Porous Structure of Waste f Ly Ashes and Their Chemical Modifications

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.Powder Technology 123 2002 5358

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Porous structure of waste fly ashes and their chemical modifications

Z. Sarbak), M. Kramer-Wachowiak

Faculty of Chemistry, Adam Mickiewicz Uniersity, Grunwaldzka 6, 60-780 Poznan, Poland

Received 1 December 2000; received in revised form 1 March 2001; accepted 6 July 2001

Abstract

Changes in the porous structure of fly ashes ZS-13 subjected to chemical treatment with solutions of NaOH, NaOH rNH HCO ,4 3

EDTA, and HCl are examined. The initial material with 3 m2rg surface area and 0.01 cm3rg pore volume after modification revealed an

increased surface area and pore volume. The greatest increase in the surface area equal to 105 m 2rg was noted for the sample treated

with HCl, while the greatest increase in pore volume equal to 0.18 cm3rg was obtained for the sample treated with EDTA. In the

modified products, meso- and macropores prevail, although in the sample treated with HCl, the presence of a small contribution of

micropores was detected. All the modifications, except that treated with HCl, were found to contain slit-shaped macropores. In the sample .treated with HCl, the pores were in the shape of nonparallel plates. A study by scanning electron microscopy SEM allowed a detailed

determination of the shape of the isles and agglomerates. Chemical treatment of the initial fly ashes resulted in a transformation of the

ball-shaped agglomerations into different products characterised by specific size and shape of particles. q2002 Elsevier Science B.V. All

rights reserved.

Keywords: Fly ashes; Chemical treatment; BET; Pore size; Pore volume; SEM

1. Introduction

Fly ashes are waste products of coal combustion in

electric and thermal power plants. The ashes are capturedelectrostatically in electrofilters or mechanically by cy-

clones and then they are deposited directly into ponds or

landfills, where they can become hazardous to the environ-w xment 1 .

Fly ashes are oxides whose composition depends on the

type of coal subjected to combustion and the combustion

conditions. The main components of ashes are mineralsw xsuch as kaolinite, mullite, quartz, calcite, and pyrite 2 .

High contents of silicates and aluminasilicates suggest

their susceptibility to transformation into zeolite-like crys-w xtalline materials as a result of chemical treatment 3 6 . It

is also expected that the obtained materials may be effi-w xcient adsorbents of environmental pollutants 7 .

The aim of the present study is to determine changes in

the surface area and porous structure of fly ashes from .Konin Power Plants Poland firing brown coal and their

chemical modifications with different chemical agents.

)

Corresponding author. .E-mail address:[email protected] Z. Sarbak .

2. Experimental

The study was performed on fly ash denoted by the

symbol ZS-13 from the Konin Power Plant combustingbrown coal.

2.1. Modification of fly ashes

Twenty grams of fly ash from the Konin Power Plant

was mixed with 160 cm3 3.5 M solution of NaOH and

heated in a water bath under reflux at 90100 8C for 24 h,

and then filtered off and washed with distilled water until

pHs 7. The obtained material was dried at 120 8C for

12 h.

The same procedure was applied to obtain other modifi-

cations of ashes using 160 cm3 3.5 M solution of NaOH

and 0.015 molar equivalent of NH HCO , 200 cm3 2.5 M4 3solution of HCl, and 0.05 molar equivalent of EDTA in

100 cm3 of water.

2.2. Chemical analysis

Chemical analysis was performed on a solution ob-

tained after melting the sample in a Philips type PW 2400

fluorescence X-ray spectrometer.

0032-5910r02r$ - see front matter q2002 Elsevier Science B.V. All rights reserved.

.P II: S 0 0 3 2 -5 9 1 0 0 1 0 0 4 3 1 -4


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( )Z. Sarbak, M. Kramer-Wachowiakr Powder Technology 123 2002 535854

Table 1

Chemical composition of ZS-13 fly ashes

.Component Content wt.%

SiO 27.372Al O 6.632 3Fe O 3.752 3TiO 0.962MgO 8.23

CaO 34.48

Na O 1.082K O 0.412

2.3. X-ray diffraction studies

Diffractograms of the starting material and the obtained

modifications were made on a TUR M-62 diffractometer .using CuK radiation and Ni filter ls 1.5418 A .a

2.4. Infrared spectroscopy studies

The studies were performed on a Perkin-Elmer Model

180 spectrometer within the range of 1800400 cmy1,

using a 1.5-mg sample mixed with 200 mg of KBr andpressed into a thin transparent tablet.

( )2.5. Scanning electron microscopy SEM

Microscopic observations were made using a Philips

type SEM 515 scanning electron microscope at the acceler-

ating voltage 15 kV. At the first stage, a suspension of the

samples was made and the smear was covered with a thin

film of gold in the atmosphere of argon, in Balzers type

SCD 050 evaporator.

2.6. Surface area and pore characterisation

To determine the surface area, specific volume, and

mean pore radius, the samples were first degassed at 350

8C and then subject to N adsorption using BET method.2The study was performed using a Sorptometer ASAP 2010

made by Micromeritics.

3. Results and discussion

Results of the chemical analyses of the initial fly ash

ZS-13 are shown in Table 1.

Table 2The effect of chemical modifications of ZS-13 fly ashes on the pore

properties and surface area

Chemical BET surface Mean pore Pore volume2 3 . . .treatment area m rg radius A cm rg

None 3 34 0.01

NaOH 59 48 0.14

NaOHr 60 43 0.13

NH HCO4 3EDTA 60 50 0.18

HCl 105 17 0.10

Fig. 1. Pore volume distribution as a function of the pore radius for fly

ash ZS-13.

According to the calculations carried out from chemical

analyses, the molar ratio of SiO rAl O was equal to2 2 3 . 7.02, whereas that of SiO q Al O r Ca q MgO q2 2 3

.Fe O was equal to 0.62, which indicates that the ashes2 3belong to the calciumsilica class.

Modification of the ash with the solutions mentioned

above brings a change in many textural parameters. Table

2 gives the pore and surface area characteristics of the

chemically amended fly ash. In all instances, the surface

area of the chemically amended fly ash is greater than in


ash ZS-13 modified with NaOH solution.


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( )Z. Sarbak, M. Kramer-Wachowiakr Powder Technology 123 2002 5358 55


ash ZS-13 modified with NaOHrNH HCO solution.4 3

.the unmodified ash ZS-13 . The greatest increase in the

surface area is observed for the ashes subjected to the

modification by hydrochloric acid. This modification also

resulted in the lowest mean pore radius, which is about

twice as small as that of the initial ash. Chemical modifica-

tion by the other solutions leads to a smaller increase in

the mean pore radius. A more dramatic influence of chemi-

cal modification is a large increase, 10-fold or even greater,

in the pore volume. The greatest pore volume increase was

noted for the sample modified by the EDTA solution. For


ash ZS-13 modified with EDTA solution.


ash ZS-13 modified with HCl solution.

this sample, the pore radius was also the largest. The

sample ZS-13 HCl showing the greatest surface area was

characterised by the smallest mean pore radius and the

smallest pore volume of all chemically modified samples.

This means that the sample has a much greater amount of

micropores than the other modifications.

A low surface area and small pore volume of the initial

ash at a relatively large mean pore radius indicates thepresence of meso- pore radius between 10 and 250 A, 10

. .As 1 nm and macropores pore radius exceeding 250 Aw xaccording to IUPAC 8 classification. This supposition is

. .Fig. 6. Nitrogen adsorption x and desorption o isotherms for fly ash

ZS-13.


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ZS-13 modified with NaOH solution.

confirmed by the dependence of the volume of the ad- .sorbed nitrogen versus the pore radius Figs. 1 5 . In this

modification, the dominant pores have a radius of 20 A. Asimilar conclusion has been drawn on the initial prepara-

tion ZS-13, in which mesopores of the radius 20 A occurin abundance. The samples obtained as a result of modifi-

cation with the solutions NaOH and NaOHrNH HCO4 3have the maximum contribution of pores of the radii 20

and 100 A. In the sample modified with EDTA, themaximum contribution of the mesopores of the radii 20

and 35 A was established.In our opinion, a greater surface area and volume of

pores in the samples modified with NaOH solution is


ZS-13 modified with NaOHrNH HCO solution.4 3


ZS-13 modified with EDTA solution.

mainly a consequence of conversion of fly ashes into

zeolite-like structures. A similar conclusion follows fromw xthe study by Howden 9 , who reported that the treatment

.of kaolinite a component of fly ashes with this solution

results in the formation of zeolite structures. The increase

in the parameters discussed upon the treatment with HCl

and EDTA solutions is related to the process of dealumina-

tion. The course of this process in the case of syntheticw xsilicaalumina was described by Scherzer 10 .

The isotherm of nitrogen adsorption obtained for the .initial ash ZS-13 Fig. 6 can be classified as type II in the

w xIUPAC 8 classification. This kind of isotherm corre-


ZS-13 modified with HCl solution.


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( )Z. Sarbak, M. Kramer-Wachowiakr Powder Technology 123 2002 5358 57

sponds to the situation when at relatively low pressures, a

monolayer of nitrogen is formed, and at relatively high

pressures, a nitrogen multilayer is formed. This character

of the isotherm is typical of macroporous materials. The

small surface area and low pore volume at a relatively

large mean pore radius indicates that the fly ash tested has

a macroporous structure. The fact that the hysteresis loop

begins at relatively low pressures also implies a small

contribution of micropores. The hysteresis loop can beclassified as H3 type, which means the overwhelming

w xpresence of slit-shaped pores 11 .

The adsorption isotherms and hysteresis loops obtained

for the samples modified with NaOH and NaOHrNH 4 .HCO Figs. 7 and 8 are similar and can be classified as3

types II and H3, respectively. For the sample modified .with EDTA Fig. 9 , the shape of the isotherm is similar

.type II , but the hysteresis loop, although also of type H3,

is much broader. A different character of the nitrogen

adsorption isotherm is observed for the sample ZS-13 HCl

.Fig. 10 . Its adsorption significantly increases at relativelylow pressure, which means that the number of micropores

in this sample is much higher than in the sample ZS-13.

. . . .Fig. 11. Scanning electron micrographs of a fly ashes ZS-13 modified with: b NaOH solution, c and d NaOHrNH HCO solution, e HCl solution4 3 .and f EDTA solution.


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The isotherm can be classified as type I with an admixture

of type II, which corresponds to microporous adsorbents,

e.g. zeolite-like crystalline solids, containing some macro-

pores. The hysteresis loop looks like type H4 characteristic

of the adsorbents with the pores formed by nonparallelw xplates 11 . A comparison of our results concerning the

surface area and porous structure of the material studied

with literature data is impossible because of the lack of the

latter.SEM observations proved the presence of well-devel-

oped particles in the shape of rather smooth balls of the

diameter of 24 mm and sometimes even 7 mm, in the .initial fly ashes Fig. 11a . Moreover, the occurrence of

scarce agglomerates of amorphous particles, of approxi-

mately 46 mm in size, was noted.

After exposure to a solution of sodium hydroxide, the

smooth balls become objects of deformed ball-like shape .Fig. 11b , sizes 45 or 1012 mm, with numerous plates

on the surface, which testifies to the occurrence of crys-

tallisation. Individual particles were found with well-devel-

oped smooth walls. In the background, there were numer-

ous small particles of the size about 5 mm, as well as a

number of short fibres of about 3 mm in length. Exposition

of the fly ashes to solutions of sodium hydroxide and .sodium bicarbonate Fig. 11c and d causes a transforma-

tion of the ball-shaped agglomerations into large particles

of the size of 1012 mm, with well-developed plates on

the surface.

The effect of HCl brings about a transformation of the

majority of ball-shaped particles into agglomerations of .undefined shape Fig. 11e . The observations also revealed

a formation of a single crystal in the form of a rod of the

size 7=15 mm. When EDTA was used for chemical

.modification of the ashes, SEM Fig. 11f shows that onlyscarce ball-shaped particles remained, while the majority

were transformed into numerous pillars of the size 6=1.2mm and individual cubic crystals with rounded edge 3=3

.mm .

As follows from the above data, each of the procedures

of chemical modification, transforms the fly ashes ZS-13

into a different product characterised by specific shape and

size of particles. It should be emphasised that the fly ashes

differ depending on the kind of combusted coal and com-

bustion conditions. Therefore, a comparison of selected

texture parameters seems pointless.

4. Conclusions

Chemical treatment of the initial fly ashes ZS-13 char-

acterised by 3 m2rg surface and 0.01 cm3rg pore volume

leads to an increase in their surface area and pore volume.

The greatest pore volume of 0.18 cm3rg occurs for the

sample treated with the EDTA solution, whereas the great-

est surface area of 105 m2rg was observed for HCl

modification. The sample ZS-13 HCl reveals a relatively

large contribution of micropores when compared to the

initial fly ashes but do not occur in the other samples. The

shape of the adsorption isotherms and hysteresis loop

suggests that the sample ZS-13 contains mainly slit-shaped

macropores. Similar pores were found in the other samples

except ZS-13 HCl, in which micro- and macropores of the

shape of nonparallel plates were found.

The modification with solutions of NaOH, NaOHrNH HCO , and EDTA leads to obtaining of new materials4 3characterised by greater surface area than that of the initial

fly ashes, which is of about 60 m2rg.

According to the results of the SEM study, the sample

ZS-13 reveals the presence of well-developed particles in

the shape of smooth balls of a diameter of 24 mm, which

under the influence of NaOH treatment, undergo deforma-

tion and numerous plates and short fibres appear on their

surface. The chemical treatment with NaOHrNH HCO4 3solution leads to the appearance of large particles of the

size of 1012 mm with well-developed plates. The modifi-

cation with EDTA leads to a transformation of the ball-

shaped particles into numerous pillars. The treatment with

HCl resulted in the appearance of agglomerations of unde-

fined shape.

In the aspect of future application of these materials as

adsorbents, catalysts, or their supports, it seems that all the

above-described modifications can find appropriate use

This conclusion is justified by the facts that chemical

modifications of the ashes have led to a significant in-

crease in the surface area, mean pore size, and transforma-

tion of ball-shaped structure of the initial ash ZS-13 into

crystalline structures observable under a scanning electron

microscope.

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Porous Structure of Waste f Ly Ashes and Their Chemical Modifications