FABRICATION OF POROUS MULLITE-ALUMINA CERAMIC USING TORREFIED RICE HUSK AS A PORE-FORMING AGENT AND SILICA

SOURCE

Vu Thi Ngoc Minh

1*, Mai Van Vo

1, 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 1250oC, the formation of mullite did not present until 1450oC. 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

samples with a dimension of 20mm x 20mm was

pressed under a pressure of 30 MPa, dried at 110oC

overnight, and then fired in an electric kiln. The

heating rate was kept at 200oC/h from room

temperature to 200oC, at 50

oC/h from 200

oC to

500oC, at 125

oC/h from 500

oC to 1000

oC, and at

200oC/h from 1000

oC to the maximum firing

temperature. Four maximum firing temperatures 1150

oC, 1250

oC, 1350

oC and 1450

oC, 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 120oC was the

evolution of water absorbed in the rice husk with an

endothermic peak presented at 105oC. There was

almost no lease of vapor and gas in the range of

temperatures from 105oC to 240

oC. A rapid

volatilization occurred in the range of temperatures

from 240oC to 490

oC with two exothermic peaks

presented at approximately 355oC and 425

oC.

Further changes in the TG and DTA curves were

not significant when the heating temperature

crossed 490oC.

Figure 1: Thermal analysis of the rice husk in air at

the heating rate of 10oC/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 240oC 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 240oC was much

easier to grind than the one heated at temperatures

less than 240oC, high weight losses made it

impractical to process. The one heated at 240oC

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

Hea

t fl

ow

(m

W)

Wei

ght

(%)

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

Wei

ght l

oss

(%

)

Wei

ght f

ract

ion

Temperature (oC)

<0.063 mm 0.5 - 0.063 mm>0.5mm Weight lost on heating

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 315oC, 406

oC and 475

oC) 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 1150

oC. 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 1250oC.

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 (1350oC and 1450

oC) 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 (oC)

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

Hea

t fl

ow

(m

W)

Wei

ght

(%)

Temperature (oC)

TGDTA

exo

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 oC.

(: cristoballite, : corundum, : mullite).

Figure 6: FESEM images taken at the fractured

surfaces of the samples sintered at 1250oC.

10 20 30 40 50 60 70

Inte

nsi

ty (

arb

itra

ry u

nit

)

2 - theta

A3RH7-1250

A4RH6-1250

A5RH5-1250

A6RH4-1250

(a)

10 20 30 40 50 60 70

Inte

nsi

ty (

arb

itra

ry u

nit

)

2 - theta

A5RH5-1250

A5RH5-1350

A5RH5-1450

(b)

10 20 30 40 50 60 70

Inte

nsi

ty (

arb

itra

ry u

nit

)

2 - theta

A6RH4-1250

A6RH4-1350

A6RH4-1450

(c)

Figure 6 (a) presents the X-ray diffraction

patterns of the samples fired at 1250oC. 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 1450

oC. At that

temperature, no cristoballite phase existed.

At 1250oC, 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 240

oC 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 1350oC, 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 1250oC,

the recrystallization of mullite did not occur until

1450oC.

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.

REFERENCES

1. I. Nettleship, Applications of Porous Ceramics, Key

Engineering Materials, Vol. 122, 305-324 (1996).

2. Z. Živcová, E. Gregorová, W. Pabst, D. S. Smith, A.

Michot, and C. Poulier, Thermal Conductivity of

Porous Alumina Ceramics Prepared Using Starch as

a Pore-Forming Agent, Journal of the European

Ceramic Society, 29(3), 347-353 (2009). 3. L. Montanaro, Y. Jorand, G. Fantozzi, and A. Negro,

Ceramic Foams by Powder Processing, Journal of the

European Ceramic Society, 18(9), 1339-1350 (1998).

4. J. She and T. Ohji, Fabrication and Characterization

of Highly Porous Mullite Ceramics, Materials

Chemistry and Physics, 80(3), 610-614 (2003).

5. C. Brinker, R. Sehgal, S. Hietala, R. Deshpande, D.

Smith, D. Loy, and C. Ashley, Sol-gel Strategies for

Controlled Porosity Inorganic Materials, Journal of

Membrane Science, 94(1), 85-102 (1994).

6. T. H. Elmer, Porous and Reconstructed Glasses, ASM International, Engineered Materials Handbook., Vol.

4, 427-432 (1991).

7. T. Watari, A. Nakata, Y. Kiba, T. Torikai, and M.

Yada, Fabrication of Porous SiO2/C Composite from

Rice Husks, Journal of the European Ceramic

Society, 26(4), 797-801 (2006).

8. K. Mohanta, A. Kumar, O. Parkash, and D. Kumar,

Low Cost Porous Alumina with Tailored

Microstructure and Thermal Conductivity Prepared

using Rice Husk and Sucrose, Journal of the

American Ceramic Society, 97(6), 1708-1719 (2014). 9. G. Wei, L. Hongbin, and F. Chunxia, Influence of

La2O3 on Preparation and Performance of Porous

Cordierite from Rice Husk, Journal of rare earths,

28(4), 614-617 (2010).

10. M. Gonçalves and C. Bergmann, Thermal Insulators

Made with Rice Husk Ashes: Production and

Correlation between Properties and Microstructure,

Construction and Building Materials, 21(12), 2059-

2065 (2007).

11. A. Kaupp, Gasification of Rice Hulls: Theory and

Praxis, Springer-Verlag, (2013).

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