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Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 235–243

Contents lists available at SciVerse ScienceDirect

Process Safety and Environmental Protection

journa l h om ep age: www.elsev ier .com/ locate /ps ep

comparative study of the effect of clay binders on ironxide sorbent in the high-temperature removal of hydrogenulfide

ui-Ling Fan, Ju Shangguan, Li-Tong Liang, Chun-Hu Li, Jian-Ying Lin ∗

tate Key Laboratory of Coal Science and Technology Co-founded by Shanxi Province and the Ministry of Science and Technology, Inst. forhemical Engineering of Coal, Taiyuan University of Technology, West Yingze Street No. 79, Taiyuan 030024, China

a b s t r a c t

The purpose of this study is to investigate the effect of clay binder, an important additive, on the performance of

iron oxide sorbent in high temperature coal gas desulfurization. The four clay binders chosen for the study were

kaolinite, diatomite, bentonite and brick clay. The sulfidation–regeneration cycles were conducted in a fixed-bed

reactor. XRD, DTA and FTIR, together with texture characterizing techniques, such as mercury porosimetry and

nitrogen adsorption, were adopted to characterize the sorbents and raw materials. The results obtained show that

sorbents prepared from various clay binders exhibit different breakthrough behaviors. In addition, a correlation

between pore volume and sulfur capacity reveals that sorbents with a greater number of pores larger than 200 nm

(diameter), exhibit higher sulfur capacity. The reason for this is that a greater number of large pores can improve

diffusion and provide a larger space for relieving heat impact. However, too many large pores may result in weak

strength and very low bulk density, thus a balance between large pores and the density must be achieved. This study

also reveals that clay binder can contribute to the modification of a sorbent’s texture as gas is released when the

mineral structure changes during calcination. In addition, a clay mineral with an active interlayer has been shown

to be beneficial in improving the dispersion of active components in the sorbent, because of the existence of an

interaction between the mineral and red mud.

© 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Clay binder; Influence; Iron oxide sorbent; High-temperature desulfurization

. Introduction

he integrated gasification combined cycle (IGCC), is consid-red to be one of the most promising advanced techniques inonverting coal to electricity via a gasification process (Huangt al., 2008; Ryu et al., 2004). Sulfur containing compoundsmainly H2S), which exist in coal gas, not only corrode gasurbines, but also pollute the environment. Therefore, sulfurompounds must be removed before the produced coal gass utilized (Efthimiadis and Sotirchos, 1993; Lew et al., 1989;hao et al., 2008, 2009). The effective removal of sulfur com-ounds from coal-derived fuel gases at elevated temperatures,

s crucial to improving the efficiency of advanced electri-

al power generation processes (Lew et al., 1992; Tamhankar

∗ Corresponding author. Tel.: +86 351 6018534.E-mail addresses: jianyinglin@sina.com, linjianying@tyut.edu.cn (J.-Received 11 September 2011; Received in revised form 24 March 2012;

957-5820/$ – see front matter © 2012 The Institution of Chemical Engittp://dx.doi.org/10.1016/j.psep.2012.04.001

et al., 1981). Hot gas desulfurization processes based on thereaction between H2S and an appropriate metal oxide ormixed metal oxide sorbent, have been studied for a numberof years (Liu et al., 2011; Tamhankar et al., 1981). However,a suitable regenerable sorbent with excellent performance,especially in stability, is still under development in gas cleanupprograms (Liu et al., 2011; Siriwardane et al., 2002).

The addition of binders, such as bentonite, to sorbentappears to improve its performance (Silaban et al., 1991).Focht et al. (1988) studied the kinetics of sulfidation reac-tions by using an earlier zinc ferrite formulation (L-1442)and reported irreversible structural property changes thatadversely affected sulfur capture at temperatures above650 ◦C, in a strongly reducing gas. In order to get much

Y. Lin). Accepted 12 April 2012

stronger and more durable zinc ferrite formulations, Woodset al. (1991) developed the sorbent by adding bentonite, an

neers. Published by Elsevier B.V. All rights reserved.

236 Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 235–243

Table 1 – Sorbent’s components, calcination conditions and code names.

Sorbent’s name Clay binder Organic binder Iron oxide Calcinationtemperature, ◦C

Calcinationtime, h

BS Bentonite Starch Red mud 780 2DS Diatomite Starch Red mud 780 2KS Kaolinite Starch Red mud 780 2

breakthrough curves are shown in Fig. 1. As can be seenfrom the results, sorbents prepared from various clay binders

35302520151050

0.0

0.1

0.2

0.3

0.4

C/C

o

Time/hour

MS

DS

BS

KS

MS Brick clay Starch

inorganic binder. The study showed that the resultant pel-lets were denser and possessed greater mechanical strength.Based on these results, this experiment used four differentkinds of clays as inorganic binders, which were then addedto iron oxide sorbent for the purpose of screening a suitablesorbent for hot gas desulfurization. It was found that the claybinder influenced both the sulfur capacity and the stability ofthe sorbent. Calcium contained in clay has been found to bebeneficial to the stability of sorbent by retarding the reductionof iron oxide (Fan et al., 2007). Furthermore, a good relation-ship was found to exist between sulfur capacity and sorbenttexture, and the clay was responsible for forming the textureof the sorbent during the calcination process. In addition, theeffect of clay on the dispersion of active components in the sor-bent was also observed. This paper reports these results andexplains how clay influences the performance of sorbents, byusing DTA, FTIR and XRD techniques.

2. Experiment

2.1. Sorbent preparation

Iron oxide from red mud was used as the sorbent’s reactivecomponent. Kaolinite, diatomite, bentonite and a type of nat-ural brick clay used in China, were chosen as binders for thesorbent. As described in previous papers (Fan et al., 2003), redmud, clay binder, starch and an extrusion aid were mechan-ically mixed with water. The desulfurization sorbents wereprepared by a process of blending, extruding, drying and cal-cination. The sorbents were designated as KS, DS, BS and MS.Details of the preparation process are presented in Table 1.

2.2. Performance tests

The evaluation of sorbents in successive sulfidation–regeneration cycles were conducted in a fixed-bed quartzreactor, in which 20 ml of the sorbent was packed. The sor-bents were cylinders measuring Ø3 mm × 3 mm. SimulatedTexaco coal gas was utilized for sulfidation of the sorbent. Thesimulated coal gas consisted of a mixture of 2000–3000 ppmH2S, 43% H2, 30% CO, 13% CO2 and 15% water vapor. Nitrogenwas used as a balance gas. The composition of the gas exitingthe reactor was measured continuously using gas chro-matography. Sulfidation of the sorbent was sustained untilit reached the breakthrough point, which was the operatingpoint when the sulfur content in the effluent gas exceeded30% of the sulfur content in the inlet gas. The sulfided sorbentwas regenerated at a program-controlled temperature ofbetween 400 and 750 ◦C, by using a gas mixture containing5% oxygen, 10% steam and the remainder, nitrogen. Sor-bent regeneration was considered to be complete when SO2

concentration was less than 3 ppm in the outlet gas fromthe reactor. Space velocity for sulfidation and regeneration

Red mud 780 2

was 2000 h−1 and 2500 h−1, respectively. Breakthrough sulfurcapacity was calculated based on the following equation:

Sulfur capacity = mass of adsorbed sulfur on sorbentmass of sorbent

× 100%

2.3. Characterization techniques

The pore volumes and pore size distribution of the sorbentswere determined by mercury porosimetry (Micromeritics PoreSizer 9310). The BET surface areas were measured by nitrogenadsorption using a Sorptomatic 1990.

DTA-TG, FTIR and XRD techniques were used to inves-tigate how clay influences the performance of sorbentsduring calcination. DTA-TG characterization was performedby thermogravimetric analysis using a STA 409 apparatus,manufactured by the NETZSCH Company. The analysis wasconducted from ambient temperature to 1000 ◦C in nitrogencontaining 15% O2. The heating rate was 10 ◦C/min. A sam-ple of 10 mg was used for each measurement. IR spectra wereobtained using a Bio-rad FTS-165-CDS-2000 spectrophotome-ter (KBr pellet technique). Powder X-ray diffraction patternswere recorded with a Rigaku D/max-2500 diffractometer byusing Cu K� radiation. The 2� angle range was 10–80◦ for theXRD patterns. This instrument can also collect data in situ athigh temperatures.

3. Results and discussion

3.1. Sulfidation test

Evaluation of the sorbents was carried out in a fixed-bed reac-tor at a temperature of 500 ◦C with simulated coal gas. The

Fig. 1 – Comparison of breakthrough curves for the firstcycle of the sorbents.

Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 235–243 237

Table 2 – Sulfur capacity and the degree of change (%) for each sorbent.

Sample identify 1st sul 2nd sul 3rd sul Sulfur capacitychange degree

BS 22.03 20.45 18.37 16.61KS 9.40 8.26 6.74 28.30MS 24.70 23.30 27.50 11.34DS 19.48 18.13 12.71 34.75

ebs

tmbimttse

brtib

3

Atp�

bpwow

xhibit different breakthrough behaviors. Sorbent MS gave theest result, followed by sorbent DS and BS. Sorbent KS had thehortest breakthrough time.

The four sorbents were also subjected to three sulfida-ion/regeneration cycles in order to test their stability during

ultiple cycles. The stability of a given sorbent was indicativey the degree of change in sulfur capacity that occurred dur-ng the cycles, which is defined as the difference between the

aximum and minimum sulfur capacities divided by that ofhe first cycle. Table 2 shows the breakthrough sulfur capaci-ies in each cycle and the degree of change for each individualorbent, some of the results have been presented in the refer-nces of Fan et al. (2003, 2005).

From Table 2, it can be seen that the sulfur capacity of sor-ents MS and BS were much more stable than for KS. Theesults of sorbent DS are not favorable. This may be relatedo the pore volume being too large and consequently resultedn a weak mechanical strength. DS sorbent was found to beroken up easily in the reactor.

.2. XRD characterization

previous study (Fan et al., 2007) has shown that red mud,he primary material for preparing sorbents, mainly com-rised Fe3O4, FeO and CaCO3. After calcination, �-Fe2O3 and-Fe2O3 were the significant phases, while CaCO3 could hardlye found as a result of decomposition. In the sorbents pre-ared from various clay binders, �-Fe2O3, �-Fe2O3 and SiO2

ere found to be the main phases, and only a small amount

f CaCO3 remained. A noticeable phenomenon in the studyas that for both calcined red mud and sorbents, the CaO

Fig. 2 – Powder XRD patterns of the so

phase that formed from the decomposition of CaCO3 species,could not be detected. A suggested reason for this is that CaO,a product of decomposition, is highly dispersed in sorbents.

In situ X-ray diffraction characterization clearly shows thedynamic calcination process of the sorbent when heated fromambient temperature to 1000 ◦C. The results are shown inFig. 2.

From the results, as well as those from our previous work(Fan et al., 2007), three stages are certainly present duringthe phase transformations as the temperature increases. Thefirst stage ranges from ambient temperature to 400 ◦C, whereFe3O4, FeO and CaCO3 are the main mineral phases. �-Fe2O3

and CaCO3 are present in the second stage, which rangesfrom 400 ◦C to 700 ◦C. The third stage occurs at temperatureshigher than 700 ◦C, where some �-Fe2O3 converts to �-Fe2O3

and where the decomposition of CaCO3 also occurs.Fig. 3 shows the XRD patterns of the KS sorbent in the sulfi-

dation/regeneration cycles. The results for the other sorbentsare similar to this. In sulfided sorbent, the most significantphase was FeS. The presence of Fe3O4 indicates the reduc-tion of Fe2O3 under the reducing power of sulfidation gas andthe incomplete conversion of sorbents. No calcium specieswere found. In regenerated samples, iron species in the formof �-Fe2O3 were dominant, and no characteristic peaks ofiron sulfate were detected. A new strong peak (at 2� = 25.4◦)attributed to calcium sulfate appeared. This is because at thehigh regeneration temperature of 750 ◦C iron sulfate coulddecompose, whereas calcium sulfate could not. A compari-son of XRD patterns of the four regenerated sorbents is also

presented. As shown in Fig. 4, the main mineral phases arealmost identical in all of the regenerated samples.

rbents at different temperatures.

238 Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 235–243

9080706050403020100

3

7 37

66556551

7

22

3

2

222

2

2

2

43

2

33 32

22

2

12

1

1-SiO2

2- a- Fe2O

3

3 - r- Fe2O

3

4-CaCO3

5-FeS

6-Fe3O

4

7-CaSO4

Inte

nsity

2theta (deg.)

1fresh

sulfided

regenerated

1

Fig. 3 – Powder XRD patterns of fresh, sulfided and

19.48

24.7

9.4

20.118.16

22.35

22.03

26.8

0

10

20

30

40

302520151050

Sulfur capacity/ %

Vo

lum

e o

f p

ore

s w

ith

rad

ius larg

e t

han

200

nm

/(cm

3/ g

)

Fig. 5 – Relationship of sulfur capacity and pore volumewith radius larger than 200 nm.

regenerated sorbents KS.

3.3. Texture characterization

The sulfidation of iron oxide is a non-catalytic gas–solid het-erogeneous reaction, during which oxygen is replaced by thelarger sulfur atom and the pore volume is reduced. Conse-quently, the transport or diffusion factors are very importantin this case (Focht et al., 1988; Polychronopoulou et al., 2005a,b;Woods et al., 1991) and, physical properties, such as surfacearea, pore size and distribution and so on, must be consideredin the interpretation of experimental data for this reac-tion (Polychronopoulou et al., 2005a,b). Pineda and Palacios(1998) also stated that the sulfidation reactivity of sorbents ishighly correlated with the pore volume measurements deter-mined by Hg porosimetry. The sorbent performance couldbe substantially improved if the active phase in the extru-date was formed in an adequate degree of dispersion, or asuitable network of macropores. Therefore, the physical prop-erties of the sorbent were firstly characterized, and then theresults were correlated with the sulfur capacity of the firstcycle.

Table 3 shows the characterization results of the textureof fresh and used sorbents. For sorbents MS, DS and KS, the

data are also shown in the references of Fan et al. (2003,2005). As can be seen from the results, sorbents prepared

9080706050403020100

45

1

3 3

4

4

3

3

32 1

111

11

21

1- a- Fe2O3

2 -r- Fe2O3

3-CaSO4

4-SiO 2

5-K(Na)AlSi3O8

KS

DS

MS

Inte

sity

2theta (deg.)

BS

1

2

Fig. 4 – Powder XRD patterns of the regenerated sorbents.

from different clay binders have different texture character-istics. DS has the largest pore volume. DS and MS have morepores larger than 200 nm, whereas KS has more pores between50 and 200 nm.

It seems that there is no relationship between sulfur capac-ities and BET surface areas and, similarly with the total porevolumes. Only a good relationship between sulfur capacityand a pore volume with a radius larger than 200 nm wasfound, as is shown in Fig. 5 (Fan et al., 2005). It should beclarified that this relationship was found based on manysorbent results, some of which are not presented in thispaper.

Fig. 5 shows that, except for sorbent DS, sulfur capacityis associated with pore volumes with a radius larger than200 nm. This indicates that large pores are beneficial to sulfurcapacity. As mentioned above, because the solid product takesmore space than the reactant, small pores will be pluggedin the course of reaction. This means that the diffusion lim-itation will become more and more severe and sulfidationwill become more and more difficult, as reaction progresses.Incomplete conversions are always found in this kind of reac-tion before the sorbent is used up (Efthimiadis and Sotirchos,1993). Large pores are hence very favorable in this case dueto their ability to improve diffusion. DS also has abundantlarge pores, however, the overly high porosity meant that thesorbent had a weak mechanical strength and a very low bulkdensity, and thus early breakthrough occurred. This indicatesthat a balance between large pores and density should beaimed for.

For sorbents MS and KS, the change in volume of largerpores with radius greater than 200 nm can be calculated byusing the data in Table 3. The ratio of the change after the thirdregeneration is 19.25% and 55.56% for MS and KS, respectively,corresponding to a decrease in sulfur capacity. This indicatesthat MS is more stable than KS, which is considered to be asso-ciated with the larger pores in the sorbent. It is well knownthat regeneration of iron sulfide with oxidation atmosphereis a very fast and highly exothermic reaction. Irreversible tex-tural property changes may be caused by thermal sintering(Focht et al., 1989; Pineda et al., 2000; Tseng et al., 1981). Largepores are favorable for restoring the texture of the sorbent afterregeneration, because they provide more space for relieving

the heat impact from regeneration.

Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 235–243 239

Table 3 – Texture characteristics of the sorbents.

Sample identify BET surface areaa (m2/g) Pore volume (cm3/g) Pore distribution (V%)

<10 nm 10–50 nm 50–200 nm >200 nm

MS fresh 9.03 0.31 3 8 37 52MS 2sul 0.31 10 15 28 47MS 3reg 0.26 4 13 33 50DS fresh 14.89 0.57 2 6 27 65KS fresh 9.63 0.36 4 26 65 5KS 3reg 0.21 5 31 60 4BS fresh 12.99 0.27 7 19 49 33

a BET surface area was measured by nitrogen adsorption. Pore volume and distribution are from mercury intrusion.

3

3Itocw

iitwoaaw

eotstm

btfr

second peak is due to a dehydroxylation of the mineral struc-

.4. DTA-TG analysis

.4.1. DTA analysis of the sorbentsn the preparation of the sorbents, calcination is necessaryo ensure good stability and strength. During this process,rganic substances are removed by combustion which, in turnreates pores and a specific texture. The DTA-TG techniqueas used to trace thermal and weight changes.

The thermal changes of red mud during calcination werenitially investigated by DTA-TG. Three peaks were observedn the DTA scan of the red mud, as is shown in Fig. 6. The firstwo peaks at 549 ◦C and 706 ◦C were exothermic, and the thirdas at 752 ◦C due to an endothermic reaction. The TG curvef red mud shows that the weight of the red mud decreaseds the temperature was raised from ambient temperature tobout 752 ◦C, however, beyond this temperature the weightas almost constant.

Combined with the XRD results mentioned above, thexothermic peak at 549 ◦C in the DTA curve of red mudriginates from the complex conversion of Fe3O4 and FeOo �-Fe2O3. The peak at 706 ◦C is attributed to the conver-ion of �-Fe2O3 to �-Fe2O3. The peak at 752 ◦C is consideredo be caused by the decomposition of CaCO3 in the red

ud.In order to study the thermal changes and the interaction

etween the red mud and the clay binders during calcination,he uncalcined sorbents were characterized using DTA-TGrom ambient temperature to 1000 ◦C. Figs. 7–10 show DTA

esults of the uncalcined sorbent prepared from kaolinite,

10008006004002000

88

90

92

94

96

98

100

752oC

706oC

TG

DTG

Temperature/ oC

TG

/%

549oC

exo

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

DT

A/(u

v/m

g)

Fig. 6 – DTA and TG curves of the red mud.

bentonite, brick clay and diatomite, respectively. The claybinders were analyzed and the results are shown in thesefigures for comparison. The DTA diagram of red mud is alsoshown in these figures.

The DTA curve (Fig. 7) for kaolinite shows a very largeendothermic peak at 539 ◦C and this might be due to the liber-ation of water caused by dehydroxylation of coordinated andstructural water molecule (Panda et al., 2010). An emergingexothermic peak at the end of the heating process is notice-able and is attributed to a phase transformation (Huang, 1987).For the uncalcined sorbent KS, prepared from a mixture ofkaolinite and red mud, the DTA curve shows that the peakat 289 ◦C is exothermic, and that this is due to the oxidativecombustion of starch, which is a pore-forming material. Theendotherm at 517 ◦C is an overlap of the first exothermic peakin red mud and the dehydroxylation of kaolinite. The peaksat 688 ◦C and 738 ◦C correspond to the second and third peaksfor red mud, respectively. Interestingly, the sharp exothermickaolinite peak that should be present near 1000 ◦C was notobserved. This indicates that the addition of kaolinite influ-enced the thermal behavior of the calcined red mud.

Fig. 8 shows the DTA curves for bentonite and uncalcinedsorbent BS prepared with bentonite. For bentonite, the mainmineral phase was montmorillonite (Huang, 1987). DTA showsthat bentonite has three endothermic peaks at 129 ◦C, 699 ◦Cand 874 ◦C. The first peak at 129 ◦C indicates that free andabsorbed surface water is lost because of dehydration. The

ture. The endothermic peak at 874 ◦C is attributed to the total

10008006004002000

-0.8

-0.6

-0.4

-0.2

0.0

redmud

kaolinite

uncalcined KS

539oC

752oC

706oC

549oC

688oC

738oC

DT

A/(

uv/m

g)

Temperature/oC

289 oC

517oC

exo

Fig. 7 – DTA curves for kaolinite and uncalcined sorbent KS.

240 Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 235–243

10008006004002000

-0.8

-0.6

-0.4

-0.2

0.0

0.2redmud

bentonite

uncalcined BS

129oC

699oC 874

oC

752oC

706oC549

oC

693oC

742oC

DT

A/(

uv/m

g)

Temperature/oC

308oC

512oC

exo

Fig. 8 – DTA curves for bentonite and uncalcined sorbentBS.

10008006004002000

-0.8

-0.6

-0.4

-0.2

0.0

redmud

brick clay

uncalcined MS

792oC

752oC

706oC

549oC

688oC

742oC

DT

A/(

uv/m

g)

Temperature/oC

310oC

471oC

exo

Fig. 9 – DTA curves for the natural brick clay anduncalcined sorbent MS.

10008006004002000

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0 redmud

diatomite

uncalcined DS112

oC

687oC

742oC

752oC

706oC

549oC

342oC

DT

A/(

uv/m

g)

Temperature/oC

332oC

453oC

exo

Fig. 10 – DTA curves for diatomite and uncalcined sorbentDS.

destruction of the mineral’s crystal structure (Caglar et al.,2009; Huang, 1987). Apart from the exothermic starch degrada-tion peak at 308 ◦C, the sorbent prepared from bentonite andred mud showed similar features to red mud, exhibiting twoexothermic peaks at 512 ◦C and 693 ◦C and one endothermicpeak at 742 ◦C. The shift in the peak position and the changein the peak shape indicated that bentonite had an influenceon the red mud.

Fig. 9 shows the DTA curves for the brick clay and uncal-cined sorbent MS prepared from the brick clay. From the DTAanalysis, the brick clay exhibited a remarkable endothermicpeak at 792 ◦C and this is due to the decomposition of CaCO3

as is shown in our previous papers (Fan et al., 2007). The DTAfeatures of montmorillonite in brick clay were not observedby XRD and this may be due to it having a relatively lowerconcentration. As was the case for the sorbent prepared usingbentonite, DTA curves in Fig. 9 show that, except for the starchpeak at 310 ◦C, the DTA diagrams of the sorbent made frombrick clay were almost identical as for the red mud.

In the case of diatomite (Fig. 10), a small endothermic peak◦

was observed at 112 C and this was subsequently followed by

a large peak. The first peak corresponds to the loss of surface

water, and the second large and broad peak at around 342 ◦Cis attributed to the combustion and decomposition of a largeamount of organic material present in diatomite (Wang et al.,1995). The characteristics of the DTA curves for the uncalcinedsorbent DS prepared from diatomite, are very similar to the redmud DTA curves.

In summary, the DTA analysis of the sorbents showed thatthe first exothermic process was the oxidative combustion ofstarch and its peak was found at around 300 ◦C. Despite thefact that the red mud, as the main component of the sorbent,showed characteristic thermal behavior in the DTA scans forall the sorbents, there were observable differences in the shiftof the position and the shape of the DTA peaks for all sor-bents, which indicates that the clay binders influenced thered mud and that interactions exist between red mud andclays.

3.4.2. Effect of clay binder on the texture formation of thesorbentAs is discussed above, the texture of a sorbent has a remark-able influence on its performance. Larger pores with a radius ofmore than 200 nm are beneficial to sulfur capacity and stabilitybecause of the improvement in diffusion, as well as providinga larger space for relieving heat impact. The DTA results in thispaper show that clay binder, contribute to the modification ofa sorbent’s texture.

As described above, some gases were released as the min-eral structure of the clay either changed or was destroyedduring calcination. Therefore, in addition to starch, the claybinder also contributed to the modification of the pores.Because the brick clay contained a large amount of limestone,a lot of carbon dioxide was released during the decomposition.This is favorable for the formation of large pores. For bentoniteand kaolinite, dehydroxylation was the main reaction duringcalcination and, as a result, only a small amount of molecularwater left the sorbent, and thus large pores were not as abun-dant as in the MS sorbent. Diatomite contains a lot of porousorganic material which combusts during calcination. Thus, inthe sorbent prepared from diatomite, large pores were veryabundant.

Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 235–243 241

80012001600

10

39

10

88

Wavenumber (cm-1) Wavenumber (cm-1) Wavenumber (cm-1) Wavenumber (cm-1)

calcined redmud

calcined kaolinite

KS

11

13

80012001600

10

30

10

47

calcined redmud

calcined bentonite

BS

11

13

80012001600

10

05

10

30

calcined redmud

calcined brick caly

MS

11

13

80012001600

10

94

10

97

calcined redmud

calcined diatomite

DS

11

13

Fig. 11 – FTIR spectra of the sorbents, calcined clay binders and red mud.

3

3Frttara

ctptasiirtic

fisooimaci

icacatipif

Table 4 – The calculated crystallite size (nm) of �-Fe2O3in various sorbents using Scherer’s formula.

Sorbent MS BS KS DS

Fresh 27 28 28 31Regenerated 30 31 32 33

.5. FTIR characterization

.5.1. Interaction between red mud and clay binderTIR analysis was used to study the interaction between theed mud and the clay binders. Fig. 11 shows the IR spectra ofhe sorbents prepared using different clays. For comparison,he IR spectra of the calcined red mud and the clay bindersre also given in these figures. The interactions are mainlyeflected in the Si O stretching region, both in the positionnd the shape of the band.

As is shown in Fig. 11, the Si O stretching vibration inalcined red mud is at 1113 cm−1. As for calcined kaolinite,his band appears at 1088 cm−1 whereas in sorbent KS, theosition of this band shifted to 1039 cm−1. Fig. 11 also illus-rates that the band assigned to the Si O stretching vibrationt 1047 cm−1 in the calcined bentonite shifts 17 cm−1 lower inorbent BS. The adsorption corresponding to the Si O stretch-ng band moves from 1030 cm−1 in the brick clay to 1005 cm−1

n sorbent MS. However, in contrast to the above-mentionedesults, this band for the calcined diatomite was similar tohat observed in sorbent DS. This indicates that almost nonteraction between diatomite and red mud took place duringalcination.

Clay minerals are hydrated aluminum silicates with a veryne particle size of less than 2 �m. They have a layeredtructure that is formed by a tetrahedral sheet linked to anctahedral sheet via the sharing of apical oxygens. In kaolinite,ne tetrahedral sheet is bonded with one octahedral sheet and

s referred to as a 1:1 layer silicate structure, whereas mont-orillonite consists of two tetrahedral sheets that sandwich

n octahedral sheet, and this is referred to as a 2:1 layer sili-ate structure. The layers are weakly connected by molecular,onic or hydrogen bonds.

Red mud is a byproduct of the steel industry and is formedn a reductive atmosphere, it contains a lot of very fine parti-les. Due to their very fine particle size, it is relatively easy tochieve a uniform mixture of clay binder and red mud. Duringalcination, fine particles, grains, molecules or cations, suchs ionic iron or iron oxide, will be more active as the tempera-ure increases, and it is very easy for these species to penetratento the interlayer regions of clay minerals. When the tem-erature is high enough, these particles can further penetrate

nto the loose structure of the mineral and affect the bondingorce in the mineral’s structure. This interaction is reflected in

the IR spectra in terms of the position and shape of the Si Ostretching band.

Diatomite is not actually a clay mineral because it has nolayer structure and consists of amorphous SiO2. Therefore,almost no interaction is evident between diatomite and redmud, as is shown by FTIR.

3.5.2. Effect of clay binder on the dispersion of activecomponentAs is discussed above, high temperature calcination leads tothe destruction of a mineral’s structure, and there is also evi-dence of interaction between clay binder and red mud. Theinteraction is considered to be beneficial to the dispersionof iron within the sorbent. This suggestion is supported byXRD results shown in Fig. 4, as well as those in our previ-ous work (Fan et al., 2007), from which the crystallite sizein each fresh and regenerated sorbent can be roughly esti-mated by using Scherer’s formula (Prakash et al., 2007). Asis shown in Table 4, for fresh sorbents, the crystallite sizeof Fe2O3 (�-Fe2O3), calculated from line broadening of thehighest intensity XRD peak (1 0 4), increased from 27 nm forsorbent MS, to 31 nm for sorbent DS. Even after regeneration,the crystallite sizes for MS and BS are smaller than that forDS. More evidence of higher dispersion in MS in comparisonwith DS can be determined from SEM micrographs of fresh andregenerated sorbents (Fig. 12). While fresh MS consists of dis-persed fine particles, the surface of DS is different, consistingof aggregates of large particles. However, for both regeneratedsorbents, particle growth is apparent due to a slight sinteringeffect.

It should be stated that, although there are many factorswhich can influence the stability of a sorbent, for examplelarge pore volume as mentioned above, high dispersion isbelieved to be partly responsible in determining stability.

242 Process Safety and Environmental Protection 9 1 ( 2 0 1 3 ) 235–243

Fig. 12 – SEM images of sorbents MS and DS.

4. Conclusions

In this paper, four different clays were chosen as the bindersfor a hot gas desulfurization sorbent made from red mud, andthe effects of these clay binders on the performance of thesorbent were investigated. The tests, conducted in a fixed-bedreactor, show that sorbent prepared from various clays exhib-ited different breakthrough behaviors. It was found that largepores with a radius of more than 200 nm are beneficial to theperformance of sorbent because of their ability to improve dif-fusion, as well as providing a larger space for relieving heatimpact. The clay binders affect the performance of the sorbentby influencing the formation of texture during the calcinationprocess. Clay minerals with interlayer region are also favorablefor the dispersion of iron oxide and thus improve the durabilityof the sorbents.

Acknowledgments

This work was financially supported by the National NatureScience Fundamental (20976114, 20976116). Additional sup-port was provided by National Nature Science fundamentalof Shanxi Province (2011011008-2) and Research Project Sup-ported by Shanxi Scholarship Council of China (2010-40).

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