1. FABRICATION OF POROUS MULLITE-ALUMINA CERAMIC USING
TORREFIED RICE HUSK AS A PORE-FORMING AGENT AND SILICA
SOURCE
Vu Thi Ngoc Minh1*
, Mai Van Vo1
, Nguyen Luong The Thinh 1
1
Department of Silicate Materials Technology, Hanoi University of Science and Technology
Received date: 05-01-2016
Abstract
Porous mullite-alumina ceramic was fabricated using torrefied rice husk as a pore forming agent and silica source.
Heat treatment of the rice husk was optimized as to balance the weight loss on heating and the grindability of the
torrefied product. The dry powder mixture contained up to 70 wt% of the torrefied rice husk with the rest being
alumina. Cane molasses was used as a binder. The effects of the sintering tempratures and the raw mix proportions on
the properties of the sintered samples were studied. Although a significant amount of the glass melt could be formed at
a temperature as low as 1250o
C, the formation of mullite did not present until 1450o
C. Depending on the rice husk
content and sintering temperature, the fabricated porous mullite-alumina samples had apparent densities in the range
from 1.0 to 2.6 g/cm3
.
Keywords: mullite, alumina, silica, porous ceramic, rice husk, torrefraction
1. INTRODUCTION
Porous ceramics are important structural
materials due to their low bulk density, high surface
area, low thermal conductivity, and high heat
resistance. Their most common applications include
gas separation, thermal insulation, chemical sensor,
catalyst, catalyst support, bacterial immobilization,
and particulate filters.1
The conventional methods to fabricate porous
ceramics include powder processing,2-4
sol-gel
processing,5
and leaching.6
Of these methods,
powder processing with the use of pore-forming
agents in the powder compact has most commonly
been used to produce ceramics with controlled
microstructure. During firing, the pore-forming
agents are burnt out, leaving voids in the final
products.
Among various types of pore-forming agents,
rice husk, the major by-product of rice processing,
has recently been used by a number of researchers
to fabricate thermal insulators and porous
ceramics.7-10
Rice husk is characterized by high ash
content (15.8 – 23 wt%), and high SiO2 content (90
– 97 wt%) in the ash.11
The newly formed SiO2
presents in the ceramic articles in forms of
amorphous phases and/or crystalline phases.
The present work focused on the fabrication and
characterization of porous mullite-alumina ceramic
from rice husk and alumina powder.
2. MATERIALS AND METHODS
The present work used the alumina powder
CT9FG produced by Almatis GmbH, Germany, as
one of the starting materials. The aluminum oxide
content of the powder was 99.5 wt%. The surface
area (BET method) of the powder was 0.8m2
/g.
Rice husk from Tien Hai (Thai Binh, Vietnam)
was used as a pore-forming agent and silica source.
Because of its fibrous structure, rice husk is a
material that is difficult to be ground. Hence, is
essential to partly decompose the biopolymers
present in the rice husk by heat treating to make it
easier to grind. Heat treatment was optimized based
on the weight loss on heating and the residue of the
heat treated rice husk on the 63 µm sieve after ball-
milling.
Porous mullite alumina ceramic was prepared
from the torrefied rice husk taken under the 63 µm
sieve and the alumina powder CT9FG at different
proportions: 70:30, 60:40, 50:50, and 40:60. Cane
molasses with a density of 1.39 g/cm3
was added to
the powder mixture at an amount of 25 wt %. Pellet
2. samples with a dimension of 20mm x 20mm was
pressed under a pressure of 30 MPa, dried at 110o
C
overnight, and then fired in an electric kiln. The
heating rate was kept at 200o
C/h from room
temperature to 200o
C, at 50o
C/h from 200o
C to
500o
C, at 125o
C/h from 500o
C to 1000o
C, and at
200o
C/h from 1000o
C to the maximum firing
temperature. Four maximum firing temperatures
1150o
C, 1250o
C, 1350o
C and 1450o
C, were applied.
The samples were kept at the maximum firing
temperature for an hour before naturally cooled down.
3. RESULTS AND DISCUSSION
3.1. Rice husk characterization
The thermal analysis curves of the rice husk are
presented in Figure 1. Below 120o
C was the
evolution of water absorbed in the rice husk with an
endothermic peak presented at 105o
C. There was
almost no lease of vapor and gas in the range of
temperatures from 105o
C to 240o
C. A rapid
volatilization occurred in the range of temperatures
from 240o
C to 490o
C with two exothermic peaks
presented at approximately 355o
C and 425o
C.
Further changes in the TG and DTA curves were
not significant when the heating temperature
crossed 490o
C.
Figure 1: Thermal analysis of the rice husk in air at
the heating rate of 10o
C/min.
Heat treatment of the rice husk for easy grinding
was investigated based on the above thermal
analysis. Figure 2 presents the weight loss of the
rice husk after heating at 240o
C for 90 minutes and
the particle size distribution of the heat treated rice
husk after a two-hour ball milling. Although the rice
husk heated at temperatures above 240o
C was much
easier to grind than the one heated at temperatures
less than 240o
C, high weight losses made it
impractical to process. The one heated at 240o
C
gained the balance between the weight loss and the
grindability. That was the temperature where a rapid
volatilization started as seen in Figure 1. The weight
loss of the rice husk heated at that temperature (for
90 minutes) was 23wt%, and 61wt% of the ball-
milled product passed the 0.063-mm sieve.
Figure 2: Illustration of weight lost on heating and
particle size distribution of the torrefied rice husk
after ball milling.
The result of chemical analysis of the RHA is
presented in Table 1. The main component was
silicon (90.11 wt% as SiO2), followed by potassium
(4.91 wt% as K2O) and calcium (2.49 wt% as CaO).
Other chemicals presented in the RHA included
Al2O3, TiO2, CaO, MgO, Fe2O3, Na2O, and SO3, in
an amount of less than 1 wt% each.
Table 1: Rice husk ash analysis
Components Weight percent
SiO2
Al2O3
TiO2
CaO
MgO
Fe2O3
FeO
K2O
Na2O
SO3
90.11
0.77
0.05
2.49
0.93
0.02
0.42
4.91
0.29
0.002
-50
0
50
100
150
0
20
40
60
80
100
0 100 200 300 400 500 600 700 800 900
Heatflow(mW)
Weight(%)
Temperature (oC)
TG
DTA
exo
0
20
40
60
80
100
0.0
0.2
0.4
0.6
0.8
1.0
180 200 220 240 260 280
Weightloss(%)
Weightfraction
Temperature (oC)
<0.063 mm 0.5 - 0.063 mm
>0.5mm Weight lost on heating
3. 3.2. Mullite-alumina ceramic from rice husk and
alumia
The thermal analysis curves of a mixture
containing alumina 50 wt%, torrefied rice husk 50
wt% and cane molasses 25 wt% are presented in
Figure 3. The thermal gravity curve of this mixture
was smoother than that of the rice husk due to the
presence and decomposition of the cane molasses.
Unlike the differential thermal analysis (DTA)
curve of the rice husk, where only two exothermic
peaks presented, there were three exothermic peaks
(at approximately 315o
C, 406o
C and 475o
C) on the
DTA curve of this mixture.
Figure 3: Thermal analysis of a mixture containing
alumina 50 wt%, torrefied rice husk 50 wt% and
cane molasses 25wt%.
Figure 4 illustrates the shrinkage and expansion
of the pellet samples fired at different maximum
sintering temperatures. Initially, all samples had a
diameter of 20 mm. The least shrinkage was
observed on samples fired at 1150o
C. At this
temperature, the more the rice husk was added to
the raw mix, the more the samples shrank.
Shrinkage and a glossy surface were observed on
the sample with 70 wt% rice husk fired at 1250o
C.
It indicated that an amorphous phase in form of a
glass melt was probably formed at a significant
amount at this temperature. That glass melt,
however, did not cause the samples to collapse at
higher temperatures (1350o
C and 1450o
C) but
closed the open pores and swelled up as the gases
inside expanded.
Pore closure and swelling made some of the
samples floated on water as indicated in Table 2. It
also affected the apparent density, water absorption
and compressive strength of the samples. The
swollen and floating samples had compressive
strength of less than 20 MPa.
Figure 4: Illustration of size change corresponding to
the raw mix proportions and sintering temperatures.
The original samples had a diameter of 20 mm.
Table 2: Properties of the sintered pellet samples.
T A D W S CS
1150 30 1.0 63.2 -12.2 -
40 1.1 59.9 -7.1 6.3
50 1.2 53.0 -5.1 5.9
60 1.3 48.4 -1.5 4.3
1250 30 1.3 9.0 -18.1 48.4
40 2.0 15.0 -23.7 118.6
50 1.8 24.5 -16.3 58.9
60 1.5 34.9 -7.1 22.3
1350 30 float - 14.0 10.6
40 1.2 2.5 -12.2 37.2
50 2.0 1.7 -19.6 136.4
60 2.0 18.7 -15.3 86.0
1450 30 float - 27.0 3.2
40 float - -4.6 16.5
50 1.8 8.6 -17.9 87.0
60 2.6 8.5 -21.1 -
T : Maximum sintering temperature (o
C)
A: Alumina content in the raw mix (wt%)
D : Apparent density (g/cm3
)
W : Water absorption (wt%)
S : Linear size change (%)
CS: Compressive strength (MPa)
-50
0
50
100
150
0
20
40
60
80
100
0 100 200 300 400 500 600 700 800 900
Heatflow(mW)
Weight(%)
Temperature (oC)
TG
DTA
exo
4. Figure 5: X-ray diffraction patterns corresponding
to the raw mix proportions and sintering
temperatures. The numbers after A and RH indicate
the proportion of the alumina powder and torrefied
rice husk in the raw mix. The number after the
hyphen (-) indicates the sintering temperature in o
C.
(: cristoballite, : corundum, : mullite).
Figure 6: FESEM images taken at the fractured
surfaces of the samples sintered at 1250o
C.
10 20 30 40 50 60 70
Intensity(arbitraryunit)
2 - theta
A3RH7-1250
A4RH6-1250
A5RH5-1250
A6RH4-1250
(a)
10 20 30 40 50 60 70
Intensity(arbitraryunit)
2 - theta
A5RH5-1250
A5RH5-1350
A5RH5-1450
(b)
10 20 30 40 50 60 70
Intensity(arbitraryunit)
2 - theta
A6RH4-1250
A6RH4-1350
A6RH4-1450
(c)
5. Figure 6 (a) presents the X-ray diffraction
patterns of the samples fired at 1250o
C. The
abbreviations A6RH4, A5RH5, A4RH4, and
A3RH7 were corresponding to samples with the
alumina contents of 60wt%, 50wt%, 40wt% and
30%. The mullite (3Al2O3.2SiO2) phase was not
formed in all samples although their linear
shrinkage rose up to 23.7%. Only cristoballite
(SiO2) and corundum (-Al2O3) were observed, but
the peaks of the cristoballite phase were much
smaller than that of the corundum phase.
Figure 6 (b) and Figure 6 (c) shows that the
mullite phase was only formed at 1450o
C. At that
temperature, no cristoballite phase existed.
At 1250o
C, the SiO2 obtained from the rice husk
presented not only in form of the crystalline phase
but also in the amorphous phase as could be seen on
Figure 7. At a low magnification, A6RH4, A5RH5,
and A4RH4 looked like loose powder compacts
with various interconnected pores. However, at a
higher magnification, the amorphous phase was
clearly shown. It was the silicate glass phase formed
by the melting of the RHA components and
possibly alumina at high temperatures. It bond
corundum particles together, and was able to
increase to compressive strength of A4RH6 to 118.6
MPa as presented in Table 2. Nevertheless, the
shrinkage of this sample was the highest.
The sample A3RH7 showed a complete different
microstructure at both magnifications compare to
the other samples. The majority was closed pores at
a wide range of sizes, from smaller than 1µm to
larger than 100µm. Unlike other images, the grain
structure in this sample was not clear.
4. CONCLUSION
Rice husk from Tien Hai, Thai Binh, was
characterized. Torrefraction of the rice husk at
240o
C for 90 minutes gained the balance between
the weight loss on heating and the grindability. The
main component of rice husk ash was SiO2 at an
amount of 90wt%.
The microstructure and strength of the sintered
samples depended strongly on the raw mix
proportions and the sintering temperature. With an
amount of more than 60 wt% of the torrefied rice
husk in the raw mix, the samples tended to swell at
temperatures above 1350o
C, forming closed pores
and being able to float on water after cooling.
Although a significant amount of the glass melt
could be formed at a temperature as low as 1250o
C,
the recrystallization of mullite did not occur until
1450o
C.
Acknowledgement. The authors are grateful to the
Vietnam Institute of Building Materials for
supplying the alumina powder. This work was
financially supported by Hanoi University of
Science and Technology.
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6. Corresponding author: Vu Thi Ngoc Minh
Department of Silicate Materials Technology
Hanoi University of Science and Technology
1 Dai Co Viet, Hai Ba Trung, Hanoi
Email: minh.vuthingoc@hust.edu.vn
Telephone number: (+84) 438692517
