Figure 1. Location of the red sediment deposits of Nolia
Nuagam, Odisha, India.
Figure 2. Sample collection points () of red sediments at
Nolia Nuagam, Odisha, India [(a): red sediment hillock, (b),
(c) & (d): heavy mineral concentrations].
Figure 3. Experimental plan for recovery of sillimanite mi-
neral from red sediments of badlands topography.
concentrate containing greater than 63% of total heavy
minerals were scrubbed initially in an alkaline medium.
The slimes were rejected. The sand was subjected to mag-
netic separation for recovery of non magnetic silli-
manite and other associated heavy minerals. The non mag-
netic fraction was subjected to sink float tests using bro-
moform (2.89 and methylin iodide (3.3 The
non magnetic methylin iodide float is sillimanite. The re-
covered sillimanite was used in the present investiga-
2.3. Effect of Microwave Treatment on
The recovered sillimanite from red sediments containing
56.67% Al2O3 was used in all the experiments. In all ex-
periments 60% sillimanite was used as constant parame-
ter. The samples in form of pellets were made with the
combination of sillimanite (Al2SiO5), alumina (Al2O3) and
additives of silicon carbide (SiC) powder in various ratios.
These samples were then placed inside the microwave
Copyright © 2012 SciRes. JMMCE
sintering furnace separately in order to study the effect of
microwave heating.
The microwave sintering furnace used in the present
investigation was G N Tech, 37.500 W, 2.45 GHz. An
adequate argon pressure is maintained in microwave fur-
nace in order to avoid any type of oxidation reaction. The
susceptor SiC placed near to sample further to facilitate
the microwave absorption and heating the sample in mi-
crowave sintering furnace [5]. The susceptor SiC is placed
diagonally to the sample in microwave sintering furnace.
Experiments were conducted in microwave furnace by
changing ratio samples such as SiC powder and alumina
(Al2O3) variables and at constant 60% sillimanite by
weight. On taking consideration that the minimum tem-
perature required to form the mullite is 1250˚C, the dif-
ferent samples were heated separately either by varying
the duration of time or by varying the power fed with
respect to variation of SiC powder and alumina (Al2O3).
Experiments were also carried out in conventional fur-
nace to achieve the mullite phase. The resulting products
were characterized by X-Ray diffraction, FESEM and
2.4. Analytical Methods
The PANalytical X-Pert X-Ray powder diffractometer
with Mo-Kα radiation (λ = 0.709Ǻ) from 6˚ to 40˚ scan-
ning angle at a scanning rate of 0.02˚/sec was used for
phase analysis of present investigation. Morphological
features of mullite were studied using the FESEM (mo-
del: Supra55; Zeiss, Germany). The FESEM has a reso-
lution of 1 nm at 30 KV which is equipped with 20 mm2.
Oxford’s Energy dispersive X-ray spectroscopy (EDS)
detector for imaging of conducting as well as non-con-
ducting samples without gold coating. SEM/EDAX stu-
dies were done by using Hitachi VP-SEM S-3400N. It
has high SE resolution of 10 nm at 3KV. The magnifica-
tion of the instrument is 5X - 300.000X; alternating volt-
age is 0.3 - 30 KV. The grains were mounted on a SEM
brass stub. The mounted grains were coated with gold in
a vacuum evaporator while the mullite sample was being
slowly rotated. Mullite and silicon carbide materials were
analyzed to know the weight % of various constituents of
atoms using EDAX.
3. Results and Discussion
3.1. Characterization of Raw Material
The chemical analysis of red sediment placer sillimanite
given in Table 1 indicate that the sample contain 56.67%
Al2O3, 25.21% SiO2, 0.51% TiO2 and 0.23% Fe2O3. The
modal analysis of red sediment feed sample is shown in
Figure 4. The data indicate that the deslimed feed sam-
ple contain 7% sillimanite, 51 % ilmenite, 1% zircon,
25% quartz, 2% rutile, 12 % slimes and 2 % others. The
XRD pattern of deslimed feed shown in Figure 5 indi-
cates that the sample contain ilmenite, quartz, sillimanite
are abundant minerals followed by rutile, zircon, hema-
3.2. Studies on Formation of Mullite from
Sillimanite Using Microwave Sintering
It is a known fact that microwave sintering furnace has a
wide range of applications in mineral and material tech-
nology. In microwave heating of minerals, the materials
which couple to microwave radiation contain dipoles
[5,6]. These dipoles align themselves in an applied elec-
Table 1. Complete chemical analysis of the sillimanite con-
Compound Name Conc. (%)
Al2O3 56.67
CaO 0.42
MgO 0.31
Figure 4. Modal analysis of red sediment feed sample.
0 10 20 30 40
Two Theta
Figure 5. XRD pattern of deslimed feed.
Copyright © 2012 SciRes. JMMCE
tric field electric
and will flip around in an alternating
field. As a consequence, the material will be heated as
the stored internal energy [5-7] is lost to friction. This
energy mode conversion has the advantage of being se-
lective to individual mineral phases within a mass by
which the mullite is formed from sillimanite.
The results of microwave power on mullite fo
om sillimanite using SiC as susceptors are given in Ta-
ble 2. It may be noted here the role of SiC in the micro-
wave sintering furnace is as a heating agent because it
couples quickly with electromagnetic radiation, creating
heating owing to the Joule effect. The decomposition of
sillimanite with addition of Al2O3 and SiC may be ex-
pected as
SiC Susceptor
23 223232
2Al O .SiOAl OSiC3Al O .2SiO
It is also an important fact to express at this point that
er led to an increase in
gy does not rely on diffusion of heat
able 2. Results of microwave energy on mullite formation
Additive Binder Microwave Max Temp Findings
e desired temperature for decomposition of sillimanite
to mullite and silica is in the temperature range of
1300˚C - 1700˚C. It is observed from Table 2 that mul-
lite is not formed with the charge sample containing sil-
limanite (60%) and Al2O3 (40%) alone. This is due to the
fact that maximum temperature of the microwave sinter-
ing furnace achieved is 805˚C only. This effect can clearly
be seen in Figure 6 where the effect of different vari-
ables such as SiC as additive and binder on the achieve-
ment of temperature (˚C) with microwave power fed (Watt)
is expressed. It is further seen that from Figure 6 that the
temperature of the microwave sintering furnace is not in-
creasing beyond 805˚C even with increasing of micro-
wave power beyond 2850 Watt.
An increase in microwave pow
e heating rate of the samples with the combination of
sillimanite, alumina, silicon carbide shows an affinity for
microwave radiation, i.e., it heats effectively with increase
of the applied power and as a consequence the tempera-
ture of the sample inside the furnace rises (measured by
infrared pyrometer).
The transfer of ener
om the surfaces and it is possible to achieve rapid and
uniform heating of sample volumetrically [6-8].
from sillimanite using SiC susceptor.
Charge Sample SiC as
[Sillimanite +
Al2O3 ] (%) (%) Power ( W) reached
60 0 2850 % + 40 %Nil 805 No
60 % + 40 % 5 5 3000
60%SiC, 10% Binder
60%SiC, 5% Binder
40%SiC, 5% Binder
5%SiC, 5% Binder
0%SiC, 0% Binder
0 500 1000 1500 2000 2500 3000 3500
ower, W
Temp, ˚C
Figure 6. Effect of different variables on achievement o
ffect of different
temperature with microwave power fed.
The results seen in Table 2 on the e
vriables such as 5% SiC and 40% SiC with 5% binder to
achieve the desired temperature around 1300˚C for mul-
lite formation from sillimanite is not seen and hence mul-
lite has not been formed in both the conditions.
It is seen from the experiments that the temperature
occurred for given microwave power to form mullite is
increasing with addition of SiC. The SiC is one of the
reducing agents, which heat rapidly in presence of ap-
plied electric field. They conduct energy into the bulk
sample so that electrons in non-dielectric materials can
become more mobile. Due to this extra mobility, there is
the formation of dipoles [5,9] in the microwave field and
the material can heat on its own. It is an established fact
that microwave energy has potential for the speedy and
efficient heating of minerals and in a commercial context
may provide savings in both time and energy. The role of
SiC powder of 250 mesh in the present investigation is
not only behave as reducing agent but also as heating
agent owing to it couples with microwaves creating
heating, essential to proceed reactions [9,10]. Also as
mentioned earlier that on increasing the microwave
power led to an increase in the heating rate of the sam-
ples shows an affinity for microwave radiation, i.e., it
heats effectively with increase of the applied power and
as a consequence the temperature of the sample inside
the furnace rises which is measured by infrared pyrometer.
Thus a mullite formation is seen when 60 % SiC and
5% binder are used with the composite charge material
i.e. sillimanite (60%) and Al2O3 (40%). The maximum tem-
perature of the microwave sintering furnace achieved is
1355˚C at 2450 W microwave power. Addition of 10 %
binder to the same charge material with 60 % SiC, the
furnace temperature achieved is 1384˚C at microwave
50 No
60 % + 40 % 40 5 2950 1080 No
60 % + 40 % 60 5 2372 1355 Yes
60 % + 40 % 60 1900 1384 Yes
Copyright © 2012 SciRes. JMMCE
power 1900 W. Hence when the sillimanite is exposed
for 25 minutes, a mullite is formed under these experi-
mental conditions. During microwave processing, the
potential energy exists to reduce processing time and en-
hance product quality as microwaves can transfer en-
ergy throughout the sample volumetrically.
3.3. Studies on Formation of Mullite from
Sillimanite Using Conventional Furnace
sillim arge
l furnace
that in the conventional fur-
central region of the pellets, resulted
The experimental results on the formation of mullite
anite using conventional furnace with the ch
sample containing sillimanite (60%) and Al2O3 (40%)
and additives with 40% SiC and 10% binder reveals that
it took three hours to form a mullite at 1300˚C.
It is important to mention here the observations once
again on the tests carried out with conventiona
d microwave furnace that a mullite formation is seen at
25 minutes of experimental duration with microwave
furnace and where as the formation of mullite is not seen
with conventional furnace during 25 minutes under op-
timum material variables. It has taken three hours of time
at 1300˚C furnace temperature for formation mullite us-
ing conventional furnace.
The heating mechanism through microwaves furnace
are distinctly different form
ce. Typical Figure 7 shows a schematic illustration of
the process steps and effect of the heating mechanism in
the carbothermal reduction reaction for microwaves fur-
nace and conventional furnace with the charge material
kaolin clay [2].
In the microwaves furnace, the heating with micro-
waves started in the
two distinct zones: darker central core completely sur-
rounded by lighter zone. It is also observed that the outer
zone for MWCR as shown in Figure 7 is primarily mul-
lite with a small percentage of Al2O3 and much less SiC
than in the central zone with primarily SiC and small per-
centages of mullite and Al2O3. Both CO and SiO gases
must diffuse out of the pellets. However, during conven-
tional heating as shown in Figure 7, the carbothermal
reaction began outer the pellets and then progressed into
Mullit e
Al O
the central core. The pellet showed two distinct zones:
lighter central region completely surrounded by a darker
zone. It was also observed that the outer zone was pri-
marily mullite with small percentages of SiC, whereas
the inner region was primarily mullite with a small per-
centage of SiO2 and much less SiC than in the outer
Thus it is concluded that during microwave processing,
the pot
2 3
Mulli t
Mulli t e
Figure 7. Effect of heating mechanism for the carbothermal
reduction for both CCR and MWCR.
additives such
as silliposite
s, it attracts Si and C atoms from the vapor to
ential energy exists to reduce processing time and
enhance product quality as microwaves can transfer en-
ergy throughout the sample volumetrically. Hence mi-
crowave heat source is much effective for value addition
to red sediment placer sillimanite to form mullite in com-
pare to conventional furnace.
3.4. Structural and Morphological
Characterization of Mullite
The XRD patterns of charge materials and
manite, alumina, silicon carbide and com
charge material (raw mullite) are shown in Figure 8. The
XRD patterns of mullite formation in both microwave
furnace and conventional furnace are also shown in Fig-
ure 9.
The XRD data indicate that the mullite formation in
both microwave furnace and conventional furnace are al-
most similar with reference to mullite phase concern. The
results evidenced that microwave is more efficient to
produce Al2O3/mullite/SiC composite at 25 min than
conventional furnace at 1300˚C/3 h duration.
The FESEM images for mullite formation from red
sediment placer sillimanite using microwave heat treated
is shown in Figure 10. It can clearly be seen that SiC
(massive form), and mullite in cluster structure, are ob-
served in the morphological features of mullite formed
from red sediment placer sillimanite using microwave
It may be noted here that as the metal melts at high
e catalyst to form whiskers. When saturation of these
atoms occurred in the liquid catalyst, the growth is side
branched type, resulting in formation of fibrous structure
[11]. Apart from this, the alumina needles are also found
and seen in the SEM image. This alumina phase is very
helpful in microwave heating because it helps to isolate
silica from silicon carbide during mullite formation.
Typical SEM EDAX data and image mapping for Al,
Si, C and O of microwave mullite are shown in Figures
11 and 12 respectively.
The image mapping for Al, Si, carbon and oxygen for
typical mullite sample prepared from red sediment placer
sillimanite using microwave sintering furnace reveals that
the sample contain both mullite and SiC materials. Thus
the data obtained from XRD, SEM EDAX confirm the
findings on the presence of mullite and silicon carbide
Copyright © 2012 SciRes. JMMCE
Copyright © 2012 SciRes. JMMCE
(a) (b)
(c) (d)
Figure 8. XRD patterns (Cu target) of (a) sillimanite (b) alumina (c) SiC and (d) raw mullite
(a) (b)
Figure 9. XRD patterns of mul) conventional face.
lite formation (Cu Target) inrowave furnace (a) and (b (a) micurn
Figure 10. Morphological features of mullite (cluster), alumina (needle shape) and silicon carbide (massive structure) at one
location. Figure 10(a). shows the image with 1K mag. image; Figure 10(b). shows the image of spot A of Figure 10(a), which is
enlarged to 10 K mag.
Figure 11. SEM-EDAX data of mullite and SiC. (a). image and spectrum analysis of mullite; (b). image and spectrum analysis
of SiC.
Figure 12. Image mapping of Al, Si, C and O for typical mullite sample prepared from red sediment placer sillimanite.
prepared from the red sediment placer sillimanite as a
The following conclusions are drawn from the stud
ed sediment placer sillimanite using
with 60% SiC, the furnace temhieved is
vue addition. 1384˚C at microwave power 1900 W.
4. Conclusions
y on Whereas under the similar additive conditions, the
mullite formed from sillimanite in conventional fur-
nace heating, it took more than 3 hours at 1300˚C.
value addition to r
microwave energy and in depth structural and morpho-
logical characterisation of mullite.
A mullite formation is seen when 60% SiC and 5%
binder are used with the composite charge material i.e.
sillimanite (60%) and AlO (4
2 30%). The maximum
temperature of the microwave sintering furnace achi-
eved is 1355˚C at 2450 W microwave power.
Addition of 10% binder to the same charge material
A mullite is formed within 25 minutes from the silli-
manite, under the above experimental conditions.
perature ac
The XRD data is shows mullite phase which rela-
tively distinct from the formation mullite using mi-
crowave furnace than conventional furnace.
FESEM image analysis shows the mullite and silicon
carbide in microwave treated sample.
The SEM EDAX and image mapping data also con-
firm the findings of mullite and silicon carbide.
Copyright © 2012 SciRes. JMMCE
Thus microwave heat source is much effective for
value addition to red sediment placer sillimanite to
(Registered at SOA
a, India) and publication of
1998, pp. 2081-2087.
form mullite in compare to conventional furnace.
5. Acknowledgements
. Satya Sai Srikant is very much thankful to the ProErf. B.
K. Mishra, Director, CSIR-Institute of Minerals and Ma-
terials Technology, Bhubaneswar for giving permission
to utilize infrastructural facilities of CSIR-IMMT and
his encouragements to do Ph.D.
versity; Bhubaneswar, Odish
[1] S. Tripathi and G. Banerjee, “Synthesis and Mechanical
Properties of Mullite from Beach sand Sillimanite: Effect
of TiO2,” European Journal Ceramic Society, Vol. 18, No.
[2] E. Fagury andullite/SiC Powders
Synthesized bthermal Re-
R. Kiminami, “Al2O3/M
y Microwave-Assisted Carbo
duction of Kaolin,” Ceramics International, Vol. 27, No.
7, 2001, pp. 815-819.
[3] T. Ebadzadeh, M. H. Sarrafi and E. S
Assisted Synthesis and Sintering of
alahi, “Microwave-
Mullite,” Ceramics
International, Vol. 35, No. 8, 2009, pp. 3175-3179.
[4] Y. Fang, Y. Chen and M. R. Silsbee, “Microwave Sinter-
ing of Fly Ash,” Material Letter, Vol. 27, No. 4-5, 1996, pp.
155-159. doi:10.1016/0167-577X(96)80007-3
[5] R. B. Rao and N. Patnaik, “Microwave Energy in Mineral
ProcessingA Review,” Journal of the Institution of Engi-
neers (India)Mining, Vol. 84, No. 2, 2004, pp. 56-61.
C. A. Pickles, “Microwaves in Extractive Metal[6] lurgy: Part
1A Review of Applications,” Minerals Engineering, Vol.
22, No. 13, 2009, pp. 1102-1111.
[7] E. T. Thostenson and T.W. Chou, “Microwave Processing:
Fundamentals and Applications,” Composites Part A: Ap-
plied Science and Manufacturing, Vo
l. 30, No. 9, 1999, pp.
[8] D. E. Clark and W.H. Sutton, “Microwave Processing of
Ceramic Materials,” Annual Review of Materials Science,
Vol. 26, 1996, pp. 229-231.
[9] S. S. Srikant, P. S. Mukherjee and R. B. Rao, “Micro-
wave Reduction of Placer Ilmenite Concentrate,” Mineral
Processing Technology, 2011.
[10] H. Kozuka and J. D. Mackenzie, “Microwa
ve Synthesis
a, E. C. Y. Lin and B. Gu-
, pp. 2283-2285.
of Metal Carbides,” Ceramic Transactions, Vol. 21, 1991,
pp. 387-394.
[11] A. C. D. Chaklader, S. D. Gupt
towski, “Al2O3/SiC Composites from Aluminosilicates
Precursors,” Journal of the American Ceramic Society, Vol.
75, No. 8, 1992
Copyright © 2012 SciRes. JMMCE