Fabrication and characterization of low cost tubular mineral-based ceramic membranes for micro-filtration from natural zeolite

Journal of Membrane Science 281 (2006) 592–599

Fabrication and characterization of low cost tubular mineral-based ceramic membranes for micro-filtration from natural zeolite Yingchao Dong a , Shaofeng Chen a , Xuebin Zhang a,b , Jiakui Yang a,b , Xingqin Liu a,∗ , Guangyao Meng a,b a

Department of Materials Science and Engineering, University of Science and Technology of China (USTC), Hefei 230026, PR China b Great Wall New Certury Membrane Technology, Hefei 230026, PR China Received 24 February 2006; received in revised form 1 April 2006; accepted 17 April 2006 Available online 27 April 2006

Abstract Ceramic multilayer micro-filtration membranes with smooth surfaces and cracks-free have been fabricated on tubular porous supports by dipcoating using natural zeolite mineral as the starting materials. The preparation process, including the powder classification, the forming and sintering of membranes, was systematically studied. The membrane thickness was determined by dipping time and solid loading of the suspension. XRD reveals that the phase compositions of the membranes were related to the sintering temperature and the final major phases were almost quartz and albite. SEM studies subsequently indicate that solids in the membrane begun to sinter at about 850 ◦ C with alkali metal oxides as the aid fluxes. The interlayers with average pore size in the range of 0.69–1.10 ␮m were obtained and the optimum firing temperature was between 850 and 950 ◦ C for 1 h. Then, the top-layer membrane with average pore size 0.54 ␮m could be prepared on the above support. Nitrogen gas permeation flux and pure water permeation flux of the resulting membrane are 1.96 × 105 and 3.20 × 103 l m−2 h−1 ×10−5 Pa−1 (1.96 × 105 and 3.20 × 103 l m−2 h−1 bar−1 ) with the trans-membrane pressure of 0.1 MPa at room temperature, respectively. © 2006 Elsevier B.V. All rights reserved. Keywords: Natural zeolite; Mineral-based ceramic membrane; Micro-filtration; Low cost; Fabrication

1. Introduction Ceramic separation membranes have increasingly been employed in a number of industrial fields because they can offer several advantages over their organic counterparts, such as the better mechanical strength, resistance to the acid or base media, superior thermal and chemical stability, narrow pore size distribution, adjustable micro-structure, low energy consumption under milder conditions and little pollution to the environment. Consequently, a great deal of research has been focused on development of new type inorganic membranes and application processes in last decade [1]. However, commercialized porous ceramic membranes with alumina, zirconia, titania, mullite, etc., as main raw materials are not suitable to a large scale application, especially in the pre-purification field of industrial wastewater, because of expensive raw materials and relatively high firing

∗

Corresponding author. Tel.: +86 551 3606249; fax: +86 551 3607627. E-mail address: [email protected] (X. Liu).

0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.04.029

temperature [2]. In recent years, the preparation and potential applications of porous mineral-based ceramic membrane whose major component is a naturally occurring mineral have increasingly attracted intention for its low cost [3–5]. The development of mineral-based ceramic membranes can lead to a critical new technological revolution that would add great economic value to natural minerals that existed widely throughout the world, many of which are not currently well utilized. Zeolite minerals, also known as natural sedimentary or natural occurring zeolites, are mainly composed of aluminosilicates with a three-dimensional framework structure bearing AlO4 and SiO4 tetrahedra [6]. At the present time, many studies are devoted to the environmental applications based on the adsorption and ion exchange performances of natural zeolite [7,8]. In our previous work, the tubular porous ceramic supports were elaborated by a polymer-aided extrusion with natural Heulandite zeolite as raw materials [9]. Natural zeolite mineral is suitable for use as the major component in the fabrication of mineralbased ceramic membrane partly because it does not swell in water and easily forms a suspension for coating membrane on

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porous support. Furthermore, the multiple compositions of natural zeolite could make its sintering differ from the conventional pure membrane materials. Therefore, inter-particle active pores with separation function in membrane could form when the dried specimens are sintered. In the current work, we have explored further probability of preparing the interlayers and top-layers on the tubular porous support by a dip-coating process from natural Heulandite zeolite mineral. An investigation on the relationship among membrane formation, phase compositions, microstructure, pore properties, permeability and the preparation conditions is presented. 2. Experimental 2.1. Pretreatment of raw mineral Natural zeolite mineral was obtained from Fanchang County (Anhui Province, China). Massive samples were pulverized by the crushing machine and then the fine powders passing through the 200 mesh sieve were collected. After drygrinding using Al2 O3 balls as medium at the fixed rotation speed of 230.38356 × 10−1 rad/s (220 rpm), the powders were wetmilled by adding a proper amount of polyacrylic acid. Then, ultrasonic agitation for 30 s followed by gravitational sedimentation classification was adopted. The particles of the suspension (5 wt% solid loading) in the specified container were left to sedimentate. Then, the powders in the upper suspension were collected followed by removal of ultra-fine powders by further sedimentation. Finally, the fractions for different sedimentation time with the sediment and ultra-fine powders discarded were tested on the photo size analyzer. 2.2. Membrane preparation Porous ceramic tubes (single-channel) elaborated with natural zeolite by extrusion in our lab were used as supports in this study, the main parameters of which are as follows: open porosity 34.5%; average pore diameter 5.86 ␮m; outer diameter 13 mm; inner diameter 10 mm; length 80 mm. For preparing zeolite particle suspensions, the classified powders (mean particle size: 2.06 ␮m for interlayer and 1.24 ␮m for top-layer) were mixed with dispersant polyacrylic acid, binder polyvinyl alcohol, plasticizer glycerol and distilled water by ball-milling for 12 h using ZrO2 balls. Then the membranes were prepared on the tubular porous support by dip-coating process, followed by drying and sintering at various temperatures. The suspension was poured into a glass dish to obtain the unsupported membranes. After dried and sintered at the same condition as the tubular membranes, the specimens were measured to obtain open porosity.

Fig. 1. The schematic diagram of the experimental bubble-point apparatus: 1, nitrogen gas pressure-relief valve; 2, 7, precision manometer; 3, ceramic membrane; 4, gas flow-meter; 5, ball cut-off valve; 6, drier device flume; 8, ball valve; 9, nitrogen gas steel cylinder.

fied by XRD (Kigaku D/MAZ-␥A rotating X-ray diffraction unit, working voltage 40 kV, working current 80 mA, scanning speed of 4◦ min−1 ). The particle size distributions for the obtained powders with different sedimentation time were measured on photo size analyzer (NSKC—1, Nanjing University of Technology, China). The surface and cross-section SEM images of membrane were obtained using a scanning electron microscope (Hitachi x—650, Japan). The maximum pore size, average pore size, pore size distribution of tubular membrane were measured on the home made bubble-point apparatus with nitrogen as gas media [10–12]. And the nitrogen permeation fluxes were also tested using dried membrane by this experimental device. Fig. 1 shows a schematic chart of this bubble-point apparatus. Pure water fluxes across micro-filtration membranes were obtained on the home made cross-flow permeation device (see the schematic diagram as shown in Fig. 2). The thickness of sintered membranes (Lm, sintered ) was determined by the weight gain method according to the formula as following [13]. Lm,sintered = (m2 − m1 )[ρA(1 − εm )]−1

2.3. Characterization techniques Chemical compositions of natural zeolite powders were analyzed on a X-ray fluorescence spectrum analyzer (XRF-1800, SHIMADZU Corporation, Japan). Phase compositions of supported membranes sintered at different temperatures were identi-

Fig. 2. The chart of cross-flow permeation device for ceramic membrane pure water flux test. 1, Glass rotor flow-meter; 2, 5, 6, precision manometer; 3, clamping fixture; 4, membrane for testing; 7, 9, cut-off valve; 8, flume; 10, stainless steel pump; 11, a set of glass rotor flow-meter.

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where m1 and m2 (g) are the weight of the support and the total one of support and sintered membrane layer, respectively. A (m2 ) is the planar area of the membrane, approximately equal to that of the outer surface area of the support; ρ (g cm−3 ) is the theoretical density of natural zeolite solid (2.17 g cm−3 ); εm is the open porosity of the sintered unsupported membrane (open porosity determined by the Archimedes’ method [14,15]). 3. Results and discussion 3.1. Fractionation of natural zeolite powders From Table 1, it is noticed that the average particle sizes of collected powders decrease with increasing the sedimentation time, which is suggested that the coarse particles have the fast sedimentation velocities and therefore the fines were left at the upper part of container. As reported by Chen [16], the movement state of spherical particles is laminar flow in the Stokes section of the actionless water and the size of settling particles is reciprocal to the square root of sedimentation time. So different size particles left in the upper suspension were obtained by controlling sedimentation time using the suspensions of 5 wt% solid loading in our experiment. The sharper decrease in mean particle sizes between the first fraction and the second fraction than others is attributed to the existence of some coarse particles (in raw powders), which mostly sedimentated quickly to the bottom of the container in 10 min. It is further suggested that the sedimentation occurred to a great extent during the early stages of the experiment. The particle size distributions of the sixth fraction and the fifth one that were used to prepare interlayer and top-layer, respectively, are shown in Fig. 3. It is obvious that both the average particle size and maximum particle size of the sixth fraction are smaller than the fifth one. Some big particles were mostly removed by sedimentation classification.

Fig. 3. Particle size distribution graphs for the fifth fraction and sixth fraction, respectively.

From X-ray diffraction patterns of the prepared membranes sintered at elevated temperature described in Fig. 4, it can be seen that the major crystalline phase of raw materials used in this study is heulandite (Ca[Al2 Si7 O18 ]·6H2 O). There are also a small amount of coexisting phases, such as quartz, albite and montmorillonite. The heavy background of patterns is due to impurities. The diffraction peaks of montmorillonite and heulandite became weak with further increasing the sintering temperature. This is because the framework structure of crystalline heulandite and the pillar structure of montmorillonite were destroyed to some extent, respectively, during the heating. However, the peak intensity of quartz and albite phases increases slightly for their further crystallization as the firing temperature increases. The major phases of prepared membranes whose sin-

3.2. Chemical composition and phase transformation As listed in Table 2, the major chemical component of natural zeolite is SiO2 (74.7 wt%). Besides Fe2 O3 and Al2 O3 , there is also a small quantity of alkali metallic oxides, such as K2 O, Na2 O, etc. Table 1 The average particle sizes of six fractions with different sedimentation time Sedimentation time (min) 0 10 20 30 50 100 Average particle diameter (␮m) 6.88 3.58 2.45 2.16 2.06 1.24

Table 2 X-ray fluorescence spectrum analysis result of natural zeolite powders (semiquantity) Components

SiO2

A12 O3

Fe2 O3

K2 O

CaO

Na2 O

Proportion (wt%) Proportion (mol%)

74.7791 84.836

10.1508 6.774

6.4138 2.728

3.4723 2.514

2.5885 3.146

0.6759 0.74

Fig. 4. X-ray diffraction patterns of the samples: (A) raw materials; (B) sintered at 750 ◦ C for 1 h; (C) sintered at 850 ◦ C for 1 h; (D) sintered at 950 ◦ C for 1 h. Q, quartz; A, albite; H, heulandite; M, montmorillonite.

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Fig. 5. Membrane thickness vs. the square root of coating time for different solid loading suspensions: (a) 8 wt%; (b) 12 wt%; (c) 16 wt%; (d) 20 wt%; (e) 24 wt% (All the membranes sintered at 900 ◦ C for 1 h; the open porosity is 48.7%.).

tering temperature is higher than 850 ◦ C are almost quartz and albite. 3.3. Membrane thickness controlling Fig. 5 shows the relationship between membrane thickness and dipping time using suspensions with different solid loading. When the solid loading of suspension is less than 16 wt%, the thickness of the sintered membrane is more or less linear dependence with the respect to the square root of dipping time, indicating a slip casting mechanism during the dip-coating process [2]. However, membrane thickness increases linearly with increasing dipping time but then keeps constant almost when the solid loading exceeded to 20 wt%. This is partly because the capillary force disappeared almost when the active pores of support were filled with the highly viscous suspension [17]. Consequently, the formation of coating ceased immediately. A large quantity of high polymer organic additives with long chains in the viscous suspensions could inhibit the diffusion of the solids

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from cake into suspension so that the reduction of membrane thickness is little. Even if the dipping time prolonged, the membrane thickness kept almost constant (see (d) and (e) in Fig. 4). In the experiments, it also found that the thicker the green membrane was (especially when exceeded to 100 ␮m), the more cracks on the surface of the prepared membrane during sintering visually there were (see Fig. 6(a)). This is because the shrinkage stress in the thick membrane was not uniform. To solve this problem, two measures were taken: the controlling of membrane thickness and using PVA as additives to increase the fracture resistance of membrane. The polyvinyl alcohol as a membrane-forming agent was dissolved into the slurry to facilitate the covering of the active pores of support and prevent the generation of cracks when green membranes were dried and fired. If too much binder, the membranes tended to generate pinholes and big pores during heating (shown in Fig. 6(b)), which was caused by the detachment of large masses of organic materials from small area. Through repeated trials, no-flaws membrane were prepared (Fig. 6(c)). 3.4. Interlayer surface morphology development Surface SEM images of the membranes fired at different temperatures are illustrated in Fig. 7. It can be seen, for the specimen made at 750 ◦ C for 1 h there are only spot contacts among solid particles around which a small amount of ultra-fine powders are distributed uniformly. With the sintering temperature increasing, the surface of particulate solid became round and smooth slightly (Fig. 7b). And the ultra-fine powders could be hardly observed, which is suggested that the fine particles had melted and precipitated on the surface of coarse ones during sintering. When heating temperature reached to 950 ◦ C, the sintering necks had grown up visibly and the particle surface surrounded by a little liquid phase became more slippery and round in shape. Liquid phase appeared without adding sintering aid can be ascribed to two factors: the major chemical composition of natural zeolite is SiO2 with low melting temperature range; the impurities of alkali metal oxides, such as K2 O, Na2 O, etc., as aid fluxes could lower the fusion point of ceramic materials so that sintering would occur at relatively low temperature

Fig. 6. Comparison for SEM micrographs of membranes influenced by thickness and PVA content: (a) cracks and big pores in the interlayer with thickness 90 ␮m and 2 wt% PVA content; (b) heterogeneous pore structure for membrane cross-section with thickness 90 ␮m and 2 wt% PVA content; (c) no flaws in the membrane with thickness 35 ␮m and 0.6 wt% PVA content.

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Fig. 7. SEM photographs of tubular membrane surfaces at different firing temperatures for 1 h: (a) 750 ◦ C; (b) 850 ◦ C; (c) 950 ◦ C; (d) 1050 ◦ C.

[7], which is consistent with the chemical composition analysis result in Section 3.2. The existence of a proper amount of liquid phase is very advantageous to accelerate mass transport and the re-arrangement of particles under the action of the capillary stress. Therefore, glass phase would form at low temperature and increase the self-bonding force between particles in the membrane. However, too much liquid phase filled into the interstices between solid particles when membranes were fired at 1050 ◦ C for 1 h. And the size of pores became uniform as a result of particle boundary movement (Fig. 7d). This could destroy pore

structure of the membrane, which is obviously unsuitable for practical application. 3.5. Pore size distribution, average pore size of interlayer The pore size distributions of tubular membranes sintered at various temperatures are displayed in Fig. 8A. When the heat treatment temperature increases from 750 to 950 ◦ C, the pore size distributions become narrow because of the restraint sintering shrinkage of the interlayer membrane on rigid support and the melt precipitation of fine particles into the coarse ones during

Fig. 8. Pore size distributions (A) and the average pore size (B) of membranes sintered at different temperatures.

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tion resistance when the sintered particles became smooth based on the Hagen–Poiseuille equation [18]. The membranes sintered at lower temperature (<800 ◦ C) had weak self-bonding force among solid state particles. So a temperature range of 850–950 ◦ C for 1 h was normally taken as membrane sintering system according to the above stated. Membranes of different characteristics could be prepared through adjusting the heat treating temperature according to the practical need. 3.6. Characterization of the resulting membranes with two layers

Fig. 9. The pore size distributions for support, interlayer and top-layer.

early stage of sintering. But, the pore size distributions become broad as temperature increasing further, which is due to the filling of much liquid glass phase into some pores among solid state particles. This is also explained by the SEM observation in Section 3.4. From Fig. 8B, the average pore size of tubular membrane rises linearly from 750 to 950 ◦ C and sharply from 950 to 1050 ◦ C with an increase in sintering temperature, respectively. The particles in the membrane can only locally grow up on the surface of support so that the membrane could hardly shrink in the direction perpendicular to the tangent plane of support, resulting in that the pores between grown-up solid particles becomes coarse. The sharp increase in pore size can be ascribable to two reasons: firstly, when increasing the sintering temperature, particles in the membrane grew up by liquid phase sintering, which led to the disappearance of small pores and formation of coarse pores; secondly, the pore size of membrane measured by the bubble-point method increased slightly because of the reduction of penetra-

The pore size distributions of the different membrane layers are shown in Fig. 9. The mean pore sizes of support, interlayer and top-layer are 5.86, 0.85 and 0.54 ␮m, respectively. It is also suggested that the pore size distribution of the top-layer becomes narrower than that of the interlayer in despite of the effect of layer thickness. Both the average pore diameter and the maximal pore size diminished after the top-layer coated while the thickness of membrane increased from 20.8 to 42.8 ␮m approximately. There was a quick membrane formation rate where there were the coarser pores in the interlayer membrane [19]. Therefore, the coarse pores and flaws were removed during the second coating. From cross-section SEM micrograph in Fig. 10(a) of the resulting membrane, it is noticed that there were no cracks and few particles permeating into pores of support. This is testified that the used suspension could meet the request for forming membrane during the dip-coating process. The support and the interlayer contact so tightly although the membrane was broken for SEM analysis. The contact between interlayer and top-layer is also good. The membrane surface SEM photograph (Fig. 10b) indicates that the pore distribution of the membrane is very uniform and few defects could be observed. Some coarse particles in the surface of membrane are due to the limitation of the classification method.

Fig. 10. SEM photographs of the resulting sintered membrane: (a) cross-section and (b) surface.

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Table 3 Permeate flux of support, interlayer and top-layer Layer

Support

Interlayer

Top-layer

Average pore size (␮m) Thickness (␮m) Nitrogen permeate flux (l m−2 h−1 ×10−5 Pa−1 (l m−2 h−1 bar−1 )) Pure water permeate flux (l m−2 h−1 ×10−5 Pa−1 (l m−2 h−1 bar−1 ))

5.86 – 2.26 × 106 26.6 × 103

0.85 20.8 5.02 × 105 6.21 × 103

0.54 22.8 1.96 × 105 3.20 × 103

The operation condition is cross-flow, 5 m/s, room temperature (17 ◦ C); membranes measured were sintered at 900 ◦ C.

not affect the pore-structure and permeability of the prepared membrane. The interlayers with pore size of 0.69–1.1 ␮m that could be controlled by adjusting the sintering temperature were prepared on the outer surface of single-channel tubular support. And the top-layer with average pore diameter centered on 0.54 ␮m had been deposited. For non-expensive natural zeolite mineral, the existence of impurities alkali metal oxides made the membrane sinter at low temperature as fluxing agent and therefore the process cost could be reduced. These low cost mineral-based membranes may be the potentially excellent candidates for use in micro-filtration application, such as the pre-purification of wastewater. Acknowledgements The authors are grateful to the Ministry of Science and Technology of China for financial support (No. 2003CB615700) and to the reviewers for their good advice. References Fig. 11. Photograph of the resulting tubular membrane made with natural zeolite.

The permeability of the resulting membrane with two layers was measured with the trans-membrane pressure of 0.1 MPa at room temperature. Table 3 shows the nitrogen gas flux and pure water flux for each layer. The tubular membranes with good permeate flux could be promising materials for the purification application (nitrogen gas flux: 1.96 × 105 l m−2 h−1 ×10−5 Pa−1 (1.96 × 105 l m−2 h−1 bar−1 ); pure water flux: 3.20 × 103 l m−2 h−1 ×10−5 Pa−1 (3.20 × 103 l m−2 h−1 bar−1 )). The material photograph of the fabricated membranes are shown in Fig. 11. 4. Conclusions In this work, it has been shown that low cost porous mineralbased ceramic membranes with an asymmetric configuration could be successfully developed by dip-coating. The powders for preparing two layers were obtained by the gravitational sedimentation classification. The framework crystalline structure of heulandite and the pillar structure of montmorillonite were both destroyed during heat treatment of membrane revealed by XRD analysis. However, the destroying of crystalline structure of this mineral did

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