ball to powder ratio

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Optimized experimental design for natural clinoptilolite zeolite ball milling to

produce nano powders

Amir Charkhi a, Hossein Kazemian b,c,⁎, Mohammad Kazemeini a

a Chemical & Petroleum Eng. Department, Sharif University of Technology, Tehran, Iranb SPAG Zeolite R&D Group. Technology Incubation Centre, Science and Technology Park of Tehran University, Tehran, Iranc Department of Chemical and Biochemical Engineering, Faculty of Engineering, The University of Western Ontario, London, Ontario, Canada N6A 5B9

a b s t r a c ta r t i c l e i n f o

Article history:

Received 29 June 2009

Received in revised form 11 December 2009

Accepted 27 May 2010

Available online 4 June 2010

Keywords:

Experimental design

Planetary ball mill

Natural zeolite nano powder

Clinoptilolite

Nano powder of natural clinoptilolite zeolite was mechanically prepared by using a planetary ball mill.

Statistical experimental design was applied to optimize wet and dry milling of clinoptilolite zeolite. To

determine appropriate milling conditions with respect to the final product crystallinity, particle size and

distribution, different milling parameters such as dry and wet milling durations, rotational speed, balls to

powder ratio and water to powder ratio (for the wet milling) were investigated. Laser beam scattering

technique, scanning electron microscopy and X-ray diffraction analyses were carried out to characterize

samples. Results showed that larger than 1 mm particle size of clinoptilolite powder may mechanically be

reduced into the size range of less than 100 nm to 30 μm by means of planetary ball milling.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Zeolites are valuable inorganic materials having wide variety of

applications including; molecular sieves, adsorbents, ion-exchan-

gers and catalysts. These materials are microporous crystalline

hydrated alumino-silicates composed of TO4 tetrahedral (T=Si,

Al) with O atoms connecting neighboring tetrahedral in which the

substitution of silicon by aluminum in framework positions leaves

a negative charge behind which in turn may be compensated by

some alkali or alkaline earth cations[1–4].

Nowadays, more than 150 different types of zeolites have been

synthesized while more than 50 types have been discovered in the

nature. Amongst natural zeolites; clinoptilolite, with the simplified

ideal formula of (Na, K)6Si30Al6O72 ∙ nH2O; is one of the most

commonly found mainly in sedimentary rocks of volcanic origin.

Clinoptilolite-rich tuff is commercially very favorable due to its

huge and easy mineable resources as well as, its high zeolite

content. This well known zeolite may be utilized for purification

and separation processes, removal of NH4+ and heavy cations from

contaminated water and wastewater, aquaculture, soil fertilizers

and conditioners as well as, for dietary supplement in animal

nutrition[4–10].

Recently there has been a considerable growing interest in

utilizing nanozeolites due to their advantages over conventional

micron sizedmaterials. In other words, the reduction of the particle

size of zeolites causes larger external surface areas available for

interaction, shorter diffusion path lengths reducing mass and heat

transfer resistances in catalytic and sorption applications, decreas-

ing of side reactions, enhancing selectivity as well as, lowering

tendencies to coke formation in some catalytic reactions. Up to

now many different methods for synthesis of nanozeolites have

been reported. All of these are based upon hydrothermal treat-

ments which is performed by adjusting effective parameters such

as temperature, process duration and ingredient concentrations in

order to increase number of durable nuclei and reduce crystal

growth [1]. It is reminded, however, that longer synthesis duration,

expensive starting materials (specially organic template), lack of

reproducibility of the synthesis processes and energy consumption

for separation of nano powders by mean of high speed centrifu-

gation are some main issues making the bottom-up chemical

synthesis of nanozeolites techno-economically unviable processes

[11]. As an alternative technique, the zeolite particle size may be

reduced mechanically using specially designed ball mills [12–17].

In previous researches, production of different synthetic zeolites

such as Y, X, A, L, ZSM-5 and mordenite was considered utilizing

this method [12–17] where possible changes of milled zeolite

characteristics when subjected to dry ball milling were well

Powder Technology 203 (2010) 389–396

⁎ Corresponding author. Department of Chemical and Biochemical Engineering,

Faculty of Engineering, The University of Western Ontario, London, Ontario, Canada

N6A 5B9. Tel.: +1(519) 661 2111x82209.

E-mail addresses: [email protected], [email protected]

(H. Kazemian).

0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.powtec.2010.05.034

Contents lists available at ScienceDirect

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investigated. However, results have ascertained that high energy

ball milling, decreased the size of aforementioned zeolites.

Furthermore, the XRD patterns revealed that the crystallinity of

the milled zeolites was also reduced. Thus, collapse of the zeolite

crystal structure renders them useless as molecular sieves,

adsorbents or shape selective catalysts. In order to overcome this

crystallinity problem, wet ball milling of the HY zeolite was

investigated. Previous results indicated that by omitting the dry

milling step, the crystalline structure of the ground HY zeolite did

not collapse completely, even at long milling durations [13].

Furthermore, higher energy efficiencies, lower magnitude of

excess enthalpies and elimination of dust formation may also be

mentioned as some other added advantages of grinding in aqueous

compared to dry medium [13].

In order to evaluate the possibility of production of natural

clinoptilolite nano-particles, in this study a combination of wet

and dry milling was investigated using a planetary ball mill. To

optimize grinding conditions, effective parameters were selected

and several sets of experiments were designed based upon

statistical methods. Zeolites were milled at different periods of

time, milling speed, balls to powder and water to powder ratios.

Characteristics of final products such as particle size and distribu-

tion as well as, crystallinity of samples were examined by Laser

particle size analyzer, scanning electron microscopy (SEM) and X-

ray diffraction (XRD) techniques.

2. Experimental section

2.1. Ball milling of clinoptilolite zeolite

Powders of natural zeolite (clinoptilolite-rich tuff) were

obtained from a mine located in north of Iran near the city of

Semnan. Ball milling of clinoptilolite zeolite was performed by

mean of a planetary ball mill (PM100; Retsch Corporation). To

optimize the milling conditions with respect to size reduction and

crystallinity retention, milling parameters such as rotational

speed, ball to powder and water to powder ratios as well as,

grinding time were varied for different experiments. In most of the

performed tests a clinoptilolite powder with particle size of larger

than 1 mm was utilized as the starting material in dry milling for a

time period of about 10–20 min. The wet milling in water media

was then carried out at different periods of time between 2 to 4 h.

All of the tests were done in a 250 ml stainless steel jar with

protective jacket of zirconium oxide. Zirconium oxide balls of 20

and 3 mm were utilized for dry and wet millings; respectively. The

grinding jars were arranged eccentrically on the sun wheel of the

planetary ball mill. The direction of movement of the sun wheel

being opposite to that of grinding jars was selected with the ratio

of 1:1. A certain amount of zeolite and balls as well as, water in wet

milling were placed in the jar at room temperature and atmo-

spheric pressure then sealed and imposed to milling. Due to lack of

appropriate accessories to control the temperature and pressure of

the jar during grinding , sampling was carried out at the end of this

period, at which time the jar was allowed to be cooled down to

room temperature. For characterization step the ground powders

was dried at 30 °C for 24 h.

2.2. Design of experiments

Different sorts of experiments were designed in order to

optimize the appropriate milling conditions for the production of

natural clinoptilolite nano-powders with higher crystallinity and

lower particle size and distributions. Selected variable parameters

and their levels are provided in Table 1. For convenience, the

shorthand nomenclature of a, b, c, d, e, f, and g were assigned in

which, (a) the dry milling speed, (b) the dry milling time, (c) the

ball to powder ratio for dry state, (d) the wet milling speed, (e) the

wet milling time, (f) the balls to powders ratio for the wet state and

finally (g) the water to powder ratio; were set. Moreover, different

conditions for each experiment are presented in this table.

2.3. Characterization

The ground clinoptilolite zeolite was characterized with differ-

ent instrumental techniques. The morphology of the ground

powders as well as its size studied by means of Scanning Electron

Microscopy utilizing a LEO 1455vp SEM instrument. XRD patterns

to evaluate the crystallinity of the ground powders determined by

mean of a STOE STAD-MPDiffractometer inwhich a copper target at

40kV and 30 mA (2 hb10_) followed. Furthermore, Particle size

measurements of the ground samples performed by laser beam

scattering technique through means of a Master sizer 2000

apparatus (MALNERN Instruments).

3. Result and discussion

In order to investigate the effect of different ball milling condi-

tions on size distribution of the ground clinoptilolite zeolite,

Table 1

Variable parameters and their level in designed experiments in this study.

Level Milling

speed

(rpm)

Balls to

powder

ratio in wet

milling (wt.%)

Ball to powder

ratio in dry

milling

( No./weight)

Wet

Milling

period of

time (h)

Dry

milling

period of

time (min)

Water to

Powder

ratio

(vol/weight)

1 450 4.5 0.1 2 10 1

2 500 5 0.2 3 20 1.2

3 550 9 4 1.5

4 600 3

Table 2

Different conditions of designed experiments in this research.

No. code Dry milling Wet milling

Zeolite powder amount

(g)

Ball

No.

Rotational speed

(rpm)

Duration

(min)

Zeolite amount

(g)

Balls weight

(g)

Rotational speed

(rpm)

Water volume

(cm3)

Duration

(h)

1 322–3311 50 10 550 20 50 225 550 50 4

2 422–4311 50 10 600 20 50 225 600 50 4

3 322–3313 50 10 550 20 50 225 550 75 4

4 422–3313 50 10 600 20 50 225 550 75 4

5 311–2221 100 10 550 10 100 500 450 100 3

6 000–1322 0 0 0 0 100 500 450 120 4

7 212–1233 50 10 500 10 50 450 450 50 3

8 312–1234 50 10 550 10 50 450 450 150 3

9 311–1213 100 10 550 10 100 450 450 150 3

10 321–2113 100 10 550 20 100 450 500 150 2

390 A. Charkhi et al. / Powder Technology 203 (2010) 389–396


volume mean size and the size span were studied. The span is a

measure of the size distribution, which may be defined as follow:

Span = d 0:9ð Þ−d 0:1ð Þð Þ=d 0:5ð Þ ð1Þ

where d(0.1), d(0.5) and d(0.9) represent sizes for which 10, 50 and

90% by volume of particles in ground samples are smaller than these

sizes, respectively. It is to be noted that smaller span corresponds to

narrower size distribution.

The particle size distribution of ground zeolite powders milled

according to conditions provided in Table 2 and measured by

means of laser beam scattering demonstrated in Fig. 1. By utilizing

these results, the span was calculated and shown in Fig. 2. The

volume weighted mean size and d(0.1) are also shown in Figs. 3

Fig. 1. Volume particle size distribution at different milling condition (experiments 1 and 3; see Table 2.).

Fig. 2. Variation of span in designed experiments.

Fig. 3. Variation of volume weighted mean size in designed experiments.

Fig. 4. Variation of d(0.1) in designed experiments.

391A. Charkhi et al. / Powder Technology 203 (2010) 389–396


and 4; respectively. The d(0.1) results indicate that through

utilizing planetary ball mill, production of submicron powders is

possible and at different ball milling conditions this value may be

varied between 0.56 and 2 μm for natural clinoptilolite zeolite with

initial size of larger than 1 mm. The volume mean sizes of these

samples were also changed in the range of 3.3 to 15 μm.

In order to evaluate the influence of each factor on the volume

weighted mean size and span, an arithmetic calculation was

carried out. The average effect of each level of variable parameters

was calculated by averaging the results of those experiments

which were carried out at that level. For example, to calculate the

average effect of second level of wet speed milling (500 rpm),

results of all experiments which were performed at that rotation

speed under wet condition (results of 311-2221 and 321-2113

experiments) were averaged. Results of these calculations are

displayed in Figs. 5 and 6. Data in Fig. 5 indicated that an increase in

Fig. 5. Influences of milling conditions on volume weighted mean size.



rotational speed under dry conditions caused initially a decrease in

volume mean size of ground zeolite while at faster speed of

600 rpm, this value increased again. Moreover, increasing of the

milling time as well as, ball to powder ratio under dry conditions

caused an enhancement in the volume mean size of ground

powders. On the other hand, increasing of the rotational speed

from 450 to 550 rpm under wet conditions, decreased the volume

mean size; while, at a little bit higher speed of 600 rpm, the volume

mean size started to rise again. In addition, the reduction in water

to powder ratio caused lowering of volume mean size. Ultimately,

form the data in Fig. 5, it is a foregone conclusion that the volume

mean size increased due to the raised ball to powder ratio. It may

further be concluded that for production of clinoptilolite powders

with the smallest volume mean size, planetary ball milling should

be performed at 1) for dry conditions: i- milling speed of 550 rpm,

ii- milling time of 10 min and iii- 5 balls of 20 mm per 50 g of

Fig. 6. Influences of milling conditions on span.



powders and 2) for wet conditions: i- milling speed of 500 rpm, ii-

ball to powder weight ratio of 4.5, iii- water to powder ratio of 1

(e.g.; 50 g of water per 50 g of zeolite) and iv- milling time of 3 h.

It is emphasized through Fig. 6 that increased in milling speed,

time and balls to powder ratio caused a raise in the span of ground

powders. Furthermore, it is seen that amongst parameters studied,

the rotation speed variations had the most influence on this span.

On the other hand, increase in speed and time of wet milling led to

a wider span. From this figure, it also is understood that increased

ball to powder and water to powder ratio under wet milling makes

narrower span.

To study probable changes of morphology for ground powders

after ball milling as well as to confirm the PSA results, samples

were subjected to SEM investigation. Some of the SEM images of

ground samples are shown in Fig. 7. SEM images indicated that

almost in all samples, zeolite powder with particles size less than

100 nm may be recognized as a separated particle or in the form of

larger agglomerates. Moreover most particles have lost their initial

octahedral shape and converted into spherical, elliptical or

irregular shapes. By careful considerations of SEM images, some

crystals with sharp edges and clean surfaces observed; which are

about 200 nm in size. Therefore, it may be concluded that careful

selection of milling conditions may result in production of nano

clinoptilolite zeolite with desirable crystalline structure. Never-

theless, mechanical production of zeolitic nano-particles by mean

of planetary ball mills may also reduce zeolite crystallinity.

As mentioned in a previous study [14], milling at high speed and

prolonged time resulted in collapsed crystalline structure of

ground zeolite. Therefore, apart from size distribution, the

crystallinity study of ground powders is also very important

when a choice for appropriate conditions for ball milling of

clinoptilolite samples has to be made. Patterns of XRD for all

samples were obtained and some of them are displayed in Fig. 8.

Results showed the increase in time and rotational speed of milling

caused reduction in crystallinity. In the ground clinoptilolite

samples, major peak observed at 2θ equal to 22.49o. With regards

to the intensity ratio of aforementioned peak in original parent

clinoptilolite to the ground samples, crystallinity reduction due to

the milling processes was calculated. Results are provided in

Table 3.

In order to determine best milling conditions amongst those

mentioned in Table 2 above, the following normalized expression;

so-called objective function (O.F.), defined to give each experiment

a weight for this purpose as a result. The biggest O.F., thus,

indicated best milling conditions in which highest crystallinity

retained while smallest volume mean size and narrowest size

distribution also obtained. The weighting factor for each term of

this expression selected according to its importance for these

authors. It is given by:

O:F: = 1−Vol weighted Mean

Max Vol weighted Mean

� �

× 0:35 + 1−Span

Max Span

� �

× 0:15 + 0:5 × Crystalinity:

ð2Þ

Fig. 7. SEM images of original and ground clinoptilolite zeolite.



As shown in Table 3, best milling conditions belonged to

experiment No. 7 with sample code of 311-2221. It meant that

choosing following conditions for milling of clinoptilolite gives the

best result with respect to crystallinity and size distribution. These

included; for dry milling i)milling speed of 550 rpm, ii) milling

duration of 10 min and iii) balls to powder ratio of 0.1 and for

wet milling; i) rotational speed of 500 rpm, ii) milling duration

of 3 h, iii) ball to powder ratio of 4.5 and iv) water to powder ratio

of 1.

To find the influence of different levels of milling parameter on

crystallinity, a simple arithmetic calculation similar to what was

done previously for size distribution performed. Results showed

that the increase in milling time and speed under dry and wet

conditions caused crystallinity to reduce. In addition, for main-

taining the crystallinity, the second level of ball to powder and

water to powder ratios in wet milling had to be selected.

Furthermore, results of experiment 000-1322 in Table 2 designed

under wet conditions indicated that omitting dry milling may

cause an enhancement in crystallinity. This issue may be explained

in terms of the following mechanism. Different methods of

grinding may impose different forces upon particles, such as

compression, shear, attrition, impact or internal forces [18]. In ball

milling, the grinding of samples is carried out by balls which are

swept along by the wall and then fall onto the load. The

fragmentation of particles in samples results from the stress,

which may be due to compression or shear, imposed upon the bed

of particles by the balls. It has been proposed that wet grinding

causes shearing along the cleavage planes whereas dry grinding

fractures the crystals [13,19]. It has also been suggested that wet

grinding proceeds with the preferential formation of new surfaces

while little bulk deformation takes place in the particles [13,20].

Thus, crystallinities of zeolite samples obtained by wet ball milling

were considerably larger than those prepared by the dry counter-

part. On the other hand, amorphization of crystalline zeolites may

be attributed to breaking the structural Si–O–Si and Si–O–Al bonds

and collapsing of the original crystal structure under the action of

intensive mechanical forces [14–17] mainly occurring at the dry

stage.

4. Conclusion

In this work the possibility of production of nano clinoptilolite

zeolite by mean of a planetary ball mill utilizing a combination of

wet and dry conditions was investigated. Statistical experimental

design incorporated in order to optimize the effect of key

parameters. Results revealed that by application of appropriate

milling conditions obtained in this study, clinoptilolite powders

with particle sizes less than 100 nm and desirable crystallinity

may be produced. Nevertheless this method led to a wide size

distribution of ground powders in the range of less than 100 nm to

30 μm and crystallinity loss of about 55–100%. Furthermore, based

upon experiments performed under wet milling conditions in

this work, it may be concluded that under certain situations

one may go thru wet milling without any needs for dry milling

pretreatments.

Acknowledgments

The Authors acknowledge Dr. Azizollah Kamalzadeh from the

Institute of Scientific-Applied Higher Education of Jihad_e_Agriculture

of Iran and Animal Sciences Research Institute in Karaj, Iran, for pro-

viding the ball mill equipment. The authors have declared no conflict

of interest.

Fig. 8. XRD patterns of a) original clinoptilolite zeolite and b) milled clinoptilolite

zeolite under condition of experiments 311-2221, c) 422-4311 and d) 321-2113.

Table 3

Milling results in terms of size distribution, crystallinity and defined objective function.

No. Exp. Code d(0.1) (μm) d(0.5) (μm) d(0.9) (μm) Volume weighted mean size (μm) span Crystallinitya O.F. Normalized O.F.

5 311–2221 0.849 2.462 6.72 3.309 2.384 0.45 0.604 1.000

6 000–1322 1.601 3.814 8.059 4.41 1.693 0.45 0.591 0.979

7 212–1233 1.199 7.935 20.03 9.598 2.373 0.38 0.419 0.694

4 422–3313 0.559 4.514 20.496 8.074 4.417 0.33 0.392 0.649

9 311–1213 1.448 8.093 22.91 10.763 2.65 0.38 0.387 0.641

10 321–2113 0.746 6.687 20.911 9.078 3.015 0.28 0.370 0.612

1 322–3311 0.8 5.347 23.37 9.466 4.221 0.30 0.351 0.580

3 322–3313 0.675 7.944 31.418 12.622 3.87 0.28 0.271 0.449

8 312–1234 1.904 8.728 35.628 15.005 3.864 0.38 0.265 0.440

2 422–4311 0.85 4.65 38.265 13.378 8.046 0 0.038 0.063

a crystallinity is the ratio of major intensity of ground sample to original Clinoptilolite zeolite.



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