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Suggested Citation format for this article:
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
I S S N 1 9 4 6 - 01 9 8
j o u r n a l h o m e p a g e : w w w. c o a l c g p - j o u r n a l . o r g
Characteristics and Composition of Magnetic Concentrates of Bottom Ash Samples from
Two 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 coal
combustion, whereas the Sambalpur plant uses circulating fluidized bed combustion boiler technology. For NT-BA, most
magnetic particles were ideal solid spheres (ferrospheres) with diverse morphological features (smooth, granular, dendritic and
skeletal, 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 occur
as fine lacy, vermicular, granular, and anastomizing veinlets; agglomerated masses; and patches intimately admixed with an
amorphous 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). Energy
dispersive X-ray analysis identified magnetite, low- and high-Fe aluminosilicate glass, native Fe, and complex oxides and
silicates. In the magnetite crystal structure, small amounts of Al, Mg, Mn, Ti, and Ca occur by isomorphous substitution of
ferrous and ferric ions. The magnetic and nonmagnetic samples contained predominantly SiO2, Al2O3, and iron oxides derived
from mineral matter in feed coal. The coarse dense ferrosphere and ferro-fragments containing large amounts of iron oxide
minerals were probably derived from decomposition and oxidation of pyrite, siderite, and ankerite in feed coal. Iron oxide
particles containing both iron oxides and glass are possibly derived from simultaneous melting of iron oxide minerals and clay
minerals 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 the
crystallization of magnetite from Fe-bearing melts in different coal combustion technologies and help evaluate environmental
issues related to ash disposal and use.
– 2016 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association
All 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. Introduction
Combustion of coal for energy production in thermal power
plants generates industrial wastes referred to as coal combustion
products (fly ash and bottom ash). Because of environmental and
disposal problems, the mineralogy, geochemistry, leaching, and
* Corresponding author. Tel.: +91-674-235-0632. E-mail: subir@immt.res.in
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; as
a 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 lightweight
mine back fill; in concrete; and for silica extraction, censosphere
separation, and low Si/Al zeolite synthesis (Ahmaruzzaman, 2010).
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.
Das / Coal Combustion and Gasification Products 8 (2016)
Fig. 1. Coal-bearing Gondwana formations in Talchir basin and Ib River basin
(Manjrekar et al., 2006).
The magnetic concentrates separated from fly ash are being considered for potential use as heavy media in coal beneficiation plants, in
dense concrete production (Vassilev et al., 2004; Sarkar et al., 2011),
as advanced magnetic materials for shielding radioactive materials
and catalytic agents, and in iron metallurgy (Yang et al., 2014a). It
is also reported, however, that magnetic particles in coal ash may
be enriched in toxic elements such as Cr, Co, Ni, Cu, Pb, and Zn
and may therefore impart an environmental pollution risk (Vassilev
et al., 2004).
Several studies have documented considerable variation in the
morphology, chemistry, texture, and origin of magnetic particles
in fly ash (Sokol et al., 2002; Sharonova et al., 2003, 2013; Vassilev
et 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 in
the feed coal, the oxidizing or reducing conditions of the combustion chamber, the retention time, the viscosity of the ferriferous
silicate melt, the temperature conditions, and the effects of postcombustion 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 fly
ashes into seven groups according to their microstructure: sheet,
dendritic, granular, smooth, ferroplerospheres, porous, and molten
drop. Zhao et al. (2006) indicated that the formation mechanisms
for the different types of ferrospheres inside the combustion chamber were also different because of the complex eutectics of Febearing minerals and clays at very high temperatures during coal
combustion. Sharonova et al. (2013) observed that, with an increase
in Fe content and a decrease in viscosity of the melt, the morphologies of ferrospheres changed from porous (foam-like) to glass-like,
fine grained (dendritic and point), skeletal-dendritic, and coarse
grained (block-like). Ferro-oxides and aluminosilicate-bearing
ferro-oxides are important sources of the initiation layer that occurs
in deposits formed in coal-burning systems (Creelman et al., 2013).
Fe occurrences in coal are classified into two parts: (1) discrete mineral matter and (2) organically associated (in pore water and organic
matter) (Zhao et al., 2006; Valentim et al., 2016). Magnetic particles
in coal combustion products are products of decomposition and oxidation of discrete Fe-bearing minerals, most commonly pyrite,
31
siderite, ferroan dolomite, and ankerite, whereas organically associated 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 products
is ,170 million tonnes/yr. The present rate of use of fly ash is
,50%, mainly in civil construction and building materials, but
with a small contribution in agriculture, and the bulk is disposed
in 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 of
these two basins are non-coking, sub-bituminous to bituminous
coals with high ash yields (up to 50%), and they are mainly used
in thermal power plants. Recently, Odisha has been witnessing rapid
growth of industrialization, and there is a huge demand for coal by
thermal 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 coal
from both deposits have been reported to vary widely within and
between the seams. The general characteristics for the Talcher
coal deposit are 2–10% moisture, 18–47% ash, 22–32% volatile
matter, 24–44% fixed C, 8–45% mineral matter, ,1% S (mineral
matter–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 and
included forms in coal samples, mostly consisted of quartz, kaolin‐
ite, and illite, with minor to trace amounts of pyrite, siderite, calcite, dolomite, montmorillonite, feldspar, magnetite goethite,
limonite, and hematite.
The mineralogy, leaching characteristics, and pozzolanic properties of bulk fly ash from the Talcher area are well known (Kanungo
and Mohapatra, 2000; Praharaj et al., 2002; Das and Yudhbir,
2006; Mishra and Das, 2010; Nayak and Panda, 2010). Prakash
et al. (2001) recovered magnetic particles from the fly ash of
Talcher thermal power plants by beneficiation using a magnetic
coating technique. Bottom ash constitutes 15–35% of the coal combustion products from the thermal power plants. A literature search
revealed a lack of studies on the morphology, internal texture and
structure, fusion temperature determination, mineral chemistry,
and leaching characteristics of the magnetic concentrates in the
bottom ash of the thermal power plants in Talcher and Sambalpur
(Nayak and Panda, 2010; Nayak, 2015). The purpose of this investigation was to use optical microscopy, X-ray diffraction, scanning
electron microscopy (SEM), energy dispersive X-ray spectrometry
(EDS), heating microscopy, chemical analysis, and leaching studies
for comprehensive physicochemical and mineralogical characterization of the magnetic particles of bottom ashes from two thermal
power plants adopting different coal-burning technologies: pulverized coal combustion (PCC) boilers and circulating fluidized bed
combustion (CFBC) boilers. The results may be helpful in interpreting the potential uses and also the possible environmental risks of
the coal ashes.
32
Das / Coal Combustion and Gasification Products 8 (2016)
2. Materials and Methods
atmosphere to measure the deformation, softening, hemispherical,
and fluid or flow temperatures.
2.1. Sampling and size analysis
2.6. Chemical analysis and leaching experiments
The bottom ash samples were collected from two thermal power
plants: a plant in Talcher (NTPC plant; sample NT-BA) and a plant
in Sambalpur (Hindalco captive power plant; HR-BA) (Figure 1).
The NTPC plant adopts PCC technology and the Hindalco power
plant uses CFBC technology for power generation. The bottom ash
in the NTPC plant is collected in water-impounded hoppers and
sluiced into a storage lagoon. The Hindalco power plant uses a system of dry ash disposal to storage silos. Approximately 20 kg of bottom ash was collected from each power plant. The samples were
thoroughly mixed following the coning and quartering method
and wet sieved down to ,25 mm in size. The magnetic particles
were separated from each size fraction by a hand-held permanent
bar magnet.
2.2. SEM and EDS
The micromorphology of the magnetic particles was studied
using a scanning electron microscope (EVO 50-06, Carl Zeiss,
Jena, Germany). The samples were sprinkled on double-sided adhesive carbon tape fixed on an aluminum metal stud, carbon coated in
a vacuum sputter coater, and viewed under the scanning electron
microscope 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 drift
detectors (Bruker AXS Microanalysis, Berlin, Germany), combined
with a pulse processor for optimal performance. Chemical analyses
on 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 polyester
resin and hardener and poured into 30-mm-diameter molds.
The resulting blocks were polished by adopting standard polished
section preparation techniques. Final polishing of the blocks was
done using 1-mm diamond paste. The sections were studied using an
incident light polarizing microscope (DM2500P, Leica Microsystems,
Wetzlar, Germany). The coarse magnetic particles were observed
under a binocular microscope (Leica Microsystems).
2.4. X-ray diffraction studies
X-ray powder diffraction patterns were recorded for magnetic
and nonmagnetic fractions of two bottom ash samples by using
an XRD unit (PW 3710, Philips, Almelo, The Netherlands) with
CuKa radiation operating at a voltage of 40 kV and a current of
20 mA.
2.5. Ash fusion temperature determination
The magnetic and nonmagnetic concentrates of the two samples
were taken for ash fusion temperature estimation by using a heating
microscope (Hesse Instruments, Osterode am Harz, Germany). The
samples were heated in a furnace at ,850uC for 5 hours, cold pressed
to cylinders (,4 mm in length and 2 mm in diameter), and mounted
on the ceramic plate in the furnace chamber. The samples were then
heated at heating rate of 25uC/min up to 1569uC under an air
Approximately 0.30 g of each sample was taken in a Teflon
beaker, and 10 ml of an HF–H2SO4 mixture was added. The beakers
were heated on a hot plate and evaporated to dryness. The residue
was dissolved by gently heating in 25 ml of an acid mixture of concentrated HCl and distilled water in a 1:1 proportion, and then the
solution was transferred to a volumetric flask and the volume was
made up to 100 ml. The sample solutions were used for analyses
of major (Al2O3), minor (Ca, Mg, Na, K, and Mn), and trace (Cr, Co,
Ni, Cu, Pb, and Zn) metals by an atomic absorption spectrophotometer (AA6300, Shimadzu, Kyoto, Japan). The concentration of Fe in
the solutions was estimated by a titrimetric method. Silica in the
samples 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 fraction (leaching by distilled water), exchangeable fraction (leaching by
1 N ammonium acetate + acetic acid, pH 5.0), and carbonate– and
surface-oxide–bound fractions (leaching by 2 N HCl) (Palmer et al.,
1995). The sample-to-solution ratio in each case was 1:20, and the
samples were shaken for 24 hours. Leachates were collected, filtered,
and acidified with HNO3. The concentrations of minor and trace elements in each solution were determined using an AA6300 atomic
absorption spectrophotometer.
3. Results and Discussion
3.1. Size analysis and magnetic concentrate
The distributions (wt%) of magnetic and nonmagnetic fractions
in each size fraction of the two bottom ash samples are given in
Table 1. The proportion of magnetic particles in sample NT-BA is
high (25.7%) compared with that in HR-BA (4.03%). Table 1 also
indicates that the NT-BA sample contained 34.20%, 11.22%, and
3.04% magnetic particles compared with 49.49%, 36.26%, and
7.87% in HR-BA in the size fractions 0.125+0.063, 0.063+0.025,
and −0.025 mm, respectively, indicating that the magnetic particles
are finer in HR-BA.
3.2. Morphology
3.2.1. NT-BA
The 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 2a
and 2b). The finer magnetic particles (,0.50 mm) are predominantly
spherical (ferrospheres) (Figures 2c–e). Backscattered electron
images of the ferrospheres indicate widely varying morphological
features 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 have
smooth surfaces with tightly packed ultrafine crystallites of magnetite intermixed with a large proportion of glass components (Figure 2f).
Smooth ferrospheres are likely to be formed via mixing of a large
volume of aluminosilicate and small Fe-mineral melts in the
combustion zone. Granular ferrospheres have rough surfaces with
Das / Coal Combustion and Gasification Products 8 (2016)
33
Table 1
Size distribution and magnetite contents in different size fractions of bottom ash samples
NT-BA
Size (mm)
+2
−2+1
−1+0.5
−0.5+0.25
−0.25+0.125
−0.125+0.063
−0.063+0.025
−0.025
Total
wt%
3.23
6.10
25.04
36.13
17.40
6.84
5.26
100
HR-BA
Magnetite
1.31
1.44
5.33
11.24
5.95
0.77
0.16
25.7
% Magnetite
wt%
Magnetite
% Magnetite
40.60
23.58
21.29
31.10
34.22
11.22
3.04
2.93
9.16
19.61
47.96
13.91
1.96
0.91
3.56
100
0.13
0.31
0.73
1.29
0.97
0.33
0.28
4.03
1.42
1.58
1.52
9.27
49.99
36.26
7.87
tightly packed fine granular magnetite grains (Figure 2g). This texture is the result of magnetite crystal deposition on the ferrosphere
surface when the temperature decreases, and it is derived from the
fusion of inherent minerals in coal (Zhao et al., 2006). The dendritic
and skeletal patterns of magnetite crystals wetted by variable
amounts of glassy matrix are shown in Figures 2h and 2i, respectively. Dendritic ferrospheres consist of fine magnetite crystallites
occurring as laths, lamellae, and streaks. Conglutination after the
oxidation of iron oxide minerals has been identified as the main
cause for the formation of dendritic and granular ferrospheres
(Zhao et al., 2006). Dendritic magnetite crystals are mostly formed
from the FeO-SiO2-Al2O3 melt, and their characteristics are determined by this system (Sharonova et al., 2013).
Minute-to-coarse pores were observed on the surfaces of the ferrospheres, indicating escape of gas during solidification of the melt.
Coarse cubo-octahedral and flower-like magnetite grains within the
glass matrix exhibit blocky surface textures, as shown in Figures 2j
and 2k, respectively. Star-like magnetite grains are found within the
glass matrix, showing a spotted texture (Figure 2l) that seems to
have formed from minute needle-like magnetite crystals assembling
together (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 have
thin shells with tightly packed octahedral magnetite grains; coarse
voids are also at times distinct (Figure 2n). Thin-shelled magnetite
cenospheres are formed owing to surface deposition of magnetite
on gas bubbles (Sokol et al., 2002). The vesicular walls of the cenospheres suggest that the gas and melt generation have a common
focus of activity (Hubbard and McGill, 1984). Frequently, ferrospheres are also partially enveloped by glassy matrix and glass
spheroids (Figure 2o).
3.2.2. HR-BA
The morphology of the magnetic particles in HR-BA is shown in
binocular microscopy images (Figures 3a and 3b) and SEMbackscattered electron micrographs (Figures 3c–l). The coarse magnetic particles are predominantly elongated, angular, and irregular
(Figures 3a and 3b), whereas the finer particles (,0.5 mm) are
subspherical, oval, subhedral, and angular (Figures 3c and 3d). In
Figure 3e, the ferrospheres exhibit surface coating by a massive
mass of magnetite with minute pores; the magnetite crystallites
occur as tightly packed grains without revealing crystal outlines.
Fine laths of magnetite crystals form triangular patterns with tiny
intergranular pores in subrounded magnetic particles (Figure 3f).
Figure 3g shows the molten drop ferro-fragments with abundant
stout cubes and fine granules on the surface. These molten drop ferrospheres/ferro-fragments form due to transformation of Fe-bearing
phases at higher temperatures (.1000uC) under reducing conditions
(Zhao et al., 2006; Xue and Lu, 2008). Coagulation or segregation of
magnetite grains is observed in globular to angular magnetite particles (Figure 3h). The broken subspherical and angular particles have
thin shells of magnetite, with the inside being composed of quartz,
amorphous glassy masses, and dispersed speckled magnetite particles with large amounts of voids (Figures 3i and 3j). Solid ferrospheres have a thin crust of magnetite on the surface, whereas the
inner materials contain intimately admixed glassy phase and magnetite crystallites (Figure 3k). The magnetite grains are globular,
stout platy, and squarish shaped, and they are tightly packed to
form a massive crust. A few ferrospheres contain thinly dispersed
magnetite 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-BA
From the SEM images showing the external surface morphology
of the sprinkled particles, it is difficult to ascertain whether the particles are hollow or solid, and it is also difficult to identify the internal intergrowth patterns of the mineral phases. General views of
polished cross sections of the magnetic particles indicate that the
ferrospheres are predominantly solid spheres (Figure 4a). The coarse
irregular particles (ferro-fragments; Valentim et al., 2016) consist of
densely packed fine magnetite crystallites forming massive bodies
with radiating cracks, and coarse and fine voids, intimately admixed
with a glass matrix (Figure 4b). This type of discrete magnetic particle possibly represents siderite or pyrite relics or partially baked or
disintegrated forms of these minerals (Valentim et al., 2016). The
agglomerated masses in the bottom ash consist of ferrospheres, glass
particles (irregular, angular, and rounded), magnetite veinlets, and
residual quartz grains that are loosely packed with high proportions
of interstitial voids (Figures 4c and 4d). The agglomerated particles
have probably been formed owing to interparticle contact or rapid
cooling (Kutchko and Kim, 2006). Thick- and thin-shelled magnetite
cenospheres mostly have undulating inner walls and contain tightly
packed magnetite euhedra with interstitial glass masses (Figure 4e).
The ferrospheres also contain diversely oriented submicrometersized magnetite laths and streaks with small amounts of aluminosil‐
icate glassy matrix (Figure 4f). Skeletal and dendritic textures
shown by magnetite crystallites within the solid ferrospheres are
demonstrated in Figure 4g. These textures are indicative of
34
Das / Coal Combustion and Gasification Products 8 (2016)
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) Ferrosphere
showing dendrites of magnetite. (j) Ferrosphere with octahedral and polygonal magnetite grains. (k) Flower-like magnetite crystals in ferrosphere. 1 and 2 = energy
dispersive X-ray spectrometry (EDS) points. (l) Star-like magnetite grains within glass matrix. (m) Thin-walled broken magnetite cenosphere with cooling cracks. (n) Thinwalled magnetite cenosphere. (o) Ferrosphere showing cubo-octahedral magnetite grains coated with glass spheres and irregular glass masses. 3 and 4 = EDS points.
magnetite crystallization under supercooling conditions (Hower
et al., 1999; Vassilev et al., 2004, 2005). Vassilev and Vassileva
(1996) concluded that the dense and vesicular spheres originate
from fully and partly degassified melts of various minerals. The ferroplerospheres contain inclusions of solid glass spheres, vesicular
ferrospheres, and small residual quartz grains (Figure 4h). Martitization of magnetite is occasionally observed in the ferro-fragments
and in the ferrospheres (Figure 4i), in which hematite also occurs
as plates and crusts. Hematite is formed by oxidation of magnetite
during cooling under oxidation conditions (Vassilev and Vassileva,
1996). The subrounded vacuolated magnetic particles contain coarse
and fine voids, densely packed cubic magnetite grains with small
amounts of interstitial glass, and thin glass rims (Figure 4j). Coarse
detrital quartz grains are often encapsulated in the ferrospheres, as
depicted in Figure 4k. In NT-BA, native Fe is rare and occurs as tabular grains and as inclusions within the porous ferrospheres (Figures
4l and 4m). The tabular native Fe contains inclusions of magnetite,
glass particles, and is partially mantled by magnetite. Under
extremely reducing condition in the combustion chamber of the
boiler, small amounts of native iron may form from the high iron–
bearing melt at a temperature of .1200uC. The paucity of native
iron in the sample NT-BA indicates formation of the ferrosphere
under conditions of high partial oxygen pressure (Sokol et al.,
2002). Figure 4n depicts a mosaic texture due to intergrowth of
rounded to subrounded magnetite grains and interstitial glass
masses, possibly indicating simultaneous crystallization. A few
Das / Coal Combustion and Gasification Products 8 (2016)
35
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+125
mm, 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) Clot
texture due agglomeration of fine magnetite crystals. (i) Thin magnetite rim around the porous spheroids. (j) Angular particle showing thin magnetite laths and intergrowth
of porous glass and magnetite grains. (k) Solid ferrosphere with surface coating of octahedral magnetite. (l) Dispersed lacy and vermicular magnetite in glass matrix.
rounded grains show composite intergrowth between euhedral and
skeletal-dendritic crystallites of magnetite with a small amount of
interstitial glass (Figure 4o). Ferrospheres of different sizes are
shown in Figure 4p; some of these forms are pseudomorphs after
pyrite 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. The
ferrospheres contain agglomerated octahedral magnetite grains
near the core surrounded by submicrometer-sized magnetite needles
dispersed within aluminosilicate glass. The morphotype shown in
Figure 4r is probably under formation (transitional form) from partially disintegrated siderites (Valentim et al., 2016).
3.3.2. HR-BA
The internal structure and texture of the magnetic particles of in
HR-BA are shown in Figures 5a–r. Polished sections of the particles
under optical microscopy and SEM reveal that the particles are predominantly solid; coarse particles are irregular (ferro-fragments);
and finer particles are subrounded, subhedral, and angular (Figures
5a–c). Reflected light microscopy studies show that the magnetite
is variously altered to hematite. A few vacuolated massive magnetite
fragments contain tightly packed fine magnetite crystals having
transverse fractures (Figure 5d). Mostly the magnetic particles contain varying proportions of an amorphous glass phase intermixed
with magnetite (Figures 5a, b, e, f, i, l, m, and o–q). In the dense particles, magnetite occurs as massive bodies containing tightly packed
fine lamellae, laths, cubes, and granules of magnetite (Figures 5g,
5h, and 5j). Fine to coarse quartz grains are frequently observed
within the magnetic grains (Figures 5f and 5i). In some rounded particles, 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 detrital
quartz grains (Figure 5i). Figure 5l depicts a rounded grain having
agglomerated and dispersed magnetite grains, with voids near the
center surrounded by a thick zone of quartz. In rounded particles,
magnetite grains occur as fine lacy, vermicular, anhedral grains
and patches (Figure 5m). In some particles, hematite grains occur
as anastomizing veinlets and as massive masses/patches exhibiting
a honeycomb texture (Figure 5n). Morphotypes shown in Figures
5m and 5n are rather common in the magnetic concentrate of this
sample. In the rounded magnetic particles (Figure 5o), patchy or
graphic intergrowth grades to coarse massive masses occur near
the core and mantle, whereas the grain is rimmed by thin magnetite
36
Das / Coal Combustion and Gasification Products 8 (2016)
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) and
photomicrographs 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 quartz
grains. 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 censophere
with undulating inner rim. (f) Ferrosphere showing closely spaced and diversely oriented magnetite crystallites. (g) Ferrosphere with dendritic and skeletal magnetite
crystallites. (h) Ferroplerosphere showing inclusions of quartz, solid ferrosphere, and glass mass. (i) Ferrosphere showing tightly packed euhedral magnetite (gray) and
hematite (white) grains. (j) Ferrosphere showing interlocking euhedral magnetite grains wetted by aluminosilicate glass. 7–10 = EDS points. (k) Detrital quartz (dark gray) in
ferrosphere. 11 = EDS point. (l) Tabular iron metal contains inclusions and partially rimmed by magnetite. 12–15 = EDS points. (m) Solid ferrosphere showing (a) inclusions
of 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.
Das / Coal Combustion and Gasification Products 8 (2016)
37
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) and
photomicrographs 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 particle
showing intimately admixed hematite and porous glass mass, +250 mm. (c) Subrounded and angular magnetic particles, 250+125 mm. (d) Fractured subrounded magnetite
fragment. (e and f) Ferro-fragments showing intergrowth of fine hematite grains and massive mass with glass matrix. Note quartz grains in (f). (g) Magnetic particle
showing massive patches of magnetite wetted by aluminosilicate glass. (h) Thin amorphous glass rim separates the magnetite zones. 22 = energy dispersive X-ray
spectrometry (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 occlusions
within 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 grains
intermixed with amorphous silicates. (n) Porous rounded particle showing interconnecting hematite masses and veinlets. (o) Fine graphic intergrowth between magnetite
and glass grading to massive mass. (p and q) Magnetite possibly representing partially baked or disintegrated form of siderite or pyrite. (r) Ferrrosphere with magnetite
euhedra.
38
Das / Coal Combustion and Gasification Products 8 (2016)
however, cristobalite is more distinct in HR-BA (CFBC plant). Glass,
magnetite, cristobalite, and mullite are the common mineral phases
in ashes from PCC plants burning low-Ca bituminous coal, where
the boiler temperature mostly exceeds 1200uC (Vassilev et al.,
2005; Xue and Lu, 2008; Valentim et al., 2016). For CFBC ash
samples, where the optimum boiler temperature is ,950uC, these
minerals 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 surface
within the combustion chamber of the plant was more than
1000uC. Mollah et al. (1999) observed that cristobalite can form
from quartz, glass, and mullite at ,950uC.
3.5. Ash fusion temperature determination
Fig. 6. X-ray diffractograms of magnetic and nonmagnetic fractions: HR-BA
nonmagnetic (a), HR-BA magnetic (b), NT-BA magnetic (c), and NT-BA
nonmagnetic (d). Q 5 quartz; M 5 mullite; Mt 5 magnetite; H 5 hematite; Cb
5 cristobalite.
ribbons. Figures 5p and 5q depict the textures of ferro-fragments
that possibly represent partially baked or disintegrated forms of sid‐
erite or pyrite (Valentim et al., 2016). Ferrospheres of typical form
shown in Figure 5r are rare in this sample; it contains tightly packed
magnetite euhedra and possibly a pseudomoph of framboidal or
massive pyrite in coal. Thick- and thin-walled magnetite cenosphere
grains are occasionally noted; however, dendritic and skeletal
magnetite grains and native Fe were not noticed in the magnetic
particles.
In the ash fusion temperature studies, the shapes of the cylinders
of magnetic and nonmagnetic particles prepared from NT-BA
and HR-BA at different temperatures are shown in Figures 7a and
7b and 7c and 7d, respectively. Magnetic NT-BA (dominantly composed of magnetite) has higher softening (1300uC), deformation
(1429uC), spherical (1463uC), and melt ($1548uC) temperatures
compared with the respective temperatures (1200uC, 1377uC,
1414uC and 1436uC) for HR-BA (mainly constituted of magnetite
and hematite) (Figures 7a and 7c). These temperatures reflect different phases of the ash melting process. The nonmagnetic fractions of
NT-BA and HR-BA have deformation temperature of 1474uC and
1556uC, respectively, and both of the samples have a very high
fusion temperature (.1569uC) (Figures 7b and 7d). Mishra and
Mohanty (2005) and Chakravarty et al. (2015) recorded widely varying flow temperatures from 1380uC to 1500uC for Talcher and from
1510uC to .1600uC for Ib Valley coal ashes. The differences in the
parameters in the ash fusion experiments are explained by the mineral compositions of the studied magnetic and nonmagnetic ash
samples. Liu et al. (2013) have reported that the three major chemical components that affect the ash fusion temperature are CaO,
Fe2O3 content, and SiO2/Al2O3 ratio (S/A). Ash fusion temperature
decreases with an increase of CaO and Fe2O3 and an increase in
the S/A ratio up to 1.5. Both of the magnetic concentrates studied
contain very high percentages of Fe2O3 (magnetite and hematite)
and higher proportions of CaO compared with the nonmagnetic
fractions, resulting in lower ash fusion temperatures.
3.4. X-ray diffraction studies
3.6. SEM-EDS studies
X-ray diffraction data for of the magnetic and nonmagnetic fractions of the bottom ash samples are shown in Figures 6a–d. The
results indicate the following: (1) the magnetic fraction of NT-BA
is dominantly constituted of magnetite with minor amounts of
quartz (Figure 6c); (2) the nonmagnetic fraction of NT-BA contains
mullite, quartz, and a trace amount of cristobalite (?) (Figure 6d);
(3) the magnetic concentrate of HR-BA contains significant
amounts of hematite and magnetite and minor amounts of quartz
and cristobalite (Figure 6b); and (4) the nonmagnetic fraction of
HR-BA contains quartz as the major mineral phase and small to
moderate amounts of hematite, mullite, and cristobalite (Figure 6a).
Magnetite is the principal iron oxide mineral in the magnetic concentrates of both NT-BA and HR-BA, although HR-BA also contains
a significant amount of secondary hematite. Mullite and cristobalite
are noted in the nonmagnetic fractions of both bottom ash samples;
Semiquantitative chemical compositions of the mineral phases in
the ashes were determined by SEM-EDS (Table 2). Typical EDS patterns are shown in Figures 8a–l. The ranges of chemical compo‐
nents for the mineral phases in the magnetic concentrates NT-BA
and HR-BA are given in Table 3. The chemical data indicate that
the 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 low
to moderate concentrations of Mg (MgO, 0–6.24% and 0–7.19%,
respectively) and Al (Al2O3: 0–3.44% and 1.38–16.62%, respectively) and small amounts of Mn (MnO, 0–2.97% and 0–1.06%, respectively), 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% and
0–1.09%, respectively). During coal combustion, a high ferrous
melt originates from melting of ferrous carbonates and Fe silicates.
Das / Coal Combustion and Gasification Products 8 (2016)
39
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).
At high temperature, Mg2+and Mn2+ substitute for Fe2+ over a wide
temperature range, and at lower temperature (Fe, Mg, Mn)O Fe2O3
preserves its homogeneity. Ca can also isomorphically substitute
for ferrous Fe in the magnetite structure, but to a limited extent
and at high temperature (Sokol et al., 2002). Al3+ isomorphically
replaces Fe3+ in the crystal structure. High proportions of Mg (spot
no. 25) and Al (spot no. 30) in some of the analyzed magnetite
grains indicate the presence of magnesioferrite and hercynite components, respectively. Calcite, dolomite, and siderite (Mn-bearing)
minerals in the feed coal are the principal source of Ca, Mg, and
Mn in the magnetite grains in the ash samples. The EDS spectra of
the magnetite grains show strong Fe and O and weak Mn, Al, and
Si peaks (Figure 8a). The elemental spectra of Al-rich magnetite
and 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, mostly
with respect to Fe, Si, and Al. Small amounts of K, Ca, Ti, Mg, and
Mn are also detected in the glass phases. The glass phases can be
classified into two types: low-Fe and high-Fe aluminosilicate glass.
The range of compositional variation for the glass phases is given in
Table 3. In NT-BA and HR-BA, the maximum values of FeO in the
low-Fe glass phases are 7.18 and 4.14% and in the high-Fe glass
phases are 68.67% and 45.23%, respectively (Table 3). The low-Fe
glass contains higher proportions of Ti, compared to the high-Fe
glass of both of the samples. K is not detected in glasses of sample
HR-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 the
melt. Mn and Ca are also detected by EDS analyses in the glass
phases. Valentim et al. (2016) observed that Ca preferentially enters
into the glass structure of the fly ash. Mg is recorded in one glass
analysis (spot no. 27). Booher et al. (1994), Vassilev and Vassileva
(1996), and Sokol et al. (2002) also noted wide variation in compositions of the glass particles in fly ash samples. Energy dispersive
spectra of the glasses in the present study revealed strong silicon
X-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 analyzed
spots. Elemental spectra of the Ti-rich (TiO2, 7.41%; spot no. 29)
and Mg-rich (MgO, 21.82%; spot no. 26) aluminosilicate glass
phases 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 quartz
particle with an FeO content of 10.76%. The heterogeneity in chemical composition of the glass phases in the particles is due to rapid
changes in crystallization of the chemically inhomogeneous drops
formed 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 encapsulated
within the ferrospheres (Figure 8j). The elemental spectra of Fe-Mn
silicate (spot no. 14) and complex Fe-Ti-Mn oxide (spot no. 13) are
given in Figures 8k and 8l, respectively.
3.7. Chemical analysis and leaching studies
Major (Fe%, SiO2%, Al2O3%), minor, and trace element data for the
magnetic and nonmagnetic fractions of NT-BA and HR-BA are given
40
Das / Coal Combustion and Gasification Products 8 (2016)
Table 2
Semiquantitative analysis (wt%) of mineral phases (by energy dispersive X-ray spectrometry) in ash particles1
Spot
MgO
Al2O3
SiO2
K2O
CaO
TiO2
MnO
FeO(t)
Total
Mineral
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
0.00
2.19
1.31
0.00
0.00
0.00
1.32
95.18
100.00
Mag.
0.00
24.32
59.17
2.09
1.12
3.00
3.18
7.12
100.00
Glass
0.00
4.31
91.13
0.77
0.00
0.00
0.00
3.78
100.00
Glass
0.00
31.93
41.96
0.44
0.52
1.16
0.00
24.00
100.00
Glass
0.00
14.71
25.86
0.51
1.34
0.97
0.00
56.61
100.00
Glass
0.00
2.50
1.05
0.00
0.00
0.00
0.00
96.45
100.00
Mag.
0.00
3.44
0.00
0.00
0.00
0.00
1.15
95.41
100.00
Mag.
0.00
25.19
38.19
0.00
4.56
1.87
0.61
29.56
100.00
Glass
0.00
0.00
89.24
0.00
0.00
0.00
0.00
10.76
100.00
Fe-qtz
0.00
10.09
85.49
1.14
0.00
0.00
0.00
3.28
100.00
Glass
6.24
0.00
0.00
0.00
2.07
0.00
1.65
90.03
100.00
Mag.
0.00
0.84
0.70
0.00
0.00
0.85
0.00
97.60
100.00
Mag.
0.00
5.42
5.42
0.00
0.23
35.92
14.26
38.76
100.00
Fe-TiMn-ox.
0.00
1.45
20.80
0.00
0.22
3.89
27.10
46.54
100.00
Fe-Mnsilicate
0.00
0.00
0.84
0.00
0.00
0.00
2.97
96.19
100.00
Mag.
Spot
MgO
Al2O3
SiO2
K2O
CaO
TiO2
MnO
FeO(t)
Total
Mineral
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
0.00
0.00
0.202
0.00
0.00
0.00
0.00
99.802
100.00
Native
Fe
0.00
1.63
1.64
0.00
0.00
0.00
0.00
96.73
100.00
Mag.
0.00
12.97
25.40
0.00
1.78
1.12
0.00
58.73
100.00
Glass
0.00
3.95
25.83
0.35
1.20
0.00
0.00
68.67
100.00
Glass
0.00
7.33
0.60
0.00
0.00
0.00
0.00
92.07
100.00
Mag.
1.89
2.68
0.00
0.00
0.00
0.00
1.05
94.38
100
Mag.
0.00
2.56
2.59
0.00
0.00
1.05
1.06
92.73
100
Mag.
0.00
31.27
27.24
0.00
0.00
0.81
0.00
40.49
100
Glass
0.00
37.68
51.34
0.00
0.00
6.84
0.00
4.14
100
Glass
7.19
1.38
0.94
0.00
1.09
0.00
0.93
88.47
100.00
Mag.
21.82
13.25
36.59
0.00
0.33
0.44
0.70
26.87
100
Glass
1.59
2.94
48.96
0.00
0.00
0.31
0.97
45.23
100
Glass
2.00
3.79
1.95
0.00
0.00
0.69
0.74
90.83
100
Mag.
0.00
37.13
52.35
0.00
0.34
7.41
0.00
2.77
100
Glass
1.85
16.62
3.12
0.00
0.43
1.58
0.00
76.41
100
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.
2
As Fe and Si metal.
in Table 4. The magnetic concentrates are strongly enriched in Fe (NTBA, ,10 times; HR-BA, ,23 times) compared with nonmagnetic
fractions, indicating dominant Fe partitioning to iron oxide minerals
(magnetite and hematite) and a small amount to the associated Ferich glass phase. The magnetic fractions of NT-BA and HR-BA contain
15.65% and 14.74% SiO2 and 7.67% and 13.38% Al2O3, respectively.
The analyzed magnetic and nonmagnetic samples are mainly composed of SiO2, Al2O3, and iron oxides (Table 4). In the feed coal, Al
occurs mainly in the clay minerals (kaolinite), whereas silica is
also found in clay minerals and quartz. The relatively low levels of
K (maximum, 1933 ppm [NT-BA-NM]; Table 4) and high percentages of Al2O3 in the samples suggest that the dominant parent
clay mineral was kaolinite rather than illite. The trace element distribution patterns in the magnetic and nonmagnetic fractions of the
studied samples show distinctly different results. The magnetic concentrate 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 nonmagnetic fraction. The magnetic concentrate of NT-BA has strong
and moderate enrichment of Mn and Co, respectively, and distinct
depletion of Cr, Ni, Cu, Zn, Pb, and K. Ca is strongly and Mg is moderately enriched in the magnetic fractions of both samples. High Mn,
Ca, and Mg concentrations in the magnetic concentrates suggests
that Fe2+-Mn2+, Fe2+-Ca2+, and Fe2+-Mg2+ diadochy are prominent
in the magnetite crystal structure; Cr3+ replaces Fe3+ in the magnetite, as well as Al3+ in the glass phase. Hower et al. (1999) noted distinct Cr enrichment and depletion in Zn, Cu, and Mn in the magnetic
fractions 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 random
element 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 occurrence
and the leaching behavior of the magnetic and nonmagnetic concentrates of NT-BA and HR-BA. The data also indicate that (1) small
amounts Mn, Na, Zn, Ca, Mg, and K are leached by water, suggesting
an occurrence as adsorbed ions on minerals and as water-soluble
salts; toxic elements, such as Cr, Co, Ni, Cu, and Pb, are not leachable
by water, corroborating the observations of Praharaj et al. (2002); (2)
metal dissolution is enhanced under acidic conditions provided by
acetic acid, leading to the leaching of variable amounts of Ni, Mn,
Cu, Zn, Ca, Mg, Na, and K occurring as ion-exchangeable components on minerals and chars, whereas Cr, Co, and Pb were not
leached; and (3) the sharp increase in element leaching under the
more extreme leaching conditions imposed by 2 N HCl is attributed
to the HCl solution extracting these elements bound to amorphous
to poorly crystalline oxides, carbonates, and Fe-Mn oxides. The
leaching studies for the magnetic and nonmagnetic fractions of
NT-BA and HR-BA from these two power plants also indicate identical leaching patterns for Cr, Ni, Mn, Pb, Ca, Na, and K (Table 4).
4. Conclusions
Magnetite particles have been successfully isolated from bottom
ash samples of the two coal-fired power plants that adopt different
combustion technologies: PCC boilers (NT-BA) and CFBC boilers
(HR-BA). The morphology, texture, and composition of the magnetic
particles in such coal combustion wastes depend on several factors,
Das / Coal Combustion and Gasification Products 8 (2016)
41
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.
most importantly the mineral matter in the feed coal, the combustion conditions, and the boiler types. The magnetic concentrates
from the two power plants show significant differences with respect
to morphology, texture, mineralogical composition, ash fusion
temperature, element concentrations, and leaching characteristics.
The magnetic particles of NT-BA are dominantly spherical (ferrospheres), and seven morphotypes have been recognized. Magnetite
is the dominant iron oxide mineral and exhibits different textures
and structures that are typical of the ferrospheres in coal
combustion wastes generated by PCC boilers burning low-Ca bituminous coal. The magnetic particles of HR-BA are subspherical
and angular and are constituted of hematite and magnetite and an
associated glass mass. The iron oxide minerals occur as massive
bodies, laths, cubes, and as granular, lacy, and vermicular grains
and anastomizing veinlets and patches. Partially baked or disintegrated forms of pyrite or siderite and a transitional form of pyrite
are recognized in the two ash samples. The coarse dense ferrospheres
and ferro-fragments, containing large amounts of iron oxide
42
Das / Coal Combustion and Gasification Products 8 (2016)
Table 3
Range of different element oxides (wt%) in magnetite and low- and high-Fe aluminosilicate glass phases
Magnetite
NT-BA
MgO
Al2O3
SiO2
K2O
CaO
TiO2
MnO
FeO(t)
HR-BA
Low-Fe aluminosilicate glass
High-Fe aluminosilicate glass
NT-BA
NT-BA
HR-BA
HR-BA
Min.
Max.
Min.
Max.
Min.
Max.
Min.
Max.
Min.
Max.
Min.
Max.
0.00
0.00
0.00
0.00
0.00
0.00
0.00
90.03
6.24
3.44
1.64
0.00
2.07
1.05
2.97
97.60
0.00
1.38
0.94
0.00
0.00
0.00
0.00
76.41
7.19
16.62
3.12
0.00
1.09
1.58
1.06
92.73
0.00
4.31
59.17
0.77
0.00
0.00
0.00
3.28
0.00
24.32
91.13
2.09
1.12
3.00
3.18
7.18
0.00
37.13
51.34
0.00
0.00
6.84
0.00
2.77
0.00
37.68
52.35
0.00
0.34
7.41
0.00
4.14
0.00
3.95
25.40
0.00
0.52
0.00
0.00
24.00
0.00
31.93
41.96
0.51
4.56
1.87
0.61
68.67
0.00
2.94
27.24
0.00
0.00
0.31
0.00
40.49
1.59
31.27
48.96
0.00
0.00
0.81
0.97
45.23
Note: Min. 5 minimum; Max. 5 maximum; FeO(t) 5 total iron as FeO.
elements—Cr, Ni, Mn, Pb, Na, and K—in the magnetic and nonmagnetic fractions of the bottom ash samples show identical dissolution
behavior patterns. In addition, the potentially toxic elements (Cr,
Co, Ni, and Pb) in the magnetic and nonmagnetic samples were
not leached by water.
The magnetic concentrates from the coal combustion products
have potential for use in mineral processing and metallurgical industries. The percentage of magnetic concentrates in the fly ash generated in the thermal power plants in eastern India ranges between
3% and 11% (Sarkar et al., 2011; Valentim et al., 2016). The present
study has indicated that the bottom ash also contains considerable
amounts of magnetic materials (4–26%). Thus, even estimating the
magnetic yield of particles at 1%, the annual production of such
minerals, were probably derived from decomposition and oxidation
of pyrite, siderite, and ankerite in the feed coal. With rising temperature, Fe-bearing minerals and clay minerals were melted and mixed
to form Fe-bearing particles (ferrospheres and ferro-fragments) containing 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 as
isomorphous substitutions for Fe2+ and Fe3+ in the crystal lattice.
The associated glass component has been classified into low- and
high-Fe glass phases.
The magnetic and nonmagnetic samples are predominantly made
up of SiO2, Al2O3, and Fe that are derived from the mineral matter
of the feed coal. Leaching studies indicate that the principal trace
Table 4
Major elements (wt%), trace element concentrations (ppm), and trace element–leaching patterns by sequential leaching with different solvents of the magnetic (M) and
nonmagnetic (NM) fractions of NT-BA and HR-BA1
ppm
Sample
Cr
wt%
Co
Ni
Mn
NT-BA-M
58
53
57
2272
NT-BA-NM
87
23
89
403
HR-BA-M
205
34
26
1232
HR-BA-NM
164
31
57
192
Leaching by water (ppm)
NT-BA-M
ND
ND
ND
12 (1)
NT-BA-NM
ND
ND
ND
22 (6)
HR-BA-M
ND
ND
ND
14 (1)
HR-BA-NM
ND
ND
ND
15 (8)
Leaching by 1 M ammonium acetate + acetic acid (ppm)
NT-BA-M
ND
ND
2 (4)
105 (5)
NT-BA-NM
ND
ND
3 (3)
29 (7)
HR-BA-M
ND
ND
1 (4)
30 (3)
HR-BA-NM
ND
ND
ND
16 (8)
Leaching by 2 N HCl (ppm)
NT-BA-M
8 (14)
3 (6)
9 (16)
168 (7)
NT-BA-NM
4 (5)
2 (9)
5 (6)
118 (29)
HR-BA-M
56 (27)
6 (18)
12 (23)
110 (9)
HR-BA-NM
4 (2)
2 (6)
2 (4)
31 (16)
Total % of elements leached in the samples
NT-BA-M
14
6
20
13
NT-BA-NM
5
9
9
42
HR-BA-M
27
18
50
13
HR-BA-NM
2
6
4
30
Note: ND 5 not detected.
1
Values in parentheses represent percentages leached.
Cu
Zn
Pb
Na
K
Ca
Mg
Fe
SiO2
Al2O3
52
94
219
44
93
195
358
106
18
40
28
35
890
853
1000
1077
735
1933
800
1800
10,200
4200
22,400
9400
660
570
1240
954
50.26
5.03
45.83
2.12
15.65
61.57
14.74
66.31
7.67
27.31
13.38
25.94
ND
2 (2)
ND
ND
ND
9 (5)
ND
8 (8)
ND
ND
ND
ND
18 (2)
8 (1)
98 (10)
27 (3)
17 (2)
9 (1)
28 (4)
20 (1)
3 (6)
13 (14)
7 (3)
3 (7)
18
13
10
27
(19)
(7)
(3)
(25)
ND
ND
ND
ND
56
34
51
29
36
18
24
12
20 (39)
13 (14)
20 (9)
4 (9)
25
17
28
16
(27)
(9)
(7)
(15)
6 (33)
9 (23)
10 (36)
5 (14)
45
30
12
16
46
21
10
48
33
23
36
14
(6)
(4)
(5)
(3)
251 (28)
82 (10)
153 (15)
89 (8)
36
15
30
14
(5)
(1)
(3)
(1)
122 (17)
80 (4)
38 (5)
36 (2)
24
6
12
4
602
65
800
54
(6)
(2)
(4)
(1)
28
15
11
14
(4)
(3)
(1)
(2)
1212
331
407
267
(12)
(8)
(2)
(3)
31
28
16
17
(5)
(5)
(1)
(2)
220 (2)
580 (14)
412 (2)
822 (9)
46
40
19
19
(7)
(7)
(2)
(2)
20
24
8
13
16
15
4
6
Das / Coal Combustion and Gasification Products 8 (2016)
material from ,100 million tonnes of coal combustion products in
Odisha would exceed 1 million tonnes. Microscopic studies of the
samples show that the magnetite crystals and masses are commonly
intimately intergrown with aluminosilicate glass. Liberation of magnetite from the gangues would be very difficult and challenging.
Hence, an integrated approach consisting of physical beneficiation
involving fine grinding, separation in a magnetic separator, and
chemical treatment of the magnetic fractions would be needed to
obtain pure magnetite concentrates for such applications.
Acknowledgments
I am grateful to Prof. B.K. Mishra, Director, Council of Scientific
and Industrial Research–Institute of Minerals and Materials Technology (CSIR‐IMMT) for encouragement and to Dr. B.B. Nayak
(CSIR‐IMMT) for help in chemical analyses of the samples. I am
indebted to Prof. J.C. Hower (University of Kentucky, Lexington)
and anonymous reviewers who provided valuable suggestions.
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