Characterization of Na and Ca Zeolites Synthesized by Various Hydrothermal Treatments of Fly Ash

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Advances in Civil EngineeringMaterials

Nevin Koshy,1

D. N. Singh,2

Bhagwanjee Jha,3

Srinivas Kadali,4

and

Jayant Patil5

DOI: 10.1520/ACEM20140048

Characterization of Naand Ca ZeolitesSynthesized by VariousHydrothermal Treatmentsof Fly Ash

VOL. 4 / NO. 1 / 2015



Nevin Koshy,1 D. N. Singh,2 Bhagwanjee Jha,3 Srinivas Kadali,4 and Jayant Patil5

Characterization of Na and Ca ZeolitesSynthesized by Various HydrothermalTreatments of Fly Ash

Reference

Koshy, Nevin, Singh, D. N., Jha, Bhagwanjee, Kadali, Srinivas, and Patil, Jayant,

“Characterization of Na and Ca Zeolites Synthesized by Various Hydrothermal Treatments

of Fly Ash,” Advances in Civil Engineering Materials, Vol. 4, No. 1, 2015, pp. 131–143,

doi:10.1520/ACEM20140048. ISSN 2165-3984

ABSTRACT

For the past several decades, researchers have studied the zeolitizationof coal fly ash (class-F) by following different methods (viz., open and

closed hydrothermal, and fusion followed by hydrothermal). In fact, these

methods involve sequential processes like (i) dissolution of silica and

alumina from the fly ash, (ii) nucleation of zeolite, and (iii) crystallization

(growth of zeolite) in the reactant solution. Also, performance of these

processes has been reported to vary with the type of alkali used as reactant

and often, NaOH has been preferred for high cation exchange capacity,

resulting in sodium zeolites. However, large scale applications of Na-based

zeolites in soil and water are questionable due to the presence of high

sodium, thereby increasing the sodicity and salinity of the soil/water. In

addition, performance of the zeolites, as adsorbent, synthesized by different

methods is expected to depend on various characteristics (viz., mineralogy,

structural bonding, specific surface area, pore volume, and morphology), of

the zeolites. In order to address the above issues, the present study is

focused to investigate the various characteristics of the synthesized zeolites

by (i) the above mentioned three methods, (ii) using Ca(OH)2 as reactant,

and (iii) considering Na and Ca present in the fly ash. Thus, the aim of the

study was to ascertain (i) a suitable method out of the three and (ii)

characteristics of the blend of Na- and Ca-zeolites from the fly ash, which can

Manuscript received December 18,

2014; accepted for publication

April 21, 2015; published online

May 20, 2015.

1

Research Scholar, Department of

Civil Engineering, Indian Institute

of Technology Bombay, Powai,Mumbai 400076, India,

e-mail: [email protected]

2

Institute Chair Professor,

Department of Civil Engineering,

Indian Institute of Technology

Bombay, Powai, Mumbai 400076,

India (Corresponding author),

e-mail: [email protected]

3

Head, Department of Civil

Engineering, Government

Polytechnic, Karad (DP), Dadra

and Nagar Haveli 396240, India,e-mail: [email protected]

4

Assistant General Manager,

Powerdeal Energy Systems

Pvt. Ltd., Nashik 422010,

Maharashtra, India, e-mail:

srinivas.kadali@powerdealenergy.

com

5

Assistant General Manager,

Powerdeal Energy Systems

Pvt. Ltd., Nashik 422010,

Maharashtra, India, e-mail:

[email protected]

CopyrightVC 2015 by ASTM International, 100 Barr Harbor Drive, P.O. Box C700, West Conshohocken, PA 19428-2959 131

Advances in Civil Engineering Materials

d o i: 1 0. 1 52 0 /A CEM 20 1 40 0 48 / Vol. 4 / N o . 1 / 2 0 15 / a va ila b le o nline a t w w w. a stm .o rg



be used as a controlled release fertilizer, as sorbent for water and soil

decontamination.

Keywords

fly ash, lime, hydrothermal treatment, calcium zeolite, sodium zeolite, pore characterization

Introduction

Over the years, fly ash from power plants and coal based industries has been estab-

lished as a potential industrial by-product for waste valorization, especially due to

the enormous quantity being produced annually across the globe and the environ-

mental impacts associated with it. Out of 500 million tons of fly ash generated glob-

ally, India alone accounts for around 130106 tons [1]. In this context, finer

fraction of raw fly ash (Class F), RFA, has been used as a source material for synthe-

sis of the microporous aluminosiliceous minerals known as zeolites, which are

well-known for their highly porous crystalline structures and cation exchange char-acteristics. Incidentally, naturally occurring zeolites (natural zeolites) have been used

in construction and building materials as anti-bacterial agent, expansion inhibitor in

concrete, and humidity controller [2,3].

During alkali activation and simultaneous heating, the amorphous glass (65 %-

78 %) and quartz present in fly ash takes part in the zeolitization by initial dissolu-

tion into the solution phase, while mullite remains as the comparatively

non-reactive mineral phase [4,5]. Among the other oxides and unburnt carbon pres-

ent in the RFA, and their role in zeolitization by using NaOH, as reactant, specific

attention has been given to its lime content, i.e., over 25 % [6] and 38 % [7], which

have been reported to affect the reactivity of the fly ash [8]. In fact, in aqueousmedia, lime (i.e., calcium oxide) reacts to form calcium hydroxide, which has less

solubility in water and is also likely to get precipitated as calcium carbonate during

zeolitization reactions [9]. However, on a positive note, lime helps to raise the pH of

the solution, along with NaOH, and also replaces the native Naþ ions in sodium zeo-

lites with Ca2þ ions; hence it may give rise to some calcium exchanged zeolites,

which have potential applications as controlled-release fertilizers, adsorbents and

catalysts [10,11]. Based on literature, large scale applications of Na-based zeolites in

soil and water are not recommended for agro- and aqua-based products due to rise

in their sodium content, which are responsible for increasing the sodicity and salin-

ity of the soil/water [12,13]. In this context, challenging issues include (i) reduction

in adversity of Na-based fly ash zeolites when applied in soil and water, and (ii)

increase in Ca-exchanged zeolites. In addition, major parameters influencing effec-

tiveness of these zeolites, as adsorbent, in soil and water remediation are their char-

acteristics (viz., cation exchange capacity, specific surface area, pore volume,

mineralogy, structural bonding, and morphology), which have also been reported to

vary differently corresponding to the adopted methods (viz., open hydrothermal,

closed hydrothermal, and fusion followed by hydrothermal) of synthesis of fly ash

zeolites. In this context, some studies were conducted by the previous researchers

using seawater [14,15], which has been ascertained to be effective for better nuclea-

tion and growth of zeolitic structure. Accordingly, the role of carbonates and sodium

chloride in the solution phase during crystallization is worthy of further

KOSHY ET AL. ON CHARACTERIZATION OF NA AND CA ZEOLITES 132




investigation since such issues have not yet been explored in detail in the past for

synthesis of a blend of Na- and Ca-based zeolites, dominated by Ca 2þ cation.

In such situations, the present study is focused on monitoring the transition in

characteristics of the synthesized zeolites as an adsorbent. In this context, specific

focus has been given to mineralogy, zeolitic structure, and pore sizes (viz., meso and

micro-pores) in the fly ash zeolites. In order to synthesize Ca2þ dominated zeolites

in the blend of Na- and Ca- zeolites, attempts have been made to use (i) lime (cal-

cium hydroxide) as a major reactant, (ii) caustic soda, and (iii) sodium chloride,

both as mineralizers, in traces. Also, efforts were made to quantify pore specific tran-

sitions with reference to physico-chemical, mineralogical, and morphological altera-

tions in the end product (the activated fly ash residue, containing zeolites).

Materials and Methods

The RFA used for this study was obtained from the hoppers of the electrostatic pre-

cipitators of the coal thermal power plant at Nashik, Maharashtra, India. It com-prises of particle sizes ranging from 0.375 to 200 lm with average particle size,

d50¼ 8.9lm. The activating reagents, Ca(OH)2, NaOH, and NaCl, were procured

from Merck Millipore Ltd., Mumbai, India. Before starting the synthesis, the initial

pH of the reaction mixture was maintained between 13 and 13.5, by adding traces of

NaOH, and homogeneously mixing it with the help of a pitched blade turbine impel-

ler (at 450 rpm). The mixtures of different initial compositions (refer to Table 1) were

treated in solution phase (L/S¼ 2.54) by resorting to two types of hydrothermal

reactors: (i) a TFE-fluorocarbon or polytetrafluoroethylene (PTFE)-lined 5-l auto-

clave reactor, at controlled temperature (100C-175C) and autogenously set pres-

sure (i.e., termed as an autoclave system, designated as AS) and (ii) an open system

(i.e., termed as water bath system, designated as WS) at controlled temperature

(98C) and atmospheric pressure. As Murayama et al. [16] have opined that the

higher reaction rate occurs either at higher temperature or longer reaction times,

TABLE 1

Composition and hydrothermal conditions.

Raw Mixture Ingredients (wt. %)

Hydrothermal Treatment RFA Ca(OH)2 NaCl T (C) t (h) End Product

Open system (using water bath) 72.73 18.18 9.09 98 6 WS1

10 WS212 WS3

24 WS4

FusionaþHydrothermal (using water bath) 98 6 FH1

72.73 18.18 9.09 12 FH2

24 FH3

Closed system (using autoclave) 72.73 18.18 9.09 100 6 AS1

125 1 AS2

150 1 AS3

175 2 AS4

a

Fusion at 600 C for 2 h followed by open hydrothermal treatment.





autoclaving was carried out for short durations (1 to 6 h), whereas a water bath was

set for longer reaction time ranging from 6 to 24 h. In order to remove unburnt car-

bon from the RFA, a third trial was performed by fusing the dry mixture with alkali

at 600C in a muffle furnace. The fused mixture was grinded and, subsequently,

open hydrothermal treatment was carried out (end products designated as FH).

After completion of the targeted treatments, both the autoclave and the water bath

were allowed to cool. Later, the end products (solid phase) were separated by vac-

uum filtration, which was followed by oven-drying at 60C for 24 h. For the sake of

clarity, the end products corresponding to variations in temperature, h, and the acti-

vation time, T , have been designated differently as WS1 to WS4 (open hydrothermal

products), AS1 to AS4 (closed hydrothermal autoclave products), and FH1 to FH3

(fusion followed by open hydrothermal products), as depicted in Table 1.

CHARACTERIZATIONOF ENDPRODUCTS

The chemical composition of both the RFA and the end products, in terms of oxides

(wt.%), was determined by X-ray fluorescence spectroscopy (Philips PW 2404, PAN-

alytical, The Netherlands). The cation exchange capacity (CEC) of various end prod-

ucts was determined by the ammonium acetate method [17] and the supernatant

obtained after washing and centrifugation was analyzed using ICP-AES (SPECTRO

ARCOS, M/s. Spectro, Germany). The amount of Na, K, Ca, and Mg ions exchanged

with the ammonium ion by the end products was used for evaluating the CEC. The

infrared transmittance spectrum was recorded by employing a FT-IR spectrometer

(Hyperion 3000 with Vertex 80 FTIR System, Bruker, Germany) in the range from

4000 to 400cm1. The mineralogical composition of the RFA and its end products

was determined by employing X-ray diffraction (XRD) spectrometer (X’Pert PRO,

PANalytical, The Netherlands), which uses a graphite monochromator and Cu-Karadiation. The pore structure characteristics, i.e., pore width, surface area, and vol-

ume of pores were investigated using a Brunauer-Emmett-Teller (BET) analyzer

(Micromeritics ASAP 2020) equipped with a patented isothermal jacket to maintain

a stable thermal profile along the full length of the sample and tubes. The BET calcu-

lation facilitates the surface area of the end products by determining the monolayer

volume of nitrogen gas adsorbed from the isotherm data. For observing the surface

morphology of the end products, representative samples were analyzed under a

scanning electron microscope (FEI ESEM Quanta 200), coupled with energy disper-

sive spectrometer in high vacuum mode.

Results and Discussion

It can be observed from Figs. 1, 2, and 3 that all the treatment methods (viz., WS,

FH, and AS) bring about a wide alteration in the FTIR spectra of the fly ash and the

end products. Generally, the bands from 4000-1600cm1 refer to vibration modes

of hydrous or hydroxyl components [6]. Incidentally, broader and deeper transmit-

tance bands are observed between 3450-3440 cm1 for the products WS2 and AS4,

which can be attributed to an asymmetrical stretching of O-H- linked to the Naþ,

Ca2þ, Si4þ and/or Al3þ. This suggests (i) bridging between Si and Al tetrahedra by

the hydroxyl ion, which also gets verified from Table 2, and which further shows sig-

nificant increase in the Na2O and CaO in the end product, WS2, and (ii) the





presence of hydrated aluminium silicates, which comprises of such OH - bonding

[6,18] in the end products. The stretching vibrations near 3450 cm1 and bending

vibrations at 1640-1620 cm1 in the end products are suggestive of zeolite crystalli-

zation and higher CEC [6].

In addition, for AS4, sharper and deeper band is observed near 1176-969 cm1

than WS2 (refer to Fig. 3), which is indicative of its enhanced crystallinity. On the

other hand, another sharper band at 1642.5 cm1 for WS2 corresponds to its higher

adsorbed surface water and hence enhanced CEC than AS4. Moreover, broader

band in WS2 between 1026 and 1014 cm1 indicates less crystallinity or presence of

more amorphous aluminosilicate gel (the -Si-O-Al- coordination) than AS4 (refer

Fig. 3). Moreover, sharper deformation bands of sodium and/or calcium bicarbonate

FIG. 1

FTIR spectra of fly ash and end

products of open hydrothermal

treatment.

FIG. 2


products of fusion-assisted

hydrothermal treatment.





can be noticed between 875 and 859 cm1 in the AS4 than the WS2 [9]. This could

be a revelation of the increased presence of carbonaceous impurities in the AS4 and

hence its lower CEC gets verified, which in turn confirms the superiority of the WS2

over AS4.

The X-ray diffractograms (see Fig. 4) exhibit the polycrystalline peaks of differ-

ent types of zeolites (viz., gismondine, phillipsite, and zeolite P) and also non-

zeolitic compounds like calcite (CaCO3) in the end products. Incidentally, WS2 can

be noticed to have lower peaks (less crystalline and near amorphous) of zeolites and

calcite, which favors maximum dissolution among all end products of the three sys-

tems. On the contrary, AS4 corresponds to the end products comprising higher

peaks of the zeolites and the calcite. Among the synthesized zeolites, gismondine

(Ca2Al4Si4O169H2O) and phillipsite ((NaKCa0.5Ba0.5)4-7 [Al4-7 Si12-9 O32] 12H2 O)

are basically Ca-based zeolites, whereas, zeolite P (Al2O3: 2.2 SiO2: 5.28 NaF: 105.6

TABLE 2

Chemical composition (% by weight) of the raw fly ash and end products.

Oxide RFA WS2 AS1

SiO2

58.011 47.546 49.951Al2O3 29.059 23.530 21.559

CaO 0.721 16.710 18.843

Fe2O3 6.091 4.311 2.952

TiO2 3.761 1.489 0.968

K2O 1.087 0.117 0.527

MgO 0.455 1.431 4.274

Na2O 0.183 3.138 0.741

P2O5 0.118 0.001 0.047

MnO2 0.050 0.035 0.034

SO3 0.038 0.001 0.047

SrO 0.034 0.027 0.031

FIG. 3


products of closed

hydrothermal treatment.





H2

O) is the only Na-based zeolite. Incidentally, Fig. 4

shows a larger peak of calcitein the AS4, which could be an important factor for its less CEC (refer to Table 3). On

the contrary, formation of gismondine (refer to the peaks at 21.7, 26.6, and 27.9

in Fig. 4), and a pair of zeolite P and phillipsite (refer to peaks at 12.55, 28.29, and

32.32 in Fig. 4) in WS2 can be attributed to dissolution of residual coal in the fly

FIG.4

XRD of fly ash and end

products with highest CECs in

brackets (in meq/100 g).

TABLE 3

CEC of the fly ash and the end products after various hydrothermal treatments.

Sample CEC (meq/100 g)

RFA 15WS1 272

WS2 394

WS3 354

WS4 288

FH1 152

FH2 139

FH3 181

AS1 189

AS2 254

AS3 327

AS4 323





ash and hence result in its high CEC (394 meq/100 g) after alkali activation. This

improvement in CEC of the zeolites synthesized by using Ca(OH)2 is closely

approaching to 420 meq/100 g reported by Berkgaut and Singer [4], and an improve-

ment over Murayama et al. [16], whom preferred NaOH for the alkali activation of

the RFA. Moreover, an overview of data in Table 4 clarifies more encouraging

improvement in the pore volume of the WS products than the AS. More specifically

for the sake of clarity, a reanalysis of data from the Table 4 has been presented in

Fig. 5.

In open hydrothermal treatment (WS1-4), as the interaction time increases, the

volume of pores ranging from 3 to 10 nm (the mesopores) reduces (refer to Fig. 5),

indicating the possible growth of zeolitic crystals. Interestingly, mesopores of

10–50 nm contribute to 69 %–78 % of the total pore volume for the closed (auto-

claved) system. It can be observed from Tables 3 and 4 that fusion treatment did not

significantly alter the pore size distribution and the CEC, whereas the amount of

macro pores are relatively higher for fusion products (45 %–47 %) compared to

other treatment methods. Furthermore, both open and closed treatments yielded sig-nificant enhancement of pore characteristics (viz., area, and volume) in the end

products (refer to Fig. 5 and Table 4).

In other words, it can be noticed from Figs. 5(a)-5(c ) that alkali activation of the

RFA significantly induces mesopores (the type-4 pores) in the end products. Macro-

pores are relatively higher for fusion products (45 %-47 %) compared to other treat-

ment methods, whereas mesopores of 10–50 nm contribute to 69 %–78 % of the

total pore volume for the closed hydrothermal system. Incidentally, contribution of

such pores in the total pore volume of the product is nearly similar for both the

WS2 (refer Fig. 5(a)) and AS4 (refer to Fig. 5( b) and Fig. 6). However, Fig. 1 ascertains

incorporation of more hydroxyl from the lime and caustic soda to the end product,and in turn cation exchange capacity of the WS2 also increases more noticeably than

the AS4 (refer to Table 3 and Fig. 3). In addition, from Table 4, higher total pore

TABLE 4

Pore size distribution for raw and activated fly ashes.

Pore Volume (%) Contribution of Various Sizes (nm)

< 2 2 to 3 3 to 10 10 to 50 >50

Sample SSA (m2/g) Total Pore Volume (cm3/g) Micro Meso Macro

RFA 0.725 0.00296 0.67 4.42 13.97 28.11 52.85WS1 63.6 0.18522 0.41 3.45 14.85 56.88 24.40

WS2 73.5 0.36067 0.13 1.23 9.44 58.55 30.65

WS3 84.6 0.2618 0.35 2.56 9.41 63.51 24.17

WS4 86.2 0.2877 0.27 2.09 8.24 68.61 20.78

FH1 16.5 0.08643 0.22 1.41 4.75 46.29 47.33

FH2 16.4 0.09053 0.20 1.42 4.29 48.73 45.36

FH3 17.2 0.08581 0.23 1.40 4.75 46.37 47.25

AS1 56.5 0.2169 0.20 1.41 6.36 78.26 13.77

AS2 79.3 0.22156 0.36 2.81 11.61 69.20 16.02

AS3 51.4 0.2438 0.15 1.62 9.11 70.71 18.41

AS4 90.5 0.28194 0.33 2.63 9.23 71.05 16.75





volume (¼ 0.36 cm3/g, i.e., 120 times that of the RFA) of the WS2 than AS4 can be

attributed to presence of majority of meso-sized pores up to 58.55 % (refer Table 4),

and also more inter-pores in this product [19]. On the contrary, higher specific sur-

face area (SSA) of the AS4 than WS2 is in line with the less macro pores (the type-5

pores) and more fines in this product (refer Fig. 5). Thus, higher CEC (¼ 394 meq/

100 g) of the WS2 could get affected by the presence of zeolites P and phillipsite (in

majority) in this product, whereas presence of gismondine in the AS4 could have

caused its less CEC. In all, alkali activation of the RFA causes significant increase in

the total pore volume of the end products from 0.0029 to 0.36 cm3/g (i.e., 120

times).

Furthermore, pore characteristics of the RFA and the end products, depicted in

Figs. 6 and 7(a)-7(c ), distinctly reveals improvements in the end products over the

RFA. Incidentally, Fig. 7 illustrates higher N2 gas absorption by the WS2 (¼231 cm3/g)

than the AS4 (¼ 187 cm3/g) and all the fusion cum hydrothermal (FH) products.

This is indicative of more porous structures present in WS2 than its counterpart,

FIG.5 Contribution of pore size type in the pore volume of the end products synthesized by different methods (a) open

hydrothermal, (b) closed hydrothermal, and (c) fusion cum hydrothermal.





FIG.6

Barrett-Joyner-Halenda (BJH)

pore distribution and SEM

images of superior end

products of hydrothermaltreatment WS2 and AS4.

FIG. 7 Variation of N2 gas adsorption with relative pressure (P/P0) for (a) open hydrothermal, (b) fusion-assisted hydrothermal,

and (c) closed hydrothermal systems.





and such improvement in the WS2 can be further inferred from Figs. 8(a)-8( b). It

can be observed from Fig. 8(a) that minerals like Na-zeolites P (group of spherical

growing crystals deposited on the activated fly ash residue surface, marked as zeolite

P), Ca-exchanged phillipsite (thin projecting crystals), and crystal of gismondine

constitute the WS2, which has undergone remarkable transition from the fly ash and

its ingredients (the spherical quartz, and oval shaped to irregular mullite). Zeolite P

and phillipsite add to its CEC, whereas AS4 (refer to Fig. 8(b)) lacks these minerals

and comprises of a majority of gismondine (small and growing prismatic crystals),

which could be responsible for its less CEC and pore volumes.

Based on the above findings, the end product of the open hydrothermal system,

WS2, gets confirmed as a superior product over the closed hydrothermal product,AS4, and all FH products. Moreover, the open hydrothermal method of activation of

the fly ash can be ascertained as a more suitable method over the closed hydrother-

mal and the fusion cum hydrothermal method.

Conclusions

The synthesis of fly ash zeolites using a combination of reagents, viz., calcium hy-

droxide, sodium hydroxide, and sodium chloride is observed to be very effective. A

blend of zeolite P and phillipsite (Na-zeolites) and gismondine (Ca-zeolite) get syn-

thesized in the open hydrothermal (water bath) system, while the majority of

Ca-zeolite gismondine gets synthesized by the closed hydrothermal (autoclave) sys-

tem. The open hydrothermal system product (WS2) obtained after 10 h of activation

at 98C attains maximum CEC as compared to activation of the fly ash in the closed

system at 175C for 2 h. The lime activation considerably results in increase in the

total pore volume to 0.36 cm3/g (i.e., 120 times of 0.0029 cm3/g for untreated fly

ash) for the end zeolitic product of the open hydrothermal method. Macropores are

relatively higher for fusion-assisted hydrothermal products (45 %–47 %) compared

to other treatment methods, while mesopores of 10–50 nm contribute to 69 %–78 %

of the total pore volume for a closed hydrothermal system. The activation of the fly

ash by using lime creates mostly mesopores in the end products at the cost of macro

FIG.8 Micrographs of superior end products of hydrothermal treatment: (a) WS2, (b) AS4.





pores, present in the fly ash. Such findings are more prominent in the case of open

hydrothermal activation of the fly ash than in autoclave and fusion assisted systems.

ACKNOWLEDGMENTS

The authors acknowledge the facilities availed to them at the Sophisticated Analyti-

cal Instrument Facility (SAIF) and the Cryo-SEM in the Department of Chemical

Engineering of IIT Bombay during the course of this study.

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Characterization of Na and Ca Zeolites Synthesized by Various Hydrothermal Treatments of Fly Ash