999) noted dis-tinct Cr enrichment and depletion in Zn, Cu, and Mn in the magneticfractions compared to nonmagnetic fractions of fly ash samples.Yang et al. (2014a) found no relationship between the Fe content

of magnetospheres and different trace elements, indicating randomelement distribution patterns in the minerals of coal ashes.The trace element concentrations in the leachates are given

in Table 4. These concentrations reveal the modes of occurrenceand the leaching behavior of the magnetic and nonmagnetic concen-trates of NT-BA and HR-BA. The data also indicate that (1) smallamounts Mn, Na, Zn, Ca, Mg, and K are leached by water, suggestingan occurrence as adsorbed ions on minerals and as water-solublesalts; toxic elements, such as Cr, Co, Ni, Cu, and Pb, are not leachableby water, corroborating the observations of Praharaj et al. (2002); (2)metal dissolution is enhanced under acidic conditions provided byacetic acid, leading to the leaching of variable amounts of Ni, Mn,Cu, Zn, Ca, Mg, Na, and K occurring as ion-exchangeable compo-nents on minerals and chars, whereas Cr, Co, and Pb were notleached; and (3) the sharp increase in element leaching under themore extreme leaching conditions imposed by 2 N HCl is attributedto the HCl solution extracting these elements bound to amorphousto poorly crystalline oxides, carbonates, and Fe-Mn oxides. Theleaching studies for the magnetic and nonmagnetic fractions ofNT-BA and HR-BA from these two power plants also indicate identi-cal leaching patterns for Cr, Ni, Mn, Pb, Ca, Na, and K (Table 4).

4.4. ConclusionsConclusions

Magnetite particles have been successfully isolated from bottomash samples of the two coal-fired power plants that adopt differentcombustion technologies: PCC boilers (NT-BA) and CFBC boilers(HR-BA). The morphology, texture, and composition of the magneticparticles in such coal combustion wastes depend on several factors,

Table 2Table 2Semiquantitative analysis (wt%) of mineral phases (by energy dispersive X-ray spectrometry) in ash particles1

Spot

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

MgO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 6.24 0.00 0.00 0.00 0.00Al2O3 2.19 24.32 4.31 31.93 14.71 2.50 3.44 25.19 0.00 10.09 0.00 0.84 5.42 1.45 0.00SiO2 1.31 59.17 91.13 41.96 25.86 1.05 0.00 38.19 89.24 85.49 0.00 0.70 5.42 20.80 0.84K2O 0.00 2.09 0.77 0.44 0.51 0.00 0.00 0.00 0.00 1.14 0.00 0.00 0.00 0.00 0.00CaO 0.00 1.12 0.00 0.52 1.34 0.00 0.00 4.56 0.00 0.00 2.07 0.00 0.23 0.22 0.00TiO2 0.00 3.00 0.00 1.16 0.97 0.00 0.00 1.87 0.00 0.00 0.00 0.85 35.92 3.89 0.00MnO 1.32 3.18 0.00 0.00 0.00 0.00 1.15 0.61 0.00 0.00 1.65 0.00 14.26 27.10 2.97FeO(t) 95.18 7.12 3.78 24.00 56.61 96.45 95.41 29.56 10.76 3.28 90.03 97.60 38.76 46.54 96.19Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00Mineral Mag. Glass Glass Glass Glass Mag. Mag. Glass Fe-qtz Glass Mag. Mag. Fe-Ti-

Mn-ox.Fe-Mn-silicate

Mag.

Spot

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

MgO 0.00 0.00 0.00 0.00 0.00 1.89 0.00 0.00 0.00 7.19 21.82 1.59 2.00 0.00 1.85Al2O3 0.00 1.63 12.97 3.95 7.33 2.68 2.56 31.27 37.68 1.38 13.25 2.94 3.79 37.13 16.62SiO2 0.202 1.64 25.40 25.83 0.60 0.00 2.59 27.24 51.34 0.94 36.59 48.96 1.95 52.35 3.12K2O 0.00 0.00 0.00 0.35 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00CaO 0.00 0.00 1.78 1.20 0.00 0.00 0.00 0.00 0.00 1.09 0.33 0.00 0.00 0.34 0.43TiO2 0.00 0.00 1.12 0.00 0.00 0.00 1.05 0.81 6.84 0.00 0.44 0.31 0.69 7.41 1.58MnO 0.00 0.00 0.00 0.00 0.00 1.05 1.06 0.00 0.00 0.93 0.70 0.97 0.74 0.00 0.00FeO(t) 99.802 96.73 58.73 68.67 92.07 94.38 92.73 40.49 4.14 88.47 26.87 45.23 90.83 2.77 76.41Total 100.00 100.00 100.00 100.00 100.00 100 100 100 100 100.00 100 100 100 100 100Mineral Native

FeMag. Glass Glass Mag. Mag. Mag. Glass Glass Mag. Glass Glass Mag. Glass Mag.

Note: FeO(t) 5 total iron as FeO; Mag. 5 magnetite; ox. = oxide; Fe-qtz 5 Fe-rich quartz.1 Points 1–21, NT-BA; points 22–30, HR-BA.2As Fe and Si metal.

40 Das / Coal Combustion and Gasification Products 8 (2016)

most importantly the mineral matter in the feed coal, the combus-tion conditions, and the boiler types. The magnetic concentratesfrom the two power plants show significant differences with respectto morphology, texture, mineralogical composition, ash fusiontemperature, element concentrations, and leaching characteristics.The magnetic particles of NT-BA are dominantly spherical (ferro-spheres), and seven morphotypes have been recognized. Magnetiteis the dominant iron oxide mineral and exhibits different texturesand structures that are typical of the ferrospheres in coal

combustion wastes generated by PCC boilers burning low-Ca bitu-minous coal. The magnetic particles of HR-BA are subsphericaland angular and are constituted of hematite and magnetite and anassociated glass mass. The iron oxide minerals occur as massivebodies, laths, cubes, and as granular, lacy, and vermicular grainsand anastomizing veinlets and patches. Partially baked or disinte-grated forms of pyrite or siderite and a transitional form of pyriteare recognized in the two ash samples. The coarse dense ferrospheresand ferro-fragments, containing large amounts of iron oxide

Fig. 8.Fig. 8. Energy dispersive X-ray spectrometry (EDS) spectra of phases observed in ash samples. x-axes: energy, kiloelectron volts; y-axes: intensity/arbitrary units (a.u.). (a)Magnetite, spot 1, see Figure 2k. (b) Magnetite, spot 30, see Figure 5l. (c) Magnetite, spot 25, see Figure 5j. (d) Glass, spot 8, see Figure 4j. (e) Glass, spot 2, see Figure 2k. (f)Glass, spot 3, see Figure 2o. (g) Glass, spot 29, see Figure 5l. (h) Glass, spot 26, see Figure 5j. (i) Fe-rich quartz, spot 9, see Figure 4j. (j) Native Fe, spot 16, see Figure 4m. (k)Fe-Mn silicate phase, spot 14, see Figure 4l. (l) Complex phase, spot 13, see Figure 4l.

Das / Coal Combustion and Gasification Products 8 (2016) 41

minerals, were probably derived from decomposition and oxidationof pyrite, siderite, and ankerite in the feed coal. With rising temper-ature, Fe-bearing minerals and clay minerals were melted and mixedto form Fe-bearing particles (ferrospheres and ferro-fragments) con-taining varying proportions of glass and magnetite.Mineral chemistry results by SEM-EDS indicate that Mn, Al, Mg,

Ti, Si, and Ca occur in varying amounts in the magnetite, partly asisomorphous substitutions for Fe2+ and Fe3+ in the crystal lattice.The associated glass component has been classified into low- andhigh-Fe glass phases.The magnetic and nonmagnetic samples are predominantly made

up of SiO2, Al2O3, and Fe that are derived from the mineral matterof the feed coal. Leaching studies indicate that the principal trace

elements—Cr, Ni, Mn, Pb, Na, and K—in the magnetic and nonmag-netic fractions of the bottom ash samples show identical dissolutionbehavior patterns. In addition, the potentially toxic elements (Cr,Co, Ni, and Pb) in the magnetic and nonmagnetic samples werenot leached by water.The magnetic concentrates from the coal combustion products

have potential for use in mineral processing and metallurgical indus-tries. The percentage of magnetic concentrates in the fly ash gener-ated in the thermal power plants in eastern India ranges between3% and 11% (Sarkar et al., 2011; Valentim et al., 2016). The presentstudy has indicated that the bottom ash also contains considerableamounts of magnetic materials (4–26%). Thus, even estimating themagnetic yield of particles at 1%, the annual production of such

Table 3Table 3Range of different element oxides (wt%) in magnetite and low- and high-Fe aluminosilicate glass phases

Magnetite Low-Fe aluminosilicate glass High-Fe aluminosilicate glass

NT-BA HR-BA NT-BA HR-BA NT-BA HR-BA

Min. Max. Min. Max. Min. Max. Min. Max. Min. Max. Min. Max.

MgO 0.00 6.24 0.00 7.19 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.59Al2O3 0.00 3.44 1.38 16.62 4.31 24.32 37.13 37.68 3.95 31.93 2.94 31.27SiO2 0.00 1.64 0.94 3.12 59.17 91.13 51.34 52.35 25.40 41.96 27.24 48.96K2O 0.00 0.00 0.00 0.00 0.77 2.09 0.00 0.00 0.00 0.51 0.00 0.00CaO 0.00 2.07 0.00 1.09 0.00 1.12 0.00 0.34 0.52 4.56 0.00 0.00TiO2 0.00 1.05 0.00 1.58 0.00 3.00 6.84 7.41 0.00 1.87 0.31 0.81MnO 0.00 2.97 0.00 1.06 0.00 3.18 0.00 0.00 0.00 0.61 0.00 0.97FeO(t) 90.03 97.60 76.41 92.73 3.28 7.18 2.77 4.14 24.00 68.67 40.49 45.23

Note: Min. 5 minimum; Max. 5 maximum; FeO(t) 5 total iron as FeO.

Table 4Table 4Major elements (wt%), trace element concentrations (ppm), and trace element–leaching patterns by sequential leaching with different solvents of the magnetic (M) andnonmagnetic (NM) fractions of NT-BA and HR-BA1

ppm wt%

Sample Cr Co Ni Mn Cu Zn Pb Na K Ca Mg Fe SiO2 Al2O3

NT-BA-M 58 53 57 2272 52 93 18 890 735 10,200 660 50.26 15.65 7.67NT-BA-NM 87 23 89 403 94 195 40 853 1933 4200 570 5.03 61.57 27.31HR-BA-M 205 34 26 1232 219 358 28 1000 800 22,400 1240 45.83 14.74 13.38HR-BA-NM 164 31 57 192 44 106 35 1077 1800 9400 954 2.12 66.31 25.94Leaching by water (ppm)NT-BA-M ND ND ND 12 (1) ND ND ND 18 (2) 17 (2) 602 (6) 28 (4)NT-BA-NM ND ND ND 22 (6) 2 (2) 9 (5) ND 8 (1) 9 (1) 65 (2) 15 (3)HR-BA-M ND ND ND 14 (1) ND ND ND 98 (10) 28 (4) 800 (4) 11 (1)HR-BA-NM ND ND ND 15 (8) ND 8 (8) ND 27 (3) 20 (1) 54 (1) 14 (2)Leaching by 1 M ammonium acetate + acetic acid (ppm)NT-BA-M ND ND 2 (4) 105 (5) 3 (6) 18 (19) ND 56 (6) 36 (5) 1212 (12) 31 (5)NT-BA-NM ND ND 3 (3) 29 (7) 13 (14) 13 (7) ND 34 (4) 18 (1) 331 (8) 28 (5)HR-BA-M ND ND 1 (4) 30 (3) 7 (3) 10 (3) ND 51 (5) 24 (3) 407 (2) 16 (1)HR-BA-NM ND ND ND 16 (8) 3 (7) 27 (25) ND 29 (3) 12 (1) 267 (3) 17 (2)Leaching by 2 N HCl (ppm)NT-BA-M 8 (14) 3 (6) 9 (16) 168 (7) 20 (39) 25 (27) 6 (33) 251 (28) 122 (17) 220 (2) 46 (7)NT-BA-NM 4 (5) 2 (9) 5 (6) 118 (29) 13 (14) 17 (9) 9 (23) 82 (10) 80 (4) 580 (14) 40 (7)HR-BA-M 56 (27) 6 (18) 12 (23) 110 (9) 20 (9) 28 (7) 10 (36) 153 (15) 38 (5) 412 (2) 19 (2)HR-BA-NM 4 (2) 2 (6) 2 (4) 31 (16) 4 (9) 16 (15) 5 (14) 89 (8) 36 (2) 822 (9) 19 (2)Total % of elements leached in the samplesNT-BA-M 14 6 20 13 45 46 33 36 24 20 16NT-BA-NM 5 9 9 42 30 21 23 15 6 24 15HR-BA-M 27 18 50 13 12 10 36 30 12 8 4HR-BA-NM 2 6 4 30 16 48 14 14 4 13 6

Note: ND 5 not detected.1Values in parentheses represent percentages leached.

42 Das / Coal Combustion and Gasification Products 8 (2016)

material from ,100 million tonnes of coal combustion products inOdisha would exceed 1 million tonnes. Microscopic studies of thesamples show that the magnetite crystals and masses are commonlyintimately intergrown with aluminosilicate glass. Liberation of mag-netite from the gangues would be very difficult and challenging.Hence, an integrated approach consisting of physical beneficiationinvolving fine grinding, separation in a magnetic separator, andchemical treatment of the magnetic fractions would be needed toobtain pure magnetite concentrates for such applications.

AcknowledgmentsAcknowledgments

I am grateful to Prof. B.K. Mishra, Director, Council of Scientificand Industrial Research–Institute of Minerals and Materials Tech-nology (CSIR‐IMMT) for encouragement and to Dr. B.B. Nayak(CSIR‐IMMT) for help in chemical analyses of the samples. I amindebted to Prof. J.C. Hower (University of Kentucky, Lexington)and anonymous reviewers who provided valuable suggestions.

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