Download MINERALOGY OPTICAL STUDIES Microscopic

MINERALOGY
OPTICAL STUDIES
Microscopic
studies
have
helped
in
identifying
many
minerals. Many are hitherto reported all over the world, some
minerals are observed for the first time in fly ashes.
They are
identified by microscopic and X-ray data. Few are confirmed solely
based on X-ray analysis.
The physical and mineralogical characters of the produced fly
ash is greatly dependent on the inorganic constituents of the feed
coal, combustion character inside the furnace and rate of cooling.
The mineralogy of fly ash can also play an important role in its
behaviour and utilization as a replacement material of cement.
In this chapter a detailed account on the mineralogy of the fly
ashes of Visakhapatnam and Kothagudem are described. Individual
crystalline phases observed in the ash samples provided some
apparent
relationship
between
chemical
composition
and
crystalline phase formation. The major mineral phases viz., quartz,
mullite,
orthoclase,
gypsum,
pyrite,
arsenopyrite,
sillimanite,
ettringite, tridymite, lepidolite and spinels etc., and also minor
76
recrystallized minerals belonging to zeolite group are identified
through
microscopic
and
X-ray
studies.
High
zirconium
concentration in the ash samples leads to identify stable oxide of
zirconium-baddeleyite for the first time. All major mineral phases
are confirmed optically as well as through X-ray data. The
mineralogical composition (Vol.%) including the total major and
minor crystalline components as well as glass content is given in
Table No.4.1.
The
minerals
orthoclase,
tridymite,
mullite,
gypsum,
heamatite, magnetitite, kaolinite are the original minerals present
or phases developed from the physico-chemical transformation of
the inorganic matter associated with coal during the combustion.
Quartz is observed in bulk samples and size fractions of both
Visakhapatnam and Kothagudem fly ashes.
Quartz reflections
have shown minor intensity variations in their X-ray patterns. This
may be due to varying quartz content in the source coal. Presence
of quartz is responsible for abrasive natureof ash of Indian coals
(Mukherjee et. al., 1983). The other abrasive minerals, iron pyrites,
siderite and zircon etc., are less in amount when compared to low –
quartz.
77
Mullite is observed in all the size fractions and bulk samples
of the studied ashes.
Mullite and sillimanite may appear as an
intermediate phase and may be derived from kaolinite (Tripathy
and Sahu, 1995). According to Diamond (1984), mullite was not
found in ashes whose A12O3 + SiO2 contents were less than 60%
and whose A12O3 was less than about 17%. In the studied fly
ashes, A12O3 + SiO2 percentage ranges between 89% and 92% with
28% to 35% A12O3. This high A12O3content may be responsible for
abundance of mullite in the studied fly ashes.
Table: 4.1 MEAN MINERAL/GLASS PHASES OF
VISAKHAPATNAM AND KOTHAGUDEM FLY ASHES
Mineral Name
Visakhapatnam Fly
Ash (%)
Kothagudem Fly
Ash (%)
53.86
54.29
Quartz
7.34
6.92
Mullite
10.51
9.64
Magnetite
6.12
5.59
Feldspar
1.61
1.58
Heamatite
1.25
1.22
Pyrite
0.61
0.52
Spinels
3.12
3.25
Glass Content
78
Sillumanite
1.25
2.61
Gypsum
1.01
1.03
Kaolinite
6.61
7.25
Zeolites (Wairakite,
Laumontite
Chabazite,
Edingtonite etc)
5.15
4.75
Baddeleyite
0.81
0.62
Graphite
0.12
0.11
Others
0.61
0.62
100.00
100.00
With the moderate CaO content in the studied fly ashes, it is
unusual to observe anhydrite (CaSO4). The availability of CaSO4 in
these ashes promotes the self pozzolanic reactions such as
ettringite
(Ca6A12(SO4)(OH)12
25
H2O)
formation
which
is
responsible for the stored ash to set-up, upon contact with water.
Mineral ettringite was identified in both the studied fly ashes
through X-ray studies.
Moderate X-ray intensity reflections of Sodalite are observed
in the coarser fraction (0.090mm) of Kothagudem fly ash sample.
79
Sodalite is generally observed as a major devitrification product
along with heamatile, pyroxene and plagioclase phases (Diamond,
1984). The mechanism of sodalite phase formation may have the
reaction of Na and Ca sulphates with the aluminosilicate glass. Xray study of magnetic separated fraction of studied fly ashes
reveals presence of heamatite and iron spinels (ferrite spinel).
Certain trace elements that were partitioned into pyrite in the coal
may also remain partitioned in the ferrite spinel phase in fly ash
(Lauf, 1981).
Opaque spheres of magnetite, translucent brownish black
spheres with finer inclusions of iron oxides, internal recrystallites
of mullite and transparent spheres of low relief alkaline silicates are
commonly observed during optical studies of both Visakhapatnam
and Kothagudem fly ashes (Plates). X-ray patterns of bottom ashes
of both the studied areas indicate that graphite is present with
moderate intensities. Graphite peaks are partially attributed to the
presence of unburnt carbon in the fly ash (Henry and Knapp,
1980).
80
X-RAY STUDIES
Fly ash samples of both Visakhapatnam and Kothagudem
contain a number of new minerals which occur as minute
segregations.
through
x-ray
These mineral occurrences are identified only
studies.
The
occurrence
of
such
mineral
assemblages from the studied fly ashes are reported for the first
time.
These zeolites and other minerals may have recrystallised
minerals due to temperature conditions at thermal power station.
Gismondine:
Rare zeolite, gismondine (CaA12Si2O84H2O) is identified from
fly ash samples of both the studied areas.
X-ray data (Table
No.4.2) is similar to that of JCPDS file number is 20-452. X-ray
patten reveals that the mineral crystallizes in monoclinic system
with possible space group P21/C.
The unit cell parameters were
determined as ao=10.021Ao(VF) and 10.028 Ao (KF); bo = 10.562 Ao
(VF) and 10.565 Ao (KF); Co= 9.828 Ao (VF) and 9.841Ao
(KF);V=1039.15Ao3(VF) and 1041.65Ao3(KF);
β =92046’(VF) and
92.465’(KF) with Z=4. The density calculated from the x-ray data is
2.222 (X) (VF) and 2.226 (X) (KF).
The strongest intensities are
from (300), (121), (202), (004) and (015) hkl values.
81
TABLE 4.2 X-RAY DATA OF GISMONDINI: (CaA12Si2O84H2O)
hkl
VISAKHAPATNAM FLY ASH
KOTHAGUDEM FLY
ASH
dAo
1/1o
dAo
1/1o
020
5.2813
5
5.2825
4
121
4.2695
2
4.2697
26
202
3.4308
24
***
***
300
3.3406
100
3.3428
100
311
3.0504
14
3.0718
8
032
2.8731
8
***
***
132
2.7423
22
2.7403
20
213
2.6971
12
2.6893
10
123
2.6558
6
***
***
004
2.4571
25
2.4603
15
221
2.1994
9
2.3963
11
142
2.2834
5
2.2734
6
214
2.1399
6
***
***
224
2.0134
12
2.0155
10
333
1.9798
8
1.9815
11
82
015
1.9329
41
1.9375
9
***
1.7819
9
***
***
***
1.6409
6
***
***
CRYSTAL SYSTEM
MONOCLINIC
SPACE GROUP
P21/C
Z
4
ao
=
10.021 AO
ao
=
10.028 AO
bo
=
10.562 AO
bo
=
10.565 AO
co
=
9.828 AO
co
=
9.841 AO
V
=
1039.15 AO
V
=
1041.65 AO
β
=
92o46’
β
=
92o465’
Dx
=
2.222 (X)
Dx
=
2.226 (X)
83
Mordenite :
Mordenite, an alkali zeolite ((Ca, Na2, K2) Al2Si10O247H2O) is
identified in the studied fly ashes. Its x-ray data (Table No. 4.3) is
comparable with that of JCPDS file No. 6-239. X-ray analysis of
mordenite discloses that the mineral crystallizes in orthorphombic
system with possible space group C*/C. The strongest diffractions
are at (202), (200), (060) and (402) hkl values.
The unit cell
parameters were calculated as ao = 18.202 Ao (VF) and 18.222 Ao
(KF); bo = 20.364 Ao (VF) and 20.393 Ao (KF), Co = 7.525 Ao (VF) and
7.518 Ao (KF); V=2789.26 Ao3 (VF) and 2793.70 Ao3 (KF) with Z=4.
Density calculated from x-ray data is 2.122 (X) (VF) and 2.127 (X)
(KF).
TABLE 4.3 X-RAY DATA OF MORDENITE:
((Ca,Na2, K2), Al2,Si10O247H2O)
hkl
VISAKHAPATNAM FLY ASH
KOTHAGUDEM FLY
ASH
dAo
1/1o
dAo
1/1o
200
9.1013
71
9.1111
70
002
3.7624
16
3.7591
15
202
3.4879
101
3.4910
100
060
3.3947
46
3.3989
18
84
402
2.8982
39
2.8953
9
152
2.7423
8
***
***
***
2.4687
8
2.4636
17
***
2.0134
12
2,0154
8
***
1.9376
13
1.9374
9
***
1.8824
4
1.8869
6
***
1.7218
6
***
***
***
1.6628
7
1.6665
8
***
1.5927
8
1.5918
5
CRYSTAL SYSTEM
ORTHORHOMBIC
SPACE GROUP
C/C
Z
4
ao
=
18.202 AO
ao
=
18.2222 AO
bo
=
20.364 AO
bo
=
20.393 AO
co
=
7.525 AO
co
=
7.518 AO
V
=
2789.26 AO3
V
=
2793.70 AO3
Dx
=
2.122 (X)
Dx
=
2.127 (X)
85
Leifite:
This mineral leifite (Na2AISi5O)2 (F,OH) 1-2 is identified in
both the studied fly ashes through x-ray analysis. X-ray data of
this mineral is given in Table No.4.4 and the data is similar to that
of JCPDS file number is 18-710. X-ray pattern indicates that the
mineral crystallizes in hexagonal system. The strongest diffractions
are at (301), (211), (101) and (220) hkl values.
The unit cell
parameters are determined as ao= 14.295 Ao (VF) and 14.296 Ao
(KF); Co = 4.815 Ao (VF) and 4.815 Ao (KF); V=852.08 Ao (VF) and
852.20 Ao (KF) with Z=3. The density calculated from x-ray data is
2.47 (X) (VF) and 2.51 (X) (KF).
86
TABLE 4.4 X-RAY DATA OF LEIFITE:
(Na2AlSi5O)2 (F,OH)1-2
hkl
VISAKHAPATNAM FLY ASH
KOTHAGUDEM FLY
ASH
dAo
1/1o
dAo
1/1o
200
6.1899
19
6.1909
11
101
4.4950
26
4.5015
21
220
3.5813
16
3.5716
28
211
3.3406
75
3.3444
81
301
3.1314
100
3.1298
100
400
3.0753
5
3.0787
10
311
2.7864
9
***
***
401
2.6014
8
2.6013
8
102
2.3963
9
2.3903
9
411
2.3666
7
2.3574
12
510
2.2134
15
***
***
212
2.1399
6
2.1475
8
***
1.9086
7
***
***
***
1.6409
6
1.6415
5
87
CRYSTAL SYSTEM
ORTHORHOMBIC
SPACE GROUP
C/C
Z
4
ao
=
14.295 AO
ao
=
14.296 AO
co
=
4.815AO
co
=
4.815 AO
V
=
852.08 AO
V
=
852.20 AO
Dx
=
2.47 (X)
Dx
=
2.51 (X)
Palygorskite
Magnesium aluminium hydroxy silicate palygorskite ((Mg.Al)5
(SiAl)5 O2 (OH)2 8 H2O)) is identified by x-ray data. The data (Table
No.4.5) is similar to that of JCPDS file No.29-855.
The unit cell
parameters are determined as ao= 12.720 Ao (VF) and 12.714 Ao
(KF);bo = 17.866 Ao (VF) and 17.835 Ao (KF); Co = 5.215 Ao (VF) and
5.213 Ao (KF); V=1185.14 Ao (VF) and 1182.07 Ao3 (KF) with Z=2.
X-ray data reveals that the mineral crystallizes in orthorhombic
system.
The strongest diffractions are at (231), (110), (200) and
(121) hkl values.
88
Apart from the commonly known assemblages in fly ashes,
four new zeolite minerals viz., wairakite, laumontite, chabazite and
edingtonite besides zirconium mineral, baddelecyite were identified
for the first time.
TABLE 4.5 X-RAY DATA OF PALYGORSKITE:
((Mg, Al)5 (SiAl)8 O2 (OH)2 , 8H2O))
hkl
VISAKHAPATNAM FLY
KOTHAGUDEM FLY
ASH
ASH
dAo
1/1o
dAo
1/1o
110
10.3432
98
10.3398
100
200
6.3602
22
63.569
19
130
5.4025
15
***
***
040
4.4667
14
4.4588
12
121
4.2623
19
4.2771
32
231
3.3454
85
3.3570
98
331
2.8982
10
***
***
89
251
2.6708
12
2.6681
15
112
2.5347
7
2.5308
12
CRYSTAL SYSTEM
ORTHORHOMBIC
Z
2
ao
=
12.720 AO
ao
=
12.714 AO
bo
=
17.866 Ao
bo
=
17.835 Ao
co
=
5.215AO
co
=
5.213AO
V
=
1185.14 AO
V
=
1182.07 AO
Dx
=
2.34 (X)
Dx
=
2.36 (X)
Wairakite:
Wairakite (Ca A12Si4O122H2O) is the calcium analogue of
analcime (Steiner, 1955).
sample
displays
an
This section of Visakhapatnam bulk
icositetrahedra
like
grain
of
wairakite.
Characteristic feature of the mineral is the presence of fine,
discontinuous lamellar twinning with two sets at right angles to
90
each other. The mineral is colourless ion plane polarised light and
first order grey under crossed nicols. It is optically biaxial with 2 Vo
=
82o. Refractive indices are α = 1.496 χ = 1.501, with birefringence
0.005.
X-ray data of wairakite was given in Table No.4.6 and the
data similar to that of JCPDS file No. is 7-326.
X-ray pattern
indicate that the mineral crystallizes in monoclinic system.
The
unit cell parameters were determined as ao = 13.671 Ao, bo =
13.661 Ao, co = 13.541 Ao; V=2528.85 Ao3 and β = 90o.4 with Z = 8.
The density calculated from the x-ray data is 2.274 (X).
strongest diffractions are at (400), (211) and (431) hkl values.
TABLE 4.6 X-RAY DATA OF WAIRAKITE:
((Ca, Al2Si4 O12 H2O))
hkl
VISAKHAPATNAM FLY ASH
dAo
1/1o
211
5.5712
75
400
3.4178
61
400
3.3888
100
420
3.0753
4
91
The
332
2.8982
8
422
2.7864
8
431
2.6971
40
600
2.2834
4
631
1.9857
7
543
1.9376
6
CRYSTAL SYSTEM
MONOCLINIC
SPACE GROUP
1’/A
Z
8
ao
=
13.671 AO
bo
=
37.661 Ao
co
=
13.541AO
V
=
2528.85 AO3
β
=
90O.4’
Dx
=
2.274 (X)
92
Laumontite:
Calcium aluminium, zeolite, laumontite (CaA12Si4O124H2O)
occurs in fly ashes expressing acicular habit and uneven fracture
with inclusions. It is colourless in plane polarized light and shows
first order pink, blue, yellow etc., colours under crossed nicols.
Optically it is biaxial negative, length-slow with 2Vα=32o. Refractive
indices are α = 1.507; β = 1.514 and χ = 1.516 with birefringence
0.009.
X-ray data of laumontite was given in Table No.4.7 and the
data comparable with that of JCPDS file No.26-1047. X-ray data
indicates that the mineral may be crystallized under monoclinic
system with probable space group Cm.
The crystallographic
constituents determined from the x-ray data for both the studied fly
ashes is as follows.
Ao = 14.710 Ao(VF) and 14.795 Ao (KF); bo=
14.072 Ao (VF) and 14.059 Ao (KF); Co = 7.525 Ao (VF) and 7.529 Ao
(KF), and cell volumes V=1448.78 Ao3 3 (VF) and 1456.58 Ao (KF)
and β = 111o55’ (VF) and 111o-56’ (KF). Calculated density is 2.310
(X) (VF) and 2.311 (X) (KF).
Strongest diffractions are at (130),
(200), (002), (040), (511) and (241) hkl values.
93
TABLE 4.7 X-RAY DATA OF LAUMONTITE:
((CaAl2Si4O124H2O))
hkl
VISAKHAPATNAM FLY
KOTHAGUDEM FLY
ASH
ASH
dAo
1/1o
dAo
1/1o
200
10.3432
100
10.3398
100
130
6.3602
20
63.569
18
002
5.4025
12
***
***
131
4.4667
13
4.4588
10
040
4.2623
18
4.2771
31
511
3.3454
80
3.3570
100
241
2.8982
8
***
***
132
2.6708
11
2.6681
13
403
2.5347
5
2.5308
9
622
2.2184
14
2.2212
12
620
2.1546
8
2.1546
10
332
2.0781
4
***
***
533
***
***
1.9879
7
94
CRYSTAL SYSTEM
MONOCLINIC
SPACE GROUP
Cm
Z
4
ao
=
14.710 AO
ao
=
14.795 AO
bo
=
14.072 Ao
bo
=
14.059 Ao
co
=
7.525AO
co
=
7.529AO
V
=
1448.78 AO3
V
=
1456.58 AO3
β
=
111o55’
β
=
111o56’
Dx
=
2.310 (X)
Dx
=
2.311 (X)
Chabazite:
A section of Kothagudem fly ash shows a fibrous shaped
chabazite ((Ca2Al4Si8O24) 12 H2O) mineral. Colour alterations at the
edges of fibres
suggests that it may be an alteration product
derived during mineral transformations. It is colourless and having
moderate dispersion.
Optically, it is negative.
Refractive indices
ranges between 1.486 and 1.492, with birefringence 0.006.
X-ray data of mineral chabazite is given in Table No.4.8 for
both the studied fly ashes. X-ray pattern reveals that the mineral
95
is crystallized under hexagonal system with possible space group R
3m. The unit cell parameters are ao= 13.780 Ao (VF) and 13.789 Ao
(KF) and Co – 15.059 Ao (VF) and 15.069 Ao (KF) with cell volume
V=2476.35 Ao3 (VF) and 2481.23 Ao3 (KF). The density calculated
from x-ray data is Dx-2.032 (X) (VF) and 2.036 (X) (KF). Maximum
intensity peaks are at (401), (101), (003), (122), (223) and (104) hkl
values and the x-ray data comparable with that of JCPDS file
No.19-208.
TABLE 4.8 X-RAY DATA OF CHABAZITE:
((Ca2Al4Si8O24) 12H2O)
hkl
VISAKHAPATNAM FLY
ASH
KOTHAGUDEM FLY
ASH
dAo
1/1o
dAo
1/1o
101
10.3431
100
10.3398
100
003
6.3601
20
63.569
18
122
5.4024
12
***
***
104
4.4666
13
4.4588
10
312
4.2622
18
4.2771
31
401
3.3453
80
3.3570
100
214
2.8981
8
***
***
96
223
2.6707
11
2.6681
13
042
2.5346
5
2.5308
9
205
2.2183
14
2.2212
12
410
2.1545
8
2.1546
10
232
2.0780
4
***
***
125
***
***
1.9879
7
116
2.3655
6
***
***
330
2.2960
9
***
***
502
2.2838
9
2.2733
5
054
2.0149
4
2.0154
9
217
1.9471
8
1.9483
7
523
1.7818
8
1.7867
6
CRYSTAL SYSTEM
HEXAGONAL
SPACE GROUP
R 3m
Z
3
ao
=
13.780 AO
ao
=
13.789 AO
co
=
15.059AO
co
=
15.069 AO
V
=
2476.35 AO3
V
=
2481.23 AO3
Dx
=
2.032 (X)
Dx
=
2.036 (X)
97
Edingtonite :
Rare barium zeolite, edingtonite (Ba Al2Si3O104 H2O) is
identified in both the studied fly ashes.
Micro photograph of
Visakhapatnam fly ash sample shows number of yellowish
cryptocrystalline grains.
Optically it is biaxial negative with 2
Va=52o. It is colourless in plane polarized light, first order grey and
yellow under crossed nicols. It’s refractive indices are α = 1.541,
β=1.552, χ=1.556 with birefringence 0.015.
Interference figures
show broad isogyres with moderate dispersion.
X-ray data of edingtonite is given in Table No.4.9 which is
comparable with the data of JCPDS file No.25-60. The strongest
peaks are at (001), (020), (200), (210) and (220) hkl values. X-ray
pattern indicates that the mineral crystallizes in orthorhombic
system with possible space group P21212. The unit cell parameters
were determined as ao = 9.548 Ao (VF) and 9.544 Ao (KF); bo = 9.639
Ao (VF) and 9.642 Ao (KF); co = 6.505 Ao (VF) and 6.495 Ao (KF); V =
598.67 Ao3 (VF) and 597.69 Ao3(KF) with Z - 2.
98
TABLE 4.9 X-RAY DATA OF EDINGTONITE:
((BaAl2Si3O104H2O)
hkl
VISAKHAPATNAM FLY
ASH
KOTHAGUDEM FLY
ASH
dAo
1/1o
dAo
1/1o
001
10.3431
100
10.3398
100
020
6.3601
20
63.569
18
200
5.4024
12
***
***
210
4.4666
13
4.4588
10
121
4.2622
18
4.2771
31
220
3.3453
80
3.3570
100
131
2.8981
8
***
***
311
2.6707
11
2.6681
13
022
2.5346
5
2.5308
9
202
2.2183
14
2.2212
12
320
2.1545
8
2.1546
10
212
2.0780
4
***
***
231
***
***
1.9879
7
032
2.3655
6
***
***
99
240
2.2960
9
***
***
322
2.2838
9
2.2733
5
023
2.0149
4
2.0154
9
203
1.9471
8
1.9483
7
CRYSTAL SYSTEM
ORTHORHOMBIC
SPACE GROUP
P 2 1 21 2
Z
3
ao
=
9.548 AO
ao
=
9.544 AO
bo
=
9.639 Ao
bo
=
9.642 Ao
co
=
6.505 AO
co
=
6.495AO
V
=
598.67 AO3
V
=
597.69 AO3
Dx
=
2.810 (X)
Dx
=
2.815 (X)
Baddeleyite:
Zirconium
oxide
baddeleyite
(Zro2)
is
identified
in
Visakhapatnam fly ash sample for the first time. It is irregular in
shape with high relief, under plane polarized light it shows deep
pinkish brown colour and under crossed nicols first order, pink
100
yellow and brown colours. It exhibits weak pleaochroism from
brownish pink to colourless.
X-ray data of baddeleyite is given in Table No.4.10, and
compared withJCTDS file No. 13-307.
the mineralcrystallize
X-ray data indicates that
inmonoclinic system with possible space
group P21/C. Unit cell parameters are a05.144 Ao, bo5.267 Ao, Cell
volume V –141.85 A°3 with β 99o.28.
ray data is 5.819 (X).
hkl values.
Density calculated from x-
Strongest peaks areat (111), (111) and (211)
The occurrence of such new mineral wassupported by
high Zr concentrations in the fly ash samples.
TABLE :4.10 X-RAY DATA OF BADDELEYITE (ZrO2)
hkl
VISAKHAPATNAM FLY ASH
dAo
1/1o
111
3.6412
100
111
2.3601
20
002
2.4024
12
020
2.4666
13
200
2.2622
18
210
2.3453
80
101
211
2.8981
8
112
2.6707
11
220
2.5346
5
221
2.2183
14
013
2.1545
8
130
2.0780
4
131
***
***
112
2.3655
6
213
1.2960
9
322
1.2838
9
023
1.0149
4
203
1.9471
8
CRYSTAL SYSTEM
MONOCLINIC
SPACE GROUP
P 21 /C
Z
4
Β
=
99O.28’
ao
=
5.144o AO
bo
=
5.267 Ao
co
=
5.215AO
102
V
=
141.85 AO3
Dx
=
5.819 (X)
DTA STUDIES
Kaolinite is present in both the fly ashes of Visakhapatnam
and Kothagudem samples. They were identified by the differential
thermal analysis at about 105°C.
This is understood to be carried
forward with the clay material inthe coals during deposition.
SCANNING ELECTRON MICROSCOPIC (SEM) STUDIES
The study of morphology as well as textural patterns of
microfined ash particles helps in understanding the binding as well
as other engineering properties. The response of fly ash particle to
the concrete type environment is individual, depending on its
specific composition and internal structure. The surface of fly ash
particle serves as the interface for toxic heavy metal adsorption and
biological interaction. Therefore it is necessary to understand the
external particle morphology.
103
Bulk samples and their fractions, and samples treated with 2
N. NaOH (at 70°C-l10°C for 6 hours) were studied under scanning
election microscope(SEM).
Most fly ash particles are spherical.
The spheres range in
size from µm to over 100 µm. Large portion of spherical particles
and plates inVisakhapatnam fly ash bulk sample can be observed.
The same view for Kothagudem fly ash bulk samples can be seen.
The Plale-3b is giving the contrast in size that may occur between
large and small fly ash spheres in Kothagudem fly ash. The
accumulation of smaller spheres in significant proportions may not
be simply random accumulation of small spheres stuck together.
These may be the fragments went through the larger particles and
bound together either by partial fusion or by layers of unburned
carbon retained between them. The attachment of spheres with
each other can be due to partial fusion (Kaufherr, et.al.,
1984).Finer fractions (0.011 mm, size) of Kothagudem fly ash show
equal sizes ofspheres in a maximum portion.
The surfaces of the particles have large variations in size,
shape and morphological features. Plate is an SEM photograph of
Visakhapatnam fly ash, indicating the deposition of sub-micron
104
size particles over the surface of a relatively very large irregular
particle, with vesicles.
The same photograph is also indicating the
accumulation of large number of sub-micron size particles around
large spheres and amorphous material.
Spherical cavities, looks
like kettle holes on a glacial deposit model, on the surfaces of larger
particles may be the gas pockets or the remmants of smaller
encapsulated spheres.
In Plate, the diameter
of the small spherical
particle,
which isattached on the top portion of a single larger particle. It is
about 1 µm and that of the larger particle, about 10 µm at X 7500
magnification. The photograph of the coarser fraction (0.09 mm,
size) of the same fly ash (Visakhapatnam) indicate the 1 µm
diameter particles with larger single particle (nearly 10 µm
diameter) at X 20,000 magnification. These two photographs
explained the wide size variations in fly ash particles.
Sometimes the underlying structure of crystalline material
embedded in glass may remain hidden. To study the surfacial
features of crystalline material, magnetically separated fly ash
particles were scanned.
105
In Plate (of Visakhapatnam) two different structures can be
observed. In the left top quadrant of the photograph, there is a
broken, relatively large, magnetic particle with the surface showing
a dendritic pattern. This dendritic pattern may indicate the mineral
iron-spinel (Diamond, 1986). X-ray diffraction analysis of the fly
ash
sample
(separated
with
a
hand
magnet)
revealed
the
occurrence of iron rich spinels such as magnetite (Fe3O4),
magnesio-ferrite (Mg Fe204) and maghemite(χ-Fe2O3) etc. Right top
corner of the same photograph contains many smaller nonmagnetic spheres.
The well rounded spheres are called "Cenospheres"" (hollow
spheres).Careful observation of these cenospheres indicates that
some of the larger spheres are having many smaller spheres and
amorphous material within it. Plate reveals that these spheres are
not empty. Hence the name "Plerosphere" (Plero-from Greek pleresfull) is suggested (Fisher el.al., 1976). Plate is like a perforated shell
of a fossil, cidaris. The surface of the plerosphere is having the
accumulation of larger number of micro crystals.
Infact the content of organic carbon residue is very low in fly
ashes, but small fragments of these residues are found in places.
106
The surface of the carbon residue particle indicates the remnants of
inorganic spheres embedded in it. This type of morphology was
described as "Swiss Cheese" morphology (Diamond, 1986).
107