Ž .Powder Technology 117 2001 232–238www.elsevier.comrlocaterpowtec

Silica white obtained from rice husk in a fluidized bed

Sheng Huang, Shan Jing), Jinfu Wang, Zhanwen Wang, Yong JinDepartment of Chemical Engineering, Tsinghua UniÕersity, Beijing 100084, People’s Republic of China

Received 17 July 2000; received in revised form 28 August 2000; accepted 5 September 2000

Abstract

In this paper, silica white, which is obtained from rice husk in a fluidized bed, is investigated. First, the experiments are carried out inŽ .the cold fluidized bed of 90-mm interior diameter I.D. . The experimental results show that the ash mixed with a little rice husk can be

fluidized when U is within the range of 0.18–0.316 mrs. Then, the experiments of combustion of rice husk are investigated in theg

fluidized bed of 84 mm I.D. The experimental results show that the combustion occurs in the dense zone of the fluidized bed. The ash inthe dense zone is easily purified to obtain the SiO powder. The quality of silica white sampled from the dense phase zone is higher than2

that of GB-precipitated silica and approaches that of pyrogenic silica except for specific surface area and iron content. q 2001 ElsevierScience B.V. All rights reserved.

Keywords: Rice husk; Fluidized bed; Silica white

1. Introduction

Rice husk is found in over 75 countries where rice isgrown and represents, according to variety, 14–35% of the

w xtonnage of harvested rice 1 . Since the world productionof rice is over 350 million tons, resulting in more than 70million tons of rice husk, it can be realized that thisproblem is very important. Some of the advantages of rice

Ž .husk can be reduced to three elements: 1 it has highŽ .energy content, 2 its silica content has amorphous charac-

Ž .teristic, 3 it has a highly cellular structure. The silicacontent in rice husk can be purified to produce highly pure

w x w x w xSiO 2 , SiCl 3 , Si N , and SiC 4 . Some authors have2 4 3 4

claimed that rice husk was combusted in a fluidized bed tow xobtain the energy content 1 . Some other authors have

described how to purify the silica content from rice huskw xash 2,3,5,6 , but the ashes are all obtained in the fixed

bed. This paper presents the experimental technology ofsilica white obtained from rice husk mixed with a largeseal of ash in a fluidized bed.

) Corresponding author. Tel.: q86-10-62772051; fax: q86-10-62772051.

Ž .E-mail address: hsanjingts@263.net S. Jing .

2. Properties of rice husk

Rice husk is an irregular boat-like powder, and itsproperties are presented in Table 1. It contains organicmaterials as the major constituent, which produces a highash content. The organic materials consist of cellulose andlignin which turn to CO and CO when rice husk burns in2

air. The ash contains 87–98% silica and a small proportionof metallic elements. In this paper, the rice husk sample iscollected in Beijing district, China.

3. Cold model

A cold experimental set-up is used to examine thefluidization qualities of rice husk, the ash of rice husk, and

Table 1Properties of rice husk

Material Shape l a b r rb s3 3Ž . Ž . Ž . Ž . Ž .m m m kgrm kgrm

Rice boat- 8–10= 2–3= 0.2= 125 500y3 y3 y3husk like 10 10 10

0032-5910r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0032-5910 00 00372-7

( )S. Huang et al.rPowder Technology 117 2001 232–238 233

Fig. 1. Experimental setup for cold model: 1—air intake; 2—gas distribu-tor; 3—fluidized bed; 4—cyclone; 5—gas vent; 6—flange.

the ash mixed with a few rice husk. As shown in Fig. 1,Ž .the interior diameter I.D. of the fluidized bed is 0.09 m,

and a cyclone is set outside the bed.The experimental results are given in Table 2 and are

summarized as follows.

1. Rice husk is very difficult to fluidize;2. Rice husk ash has the good quality of fluidization

and the ash powder appears to have the tendency ofsegregation with increasing of fragmentation;

3. The ash mixed with a few rice husk also showsbubbling fluidization, but after 5 min, segregationoccurs.

ŽIn order to purify the products, the ash of rice husk andw x.not the quartz and sand particles 1 is chosen to help the

rice husk to be fluidized. The key to the question of how to

feed the rice husk into the fluidized bed is given in Section4.2.

4. Thermal experiments

4.1. The process of silica white obtained from rice husk

The process of obtaining silica white from rice husk isshown in Fig. 2.

This paper focuses on the step of combusting rice huskin a fluidized bed.

4.2. Experimental apparatus

A schematic diagram of the experimental apparatus isshown in Fig. 3. The interior diameter of the fluidized bedis 84 mm. An electric furnace is surrounded out of the bed.To avoid temperature runway, the top of the bed is ex-posed to air. The height of the bed above the gas distribu-tor is 1.8 m, while that of the exposed section is 0.5 m.Three thermocouples were inserted to the bed at 1.7, 1.2and 0.7 m above the distributor, which are marked asthermocouples A, B, and C, respectively. Thermocouple Cis used to measure and control the temperature in the densephase zone. Air flows through the gas distributor and thenmixes with the ash. The carryover ash goes through thecyclone, and is collected in the collecting tank. The cy-clone is wrapped by heat belts, so the temperature is higherthan 1008C to prevent water from condensing and blockingit. The operation conditions of the fluidized bed are shownin Table 3.

The screw feeder with pressurized gas is used to pre-vent the high-temperature gas in the fluidized bed fromentering the screw feeder, burning with the rice husk, andleading to discontinuous feeding. The feeder is located inthe dilute zone, as shown in Fig. 3. In the dilute phasezone, the density of rice husk—which first drops andmixes with the hot air upward, then rapidly ignites andburns to remove the organic component during the falling

Table 2Experimental results in the cold fluidized bed

Materials Rice husk Orbicular ash The orbicular ash mixed with a few rice husk

Superficial gas velocity – 0.18–0.316 mrs 0.18–0.316 mrsQuality of fluidization channeling bubbling fluidization, rice husk subsiding, the ash in

segregation with the bubbling fluidization and after 5increase of fragmentation min or so, segregation

( )S. Huang et al.rPowder Technology 117 2001 232–238234

Fig. 2. Technology processing.

Fig. 3. Experimental setup for thermal model: 1—air intake; 2—electricfurnace; 3—flange; 4—thermocouple B; 5—motor; 6—screw feeder; 7—thermocouple A; 8—thermocouple C; 9—gas vent; 10—cyclone; 11—collecting tank; 12—gas distributor.

process—decreases, thus the tendency of segregation inthe dense phase zone reduces relatively.

5. Results

5.1. Temperature distribution

In the fluidized bed, heat is supplied by the furnacebefore the rice husk burns. In the steady state, heat is

Table 3Operating conditions of the fluidized bed

L U T Gg sy5Ž . Ž . Ž . Ž .m mrs K 10 kgrs

0.3 0.18–0.316 973–1073 3.25–7.40

Fig. 4. Axial temperature profile in the bed; U s0.18 mrs, G s5.35=g s

10y5 kgrs, ts3 h.

carried out by air and through the exposed section at thetop of the bed, and temperature fluctuation of point C iswithin the range of 208C. Fig. 4 shows that temperaturereduces sharply along the height of the fluidized bed. Themajor reaction occurs in the dense phase zone because thetemperature of the dense phase zone is higher than 7008C.

The variations of temperature also give the informationon the steady state of the bed, as shown in Fig. 5. Fig. 5shows that when rice husk is fed into the bed, the fluidizedbed shortly goes into the stable operating state. After 1 h,the values of temperature at three points remain constant,i.e., the steady state of the bed begins.

Figs. 6 and 7 show the variations of temperature withthe feed rate of rice husk and superficial gas velocity in the

Fig. 5. Temperature history in the bed; U s0.18 mrs, G s5.35=10y5g s

kgrs.

( )S. Huang et al.rPowder Technology 117 2001 232–238 235

Fig. 6. Variations of temperature with gas velocity in the bed; G s5.35s

=10y5 kgrs, Ts973 K.

Fig. 7. Variations of temperature with feed rate in the bed U s0.18g

mrs, Ts973 K.

bed. From Figs. 6 and 7, it can be seen that feed rate orsuperficial gas velocity in the experimental range has littleeffect on the variations of temperature in the fluidized bedduring this experiment.

Fig. 8. Variations of CO content with U in the bed.2 g

Fig. 9. Variations of CO content with U in the bed.g

Fig. 10. Carbon residue in the carryover ash.

5.2. Analysis of products

From Fig. 8, it can be seen that the content of CO ,2

which is analyzed by means of the Orsat apparatus, is

Fig. 11. Silicon dioxide content in the furnace ash.

( )S. Huang et al.rPowder Technology 117 2001 232–238236

Fig. 12. X-ray diffraction of silica white from carryover ash; G s5.35=10y5 kgrs, ts3 h.s

reduced slightly when the temperature and superficial gasvelocity are increased, while that of CO remains constantŽ .see Fig. 9 because of oxygen excess in the fluidized bed.

In order to obtain silica white powder, variations ofcarbon residue in the combustion are very important. Afterrice husk ash is leached to get rid of metallic elementswith a solution of HCl and carbon residue is removed byroasting, sampling results of carryover ash from the flu-

Ž .idized bed are obtained as shown in Fig. 10 . The contentof carbon residue in the carryover ash reduced quicklywith the increase of temperature or gas velocity. However,the content of carbon residue is 14.40% in the dense phasezone of the bed. All these show that the carbon in rice

husk is very difficult to remove entirely during the 4-hperiod.

Silica white was analyzed by means of hydrofluoricacid. The SiO content of furnace ash when superficial gas2

velocity is increased is shown in Fig. 11. Experimentalresults show that the operating condition of the fluidizedbed has no effect on the content of silicon dioxide, and thecontent remains at 97–99% during the 4-h period.

5.3. Analysis of silica white obtained from rice husk

After the ash is purified and roasted to obtain silicawhite, the quality of silica white is analyzed and compared

Fig. 13. X-ray diffraction of silica white from furnace ash; G s5.35=10y5 kgrs, ts3 h.s

( )S. Huang et al.rPowder Technology 117 2001 232–238 237

Table 4Silica white samples

Silica obtained from Silica obtained fromcarryover ash furnace ash

Appearance fluffy and white fluffy and whiteSpecific surface 108 110

2Ž .area m rgŽ .45 m screenings % 0.5 0.5

Apparent density 40.34 38.623Ž .kgrm

pH 5.5–6 5.5–6Ž .Heating loss % 1 1

Ž .Loss on ignition % 2 2Ž .SiO % 97.65 98.312

Ž .Fe % 0.52 0.15

Ž .with that of GB-precipitated silica ISO5794r1 and thatof pyrogenic silica standard. The standard is shown inAppendix A.

Figs. 12 and 13, which are XRD spectrograms obtainedŽby a Japan Drmax-RB X-ray diffraction instrument Cu.Target, 40 kV and 120 mA, 88rmin speed, 0.028 step ,

obviously show the amorphous component of SiO in ash2

in the collecting tanks and the dense phase zone. Further-more, the quality of silica white, including the elementcontent analyzed by an XRF-1700 fluorescence X-rayspectrum, the specific surface area determined by anAmerican CHEM-3000 specific surface area spectrum, and

Ž .the other parameters in GB10517-89 ISO5794r1 areshown in Table 4. Table 4 shows that the quality of silicawhite obtained in the fluidized bed from rice husk ishigher than that of GB-precipitated silica and approachesthat of pyrogenic silica, but not in specific surface area andmetal content.

6. Conclusions

Our experimental study on silica white obtained fromrice husk in a fluidized bed has shown that:

1. The ash with a little rice husk can be fluidized whenthe values of U is within the range of 0.18–0.316g

mrs.2. The screw feeder with pressurized gas can continu-

ously feed the rice husk into the fluidized bed and thescrew feeder located in the dilute zone can reduce thetendency of segregation of particles in the densezone.

3. Silica white is obtained in the fluidized bed becauseof the oxygen-excess combustion when the tempera-ture of dense zone is maintained at 973–1073 K.

4. The quality of silica white sampled from the flu-idized bed from rice husk is higher than that ofGB-precipitated silica and approaches that of pyro-genic silica but not in specific surface area and ironcontent.

7. List of symbols

a Ž .Width of the rice husk mGs Ž .Feed rate kgrsl Ž .Length of the rice husk mt Ž .Time hUg Ž .Superficial gas velocity mrsrs Ž 3.Particle density kgrmb Ž .Thickness of the rice husk mH Ž .Height mL Ž .Stagnatic height of rice husk mT Ž .Temperature Kr b Ž 3.Bulk density kgrm

Appendix A. Standard of silica white

Ž .The table shows the Degussa pyrogenic silica standard and GB-precipitated silica standard ISO5794r1 .

Degussa pyrogenic silica standard GB-precipitated silica standard

R972 130 200 300 380

Appearance fluffy and white2Ž .Specific surface area m rg 120 130 200 300 380 70–200

Ž .45 m screenings % 0.53Ž .Apparent density kgrm 50 40 –

pH 4–6 3.5–6 3.5–5.5 5–8Ž .Heating loss % 3 1.5 4–8

Ž .Loss on ignition % 5 3 7Ž .SiO % 98.3 99.8 902

Ž .Al % -0.05 0.5Ž .Fe % -0.01 -0.003 0.15

XRD analysis amorphous

( )S. Huang et al.rPowder Technology 117 2001 232–238238

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