Elaboration and chemical corrosion resistance of tubular macro-porous cordierite ceramic membrane supports

Journal of Membrane Science 304 (2007) 65–75

Elaboration and chemical corrosion resistance of tubular macro-porous cordierite ceramic membrane supports Yingchao Dong a , Xuyong Feng a , Dehua Dong a , Songlin Wang a , Jiakui Yang b , Jianfeng Gao a , Xingqin Liu a,∗ , Guangyao Meng a a

USTC Lab for Solid State Chemistry & Inorganic Membranes, Department of Materials Science and Engineering, University of Science and Technology of China (USTC), Hefei 230026, PR China b Hefei Great Wall New Century Membrane Science & Technology Co. Ltd., Hefei 230601, PR China Received 20 April 2007; received in revised form 19 June 2007; accepted 23 June 2007 Available online 3 July 2007

Abstract Tubular macro-porous supports for ceramic micro-filtration membranes were prepared by extrusion followed by sintering from low cost commercial grade cordierite powder. The effects of sintering temperature on the micro-structure, fracture strength, LTEC (linear thermal expansion coefficient), pore size distribution and permeation flux were studied. Our results indicate that the optimized sintering temperature was in the vicinity of 1380 ◦ C. The fabricated cordierite supports showed an average pore diameter of 8.66 ␮m, open porosity of 36.20%, nitrogen gas flux of 1.45 × 104 m3 m−2 h−1 and LTEC of 4.34 × 10−6 ◦ C−1 (between 25 and 1000 ◦ C). To verify the practicability in the strong corrosive environment, the chemical resistance tests were carried out in the sodium hydroxide and sulfuric acid solutions under the boiling condition, respectively. Compared with the as-used alumina supports, the cordierite supports exhibited better thermal alkali resistance but poorer acid resistance according to the mass loss results. In addition, the corroded cordierite supports were characterized by fracture strength, open porosity and pore size distribution. The fabricated macro-porous supports are expected to have potential applications in the pre-treatment of strong alkali media and dust-containing hot gas. © 2007 Elsevier B.V. All rights reserved. Keywords: Ceramic membrane; Porous support; Cordierite; Chemical resistance; Low cost

1. Introduction Various types of porous ceramic separation membranes such as MF (micro-filtration), UF (ultra-filtration) and NF (nano-filtration) are extensively studied due to their excellent mechanical strength, chemical resistance, thermal stability and long working time. Nowadays, MF ceramic membranes are commercially available for some industrial separation applications. For this membrane type, single layer or multi-layer membrane is coated on a macro-porous support to form an asymmetric structure with a pore size gradient [1–3]. Such kinds of supports should offer sufficient mechanical strength for system to withstand pressure gradients imposed during the practical applications [4]. Large pore size and high flux are also essential to

∗

Corresponding author. Tel.: +86 551 3606249; fax: +86 551 3607627. E-mail addresses: [email protected] (Y. Dong), [email protected] (X. Liu). 0376-7388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2007.06.058

keep the flow resistance of membrane as low as possible. In addition, the preparation and performance of follow-up membrane layers are directly affected by their supports. At present, unfortunately, the preparation of ceramic membrane supports is prevalently reported as a key technical secret in the form of patent. Conventionally, alumina (␣-Al2 O3 % ≥ 99%) is considered as the main body materials for commercialized porous ceramic membrane supports. However, both expensive starting materials and high cost of sintering limit their further applications in more industrial fields. Moreover, it is quite difficult to obtain the porous supports with large pore size and good thermal shock resistance from alumina because of its high melting point (2050 ◦ C) and high thermal expansion coefficient (theoretical value: 8.80 × 10−6 ◦ C−1 at 200–1000 ◦ C). In order to be applied on a large scale, a major concern in ceramic membrane supports today is to continue to lower their fabrication costs using three strategies: (a) exploitation of inexpensive starting materials [5–11], (b) reduction of sintering temperature by adding effective sintering aids [12,13] and

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(c) adoption of simple fabrication processes such as centrifugal casting [14,15]. As a popular ceramic material, readily available and easily massively produced, cordierite (stoichiometric formula: 2MgO·2Al2 O3 ·5SiO2 ) has been widely studied in last decades for its interesting properties, especially low LTEC. This uniqueness of cordierite is caused by the negative lattice expansion coefficient in the c-axis direction, which provides excellent resistance to thermal shock as well as low thermal conductivity [16–18]. This makes it suitable to be applied where the environmental temperature is rapidly and severely changed such as refractory products for industrial furnaces and electric heaters [19], heat exchangers for gas turbines, thermal shock-resistant tableware, and monolithic catalyst supports for diesel automobiles [20–23]. Recently, porous cordierite has been used as the supports for porous ceramic separation membranes by some researchers [24–29]. However, no elaborate and comprehensive research has been done to prepare cordierite supports and characterize their acidic and alkali corrosion resistance. The use of cordierite as porous support materials offers some possible advantages: (1) low fabrication cost including not only inexpensive raw materials which could be obtained from sintered natural clays, but relatively low heat treatment temperature because of its low melting point (1465 ◦ C); (2) excellent resistance to thermal shock due to its low LTEC (theoretical value: (1.0–2.0) × 10−6 ◦ C−1 between 20 and 800 ◦ C); (3) as reported in this study, the cordierite supports are expected to have good resistance to thermal alkali solution. In the current work, therefore, tubular macro-porous cordierite supports were prepared by extrusion followed by sintering using low-cost industrial grade powder. The properties of the porous cordierite were discussed as a function of sintering temperature in order to optimize the preparation conditions. For the corrosion resistant application, the fabricated supports were characterized in terms of mass loss, fracture strength and the variations in open porosity and pore size distribution before and after chemical corrosion using alkali (NaOH) and acidic (H2 SO4 ) solutions under the boiling state, respectively. In particular, a corrosion mechanism was proposed according to the SEM-EDS (scanning electronic microscopy coupled with energy dispersive spectroscopy) and XRD (X-ray diffraction) results. 2. Experimental procedure 2.1. Starting materials In this work, industrial grade cordierite powders were used as the main starting materials. The coarse powder was obtained by sieving as-received cordierite using different sieve types. After sieving, the powder with particle diameter ranging from 100 to 250 ␮m was used to prepare macro-porous supports. High-quality kaolin (−500 mesh) was purchased from Rizhao Ceramic Science & Technology Co. Ltd. The mixture of fine kaolin and basic carbonate magnesium ((MgCO3 )4 · Mg(OH)2 ·5H2 O), referred as KM (K: kaolin; M: basic carbonate magnesium), was used as sintering aid. Another function of

adding kaolin was to improve the plasticity of the paste which consisted of coarse cordierite. The composition of the KM mixture (K:M = 3.25:1, in mass proportion), which was calculated according to the ternary phase diagram of MgO–Al2 O3 –SiO2 system, located in the phase-forming region of cordierite [30]. 2.2. Fabrication of the samples The sieved cordierite powder, sintering aid (KM) and organic additives were uniformly mixed for 24 h using polyurethane-coated steel balls in a polyethylene pot. Cornstarch (d50 = 18.10 ␮m) was used as pore-forming agent and MC-400 (methylcellulose) as organic binder. After adding tap water, glycerol, surfactant and lubricant, the mixture was pugged for 1 h. Then, the obtained paste was kept in a closed plastic bag for 2 days under high humidity environment to avoid premature drying and ensure homogeneous distribution of moisture and organic additives. After vacuum pugging and aging twice, the paste was extruded into the tubular specimens. The wet tubes were set on rollers to ensure homogenous drying and avoid twisting at room temperature. Afterwards, the dried samples were heated from room temperature to 450 ◦ C at a rate of 1 ◦ C min−1 . An hour was held at 450 ◦ C in order to remove organic materials. Then the pre-fired samples were heated at a rate of 3 ◦ C min−1 up to the final sintering temperature, and was maintained for 2 h and then cooled to room temperature naturally. For mechanical strength measurements, the porous cordierite bars were prepared by uniaxial cold-pressing at 200 MPa from the mixture with the same formulation as the extrudates. The specimens were polished by 200 mesh and then 800 mesh metallographic sandpapers before test. 2.3. Corrosion resistance tests Chemical resistance of the cordierite supports sintered at 1380 ◦ C for 2 h was characterized in terms of mass loss, fracture strength (the strip samples), open porosity and pore size distribution (the tubular samples) after boiling in the acidic and alkali solutions. The specimens were cut and then placed in H2 SO4 (20 wt.%) and NaOH (1, 5 and 10 wt.%) solutions, respectively, and afterwards boiled at 105–107 ◦ C for different time (2, 4, 6 and 8 h). Mass loss of tubular porous alumina supports (average pore diameter: 2.96 ␮m; open porosity: 39.80%; sintered at 1620 ◦ C for 2 h) was also characterized under the same conditions to be compared with the cordierite supports. In this study, the alumina supports were supplied by our Great Wall New Century Membrane Science & Technology Co. Ltd. (Hefei, China). 2.4. Characterization techniques The as-received powders (cordierite and kaolin) were characterized by an X-ray fluorescence spectrometer (XRF-1800, Shimadzu Corporation, Japan). Crystalline phase composition was analyzed by XRD (Philips X’ Pert PRO SUPER, Philips Corporation, Netherlands; Cu K␣ radiation, working voltage 40 kV, working current 50 mA, scanning speed of

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4◦ 2θ/min). Cross-sectional micro-structures of the prepared porous cordierite were observed by using SEM (Scanning Electron Microscope; KYKY1010B, China). Porosity and bulk density were determined using the standard Archimedes method with distilled water as an immersion medium. All the measurements were conducted four times to get average values. Fracture strength was determined by the three point bending method in an apparatus, named as universal testing machine (DCS-5000, Shimadzu Corporation, Japan), with a span length of 30 mm and crosshead speed of 0.5 mm/min. Then fracture strength was calculated according to the following expression (ISO9693 1999). σ=

3Pl 2bh2

where σ is the fracture strength (Pa), P the fracture load (N), l the span length (m), b the width of samples (m), and h is the height of samples (m). Average fracture strength derived from three samples. LTEC measurement of the sintered cordierite bars was carried out in a horizontal dilatometer (DIL 402C, Netzsch, Germany). All the samples were heated from room temperature (25 ◦ C) to 1000 ◦ C at a heating rate of 10 ◦ C min−1 . Pore size distributions and nitrogen gas fluxes were examined, respectively, by the bubble point method on the home-made

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equipment with nitrogen gas as a permeation medium. Pure water fluxes across tubular porous cordierite were obtained on the home-made cross-flow permeation device. All the samples were measured at room temperature and the gas and liquid fluxes were calculated at a trans-membrane pressure of 0.10 MPa. For pure water flux measurement, the flow velocity of pure water across cordierite supports is 5 m/s. The thickness is 2.47 mm for the 1350 ◦ C sample, 2.42 mm for the 1380 ◦ C sample and 2.21 mm for the 1400 ◦ C sample. To study the corrosion mechanism in the strong alkali and acidic media, SEM-EDS (INCA300, Oxford Instrument, England) micro-region composition analysis was carried out on the three randomly selected points in the surface of the samples un-corroded, corroded in 10 wt.% NaOH for 4 h and corroded in 20 wt.% H2 SO4 for 4 h. In addition, crystalline phase was detected by XRD for the samples before and after corrosion by 20 wt.% H2 SO4 for 8 h and 10 wt.% NaOH for 8 h, respectively. 3. Results and discussion 3.1. Characterization of starting materials The as-received cordierite powder was characterized by XRF and its chemical composition (in wt.%) is displayed as follows:

Fig. 1. Cross-sectional micrographs of the tubular samples sintered at different temperatures for 2 h: (a) 1300 ◦ C; (b) 1350 ◦ C; (c) 1380 ◦ C; (d) 1400 ◦ C.

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Table 1 Porosities and bulk densities of the cordierite supports sintered at different temperatures for 2 h Sintering temperature (◦ C)

Total porosity (%)

Open porosity (%)

Closed porosity (%)

Bulk density (g cm−3 )

1300 1350 1380 1400

45.77 43.85 37.69 17.69

44.97 42.84 36.20 14.46

0.80 1.01 1.49 3.23

1.41 1.46 1.62 2.14

51.25% SiO2 , 31.68% Al2 O3 , 13.90% MgO, 0.33% CaO, 0.90% Fe2 O3 , 0.39% TiO2 , 0.92% K2 O, 0.42% Na2 O and 0.21% other oxides (Cr2 O3 and ZrO2 ). The SiO2 /Al2 O3 /MgO mass ratio of the as-used cordierite is close to that of stoichiometric cordierite (51.37 wt.% SiO2 , 34.93 wt.% Al2 O3 and 13.70 wt.% MgO). The XRD result (not shown) indicates that only the diffraction peaks of ␣-cordierite are observed. Other minor phases are almost non-existent. The content of cordierite is about 95%, which was provided by the manufacturer. The chemical composition (in wt.%) of the as-used kaolin obtained by XRF is: 40.97% Al2 O3 , 47.49% SiO2 , 6.28% CO2 , 2.10% SO3 , 0.65% PbO, 0.56% P2 O5 , 0.54% K2 O, 0.49% Fe2 O3 , 0.34% TiO2 , 0.28% CaO, 0.12% ZnO and 0.18% other minor oxides (Na2 O, SrO, NiO and ZrO2 ). Fine kaolin powders, together with basic carbonate magnesium, were ball-milled for 24 h using zirconia balls with ethanol as a liquid medium. The ground mixture exhibited an average particle diameter of 1.82 ␮m. 3.2. Fabrication of cordierite supports 3.2.1. Micro-structure observation Fig. 1 illustrates the cross-sectional micrographs of the tubular samples sintered at different temperatures for 2 h. It is found that the micro-structures depend on sintering temperature greatly. At 1300 ◦ C, some fine particles derived from the KM mixture are observed, which were distributed on the surface of coarse cordierite particles at random. This suggests low bonding strength of cordierite supports because no significant sintering took place between these fine particles and coarse cordierite. For the sample made at 1350 ◦ C, the amount of the randomly distributed fine particles decreases (Fig. 1b). Most of the added KM fine powder dissolved and then precipitated onto the coarse cordierite solids. As sintering temperature increases further, the surface of coarse cordierite becomes smooth. This is due to that more liquid glassy phase was produced on the particulate surface preferentially where there were prisms and corners with high surface energy. According to the liquid isotherm line in the ternary phase diagram of MgO–Al2 O3 –SiO2 system, it is noted that above 1355 ◦ C the amount of liquid phase increases quickly with increasing temperature based on the lever principle [30]. The presence of liquid phase led to enhanced densification through enhanced rearrangement of the coarse particulate solids under the influence of capillary stress gradients and enhanced matter transport through the liquid phase. The liquid wet and spread over the coarse cordierite particulate surface, and therefore the solid–vapor interface area of the particulate system was decreased and pores were formed. This provided a driving

force for densification. However, some approximately spherical closed pores were also formed in the melting liquid during the sintering of the system, which can be observed for the sample sintered at 1400 ◦ C (Fig. 1d). The formation of these closed pores and excessive liquid phase should be avoided during the fabrication process of porous supports. 3.2.2. Porosity and bulk density Table 1 presents the porosities and bulk densities of the tubular porous cordierite supports sintered at various temperatures for 2 h. As seen, the open porosity decreases slightly from 1300 to 1350 ◦ C but sharply from 1380 to 1400 ◦ C, the latter is due to the significant sintering shrinkage, as well as the filling of the gaps between cordierite particles by liquid glassy phase. The closed porosity increases with sintering temperature because some closed pores formed in the liquid phase of low viscosity at higher temperatures. This is in accordance with the SEM observation in Section 3.2.1. At 1400 ◦ C, the closed porosity reaches 3.23%. As expected, the bulk density increases with sintering temperature due to the volume shrinkage induced by sintering. A sharp increase could be observed between 1380 and 1400 ◦ C because of the vast sintering shrinkage in this temperature range. When the samples were sintered at 1380 ◦ C for 2 h, the bulk density is only 1.62 g cm−3 . 3.2.3. Fracture strength and LTEC Fig. 2 shows the fracture strength and LTEC of the porous cordierite supports as a function of sintering temperature. It is noted that the fracture strength increases slightly between 1300

Fig. 2. Fracture strength and LTEC (linear thermal expansion coefficient) of the sintered cordierite bars as a function of sintering temperature with a holding time of 2 h.

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and 1350 ◦ C but then sharply between 1350 and 1400 ◦ C (more obviously from 1380 to 1400 ◦ C). The latter is due to that the sufficient sintering of porous cordierite supports resulted in the further growth of more sintering necks and enhanced densification. This supports the result of the decreasing trend in total porosity with sintering temperature (Table 1). After sintering at 1380 ◦ C for 2 h, the porous cordierite exhibits an average fracture strength of 31.03 MPa. As seen in Fig. 2, the effect of sintering temperature on the thermal expansion performance is relatively little. The LTEC decreases from 4.84 to 4.43 × 10−6 ◦ C−1 from 1300 to 1350 ◦ C, and then maintains in the range of (4.30–4.43) × 10−6 ◦ C−1 from 1350 to 1400 ◦ C. Between 1300 and 1350 ◦ C, the amount of formed cordierite, derived from the sintering aid KM, increased quickly. This resulted in the decrease in LTEC. Although the total porosity decreases dramatically from 1350 to 1400 ◦ C (Table 1), the LTEC decreases very slightly. This suggests that all the gas pores dispersed relatively uniformly in the prepared cordierite supports. The porous cordierite sintered at 1380 ◦ C shows an average LTEC of 4.34 × 10−6 ◦ C−1 between 25 and 1000 ◦ C. This moderate LTEC value is mainly due to the existence of other compositions in the starting materials such as silica-rich glassy phase, alkaline and alkaline-earth metal oxides (Na2 O, K2 O, MgO, CaO, etc.) [31]. 3.2.4. Pore size distribution, nitrogen gas and pure water fluxes A pressure drop should be minimized in any filtration application because an additional energy is required to enable the fluid not only to flow but also to cross the physical barrier. Therefore, the support should be porous and have large pore size. Fig. 3 shows the pore size distributions of the tubular cordierite supports sintered at different temperatures for 2 h. In this study, the pore size distribution of the sample at 1300 ◦ C could not be obtained due to its very low mechanical strength. The samples were very fragile during the measurements. With increasing sintering temperature from 1350 to 1380 ◦ C, the average pore

Fig. 3. Pore size distribution of the tubular cordierite ceramic membrane supports sintered at various temperatures (the thickness is 2.47 mm for the 1350 ◦ C sample, 2.42 mm for the 1380 ◦ C sample and 2.21 mm for the 1400 ◦ C sample).

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diameter decreases slightly from 9.49 to 8.66 ␮m. This could be ascribed to the pore shrinkage induced by the solid state particulate sintering. The effect of thickness of the measured samples on pore size distribution can be neglected because the thickness difference is only 0.05 mm (according to the Hagen–Posieuille equation [32]). But at 1400 ◦ C, the pore size decreases dramatically and its distribution becomes more discrete in the range of 2–18 ␮m. There are two factors: (a) significant shrinkage of gas pores due to liquid phase sintering; (b) more liquid glassy phase of low viscosity flowed into the large gaps between coarse cordierite solids. From 1350 to 1380 ◦ C, the nitrogen gas flux decreases slightly from 1.57 × 104 to 1.45 × 104 m3 m−2 h−1 and pure water flux from 103.37 to 94.79 m3 m−2 h−1 . A significant decrease occurred for both nitrogen gas and pure water permeation fluxes when sintering temperature increased from 1380 to 1400 ◦ C. The 1400 ◦ C sample exhibits a nitrogen gas flux of only 1.28 × 103 m3 m−2 h−1 and pure water flux of only 10.44 m3 m−2 h−1 . The dramatic reduction of pore size, combined with the considerable decrease in open porosity, resulted in this significant decrease in gas and pure water permeation fluxes. A small variation in sintering temperature led to a big variation in the main properties of the porous cordierite supports. Therefore, it is concluded that the tubular cordierite supports sintered at 1380 ◦ C for 2 h have the optimum properties such as enough porosity, moderate LTEC, sufficient mechanical strength, narrow pore size distribution and high permeation flux. 3.3. Chemical corrosion resistance of porous cordierite 3.3.1. Mass loss Cordierite and alumina supports were corroded respectively under a boiling state for different time in the thermal H2 SO4 solution (20 wt.%). The mass loss is shown as a function of boiling time in Fig. 4. An approximately linear relationship exists between mass loss and boiling time for both alumina and cordierite. Compared with the alumina supports, the cordierite

Fig. 4. Mass loss of the tubular porous cordierite and alumina supports boiled in the 20 wt.% sulfuric acid solutions as a function of time (the solution temperature was in the range of 105–107 ◦ C).

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Fig. 5. Mass loss of the tubular porous cordierite and alumina supports boiled in different concentration alkali solutions for various boiling time (the solution temperature was in the range of 105–106 ◦ C).

supports show a poorer acid-corrosion resistance. For the same corrosion time, the mass loss of cordierite supports is far greater than that of alumina supports. Also, the mass loss rate of cordierite supports (0.026 h−1 ) is greater than that of alumina (0.0029 h−1 ). The poor acid resistance of the cordierite supports is mainly due to the nature of cordierite, as well as the existence of alkaline metal or alkaline earth metal oxides in the starting materials (see Section 3.1). The corrosion mechanism would be discussed in detail below. Even after boiling for 8 h, the alumina supports have a mass loss of only 1.25 wt.%. But for porous cordierite, the mass loss is as high as 17.01 wt.%. Fig. 5 displays the mass loss of the cordierite and alumina supports after boiling in the sodium hydroxide solutions of different concentrations (1, 5 and 10 wt.%). As can be seen, the cordierite supports exhibit a better thermal alkali resistance than the alumina supports. With increasing boiling time, the mass loss of all the samples is increased slightly for low concentration alkali solutions, but rapidly for high concentration alkali solutions. This is due to that more hydroxide ions reacted with the samples in the high concentration alkali media at the same corrosion time. Even if corroded in the 10 wt.% NaOH solution, the cordierite supports still show lower mass loss than the alumina supports that were corroded in the 5 wt.% NaOH solution. Kaolin clay was added as sintering aid and plasticizer during the fabrication process of alumina supports. Sintered kaolin was first corroded in the thermal alkali solution due to its impurities and abundant free silica. And then the chemical reaction between alumina grains and OH− took place, just shown as follows: Al2 O3 + OH− → AlO2 − + H2 O This corrosion reaction first occurred between the grain boundaries of alumina where the impurities and defects were centralized. The produced sodium meta-aluminate dissolved into water easily and therefore the mass of alumina supports decreased quickly. For high concentration NaOH solution, it was also observed that after boiling for a long time some fine alumina particles from the supports appeared at the bottom of flask (corrosion container) due to the corrosion of sintering necks.

Fig. 6. The variation in fracture strength of the cordierite porous supports before and after boiling in the 10 wt.% NaOH and 20 wt.% H2 SO4 solutions for different time, respectively. The fracture strength loss ratios are also depicted in this figure.

3.3.2. Mechanical strength variation To verify strength resistance in different corrosion media, the fracture strength of the cordierite bars was measured after corroding in the 20 wt.% H2 SO4 and 10 wt.% NaOH solutions, respectively, and the result is shown in Fig. 6. The fracture strength loss ratios are also depicted in this figure. After acid corrosion, the residual fracture strengths are 20.79 MPa for 2 h and 14.00 MPa for 4 h, respectively. This also suggests that the cordierite supports dissolved into the corrosion medium partly and therefore a structure of more porous was formed. After boiling for a long time, the porous cordierite samples became fragile because the fracture strength decreased greatly at high acid leaching levels. The corrosion of NaOH aqueous solutions resulted in a slight decrease in fracture strength, just shown in Fig. 6. When corroded in 10 wt.% NaOH for 4 h, the cordierite support still exhibits an average fracture strength of 28.49 MPa. The strength loss ratio is only 8.16%, which is lower than that of 20 wt.% H2 SO4 corrosion (54.87%) at the same boiling time. Even after corroding for 8 h, the loss ratio of fracture strength is still 12.84%. Despite similar open porosity, the 10 wt.% alkali corroded samples show higher fracture strength than the 20 wt.% acid corroded samples. This is partially because, after alkali corrosion, glassy phase was first removed and therefore cordierite solids were still bonded by a lot of interlocked cordierite crystals of flake-like shape (explained below), which provided sufficient mechanical strength. Therefore, it is testified that the porous supports made from industrial cordierite are alkali resistant. 3.3.3. Open porosity and pore size distribution variations Table 2 shows the open porosity of the acidic and alkali corroded tubular cordierite samples. As expected, after acid corrosion, the open porosity increases quickly with boiling time because of the partial dissolution of the samples. As a result, a more porous structure was generated. As boiling time and the concentration of alkali solution increase separately, the open porosity increases slightly. The porous structure of tubular

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Table 2 Open porosity of the cordierite supports after corrosion in H2 SO4 (20 wt.%) and NaOH solutions (1, 5 and 10 wt.%), respectively, for different boiling time Corrosive medium

Concentration (wt.%)

Boiling time (h) 2

4

6

8

H2 SO4 NaOH NaOH NaOH

20 1 5 10

38.56 36.75 37.61 39.68

41.38 37.21 39.22 40.52

43.88 37.56 40.23 42.06

45.57 37.97 41.48 43.85

cordierite could be tailored by choosing the leaching conditions such as boiling time and the concentration of corrosion medium. Fig. 7 shows the pore size distributions of the tubular cordierite supports before and after acid (20 wt.% H2 SO4 ) and alkali (10 wt.% NaOH) corrosion for 4 h. It is observed that the pore size obviously increases and its distribution is typically bimodal for the acid corroded sample. After corrosion for 4 h, the average pore diameter increases up to 13.64 ␮m. In addition, some pores with diameter ranging from 4.40 to 6.00 ␮m were formed due to the generation of small holes in the coarse cordierite solids which were corroded by the acid solution. For the alkali-corroded sample, however, the pore size decreases slightly because the roughness of the pore wall increased after the removal of glassy phase. The average pore diameter is 8.37 ␮m after corrosion for 4 h. Similar with the un-corroded

Fig. 7. Pore size distribution of the tubular cordierite supports before and after corrosion in different media for 4 h.

sample, the alkali-corroded support also shows a slight bimodal distribution of pore size. 3.3.4. Corrosion mechanism analysis To study the corrosion mechanism in the strong alkali and acid media, SEM coupled with EDS micro-region composition analysis was carried out and the results are shown in Figs. 8 and 9. Three points were selected at random on the particulate sur-

Fig. 8. SEM microphotographs of selected particulate surface of the tubular porous cordierite supports before and after alkali and acid corrosion, respectively: (a) un-corrosion; (b) 10 wt.% NaOH solution for 4 h, low magnification; (c) 20 wt.% H2 SO4 for 4 h; (d) 10 wt.% NaOH solution for 4 h, high magnification. The regions of cross-mark in the photos were analyzed by EDS (shown in Fig. 9 and summarized in Table 3).

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Fig. 9. EDS microanalysis of the selected points (Fig. 8) on the particulate surface of the tubular porous cordierite supports: (1–3) un-corrosion; (4–6) alkali corrosion for 4 h (10 wt.% NaOH solution); (7–9) acid corrosion for 4 h (20 wt.% H2 SO4 solution).

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Table 3 Average composition of detected elements of the cordierite supports before and after acid and alkali corrosion (derived from Fig. 9) Elements (wt.%)

Stoichiometric Un-corrosion Alkali corrosion Acid corrosion

O

Mg

Al

Si

Fe

K

Ti

Zr

Ca

49.32 44.99 48.23 45.80

8.22 6.66 8.14 1.97

18.49 16.73 18.07 5.72

23.97 26.97 24.73 45.38

– 1.37 0.83 0.54

– 1.00 – 0.59

– 0.74 – –

– 1.24 – –

– 0.30 – –

The composition of stoichiometric cordierite is also shown for comparison.

face from each sample. The average element composition of every three point was calculated and then summarized in Table 3. Also, the composition of stoichiometric cordierite is added for comparison. After corrosion in the thermal NaOH solution, the surface of cordierite solids became slightly rough (Fig. 8b). Some small corrosion holes are observed. By comparing the average element composition before and after alkali corrosion (Table 3), it is noted that the relative content of Si element decreases slightly while that of Mg and Al increases. This reveals that a small amount of silica-rich glassy phase first dissolved into thermal NaOH aqueous solution to form sodium silicate. This resulted in that more interlocked cordierite crystals of a flakelike shape were exposed (Fig. 8d). As also seen from Table 3, the composition of the alkali-corroded sample is closer to that of stoichiometric cordierite than the un-corroded cordierite sample because of the dissolution of glassy phase. Therefore, the supports of purer cordierite phase were obtained. As seen in Fig. 9, the elements of K, Ca, Ti and Zr were not detected after alkali corrosion due to their slow dissolution into the thermal solution companied with glassy phase. SiO2 + 2OH− → SiO3 2− + H2 O Compared with the alkali-corroded sample, a structure of more porous is observed for the sample corroded in thermal H2 SO4 solution (Fig. 8c). This supports the above-mentioned results of quick loss in mass and fracture strength after acid corrosion (Sections 3.3.1 and 3.3.2). From Fig. 9, it is noted that the peak intensity of detected Al and Mg elements is weaker than that of the un-corrosive sample. This indicates that Al and Mg elements were removed from the cordierite structure after acid corrosion. The average contents before and after corrosion are 6.66 and 1.97 wt.% for Mg, 16.73 and 5.72 wt.% for Al, respectively. The [AlO]4 tetrahedra of six-membered rings which consisted of [SiO]4 and [AlO]4 tetrahedra were easily corroded by H+ in the thermal strong acidic solution because of the openness of cordierite structure. In the H2 SO4 aqueous solution, hydrogen ions (radius = 0.15 nm) easily entered some channels of six-membered rings (radius = 0.28 nm) and then corroded the [AlO]4 tetrahedra. And the [MgO]6 octahedra shared by six-membered rings were corroded by H+ to form solvent magnesium sulfate. Al2 O3 ([AlO]4 ) + 6H+ → 2Al3+ + 3H2 O MgO ([MgO]6 ) + 2H+ → Mg2+ + H2 O

Fig. 10. XRD patterns of the cordierite supports before and after boiling in the 20 wt.% H2 SO4 and 10 wt.% NaOH solutions for 8 h, respectively.

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As a result of the dissolution of Al and Mg oxides, the relative content of Si element is increased from 26.97 to 45.38 wt.% after boiling in the 20 wt.% H2 SO4 solution for 4 h (Table 3). For the acid corroded cordierite support, acid-resistant free amorphous silica was generated on the surface of cordierite solids. This is also testified by the XRD result, shown in Fig. 10. This figure reveals that, for the acid corroded sample, amorphous silica phase (Fig. 10b) is indicated by a broad underlying structure in the diffraction angle (2θ) range of 20–30◦ . This is consistent with the acid leaching results concluded by Elmer [33] (using nitric acid) and Shigapov et al. [34] (using hydrochloric acid). There is only cordierite phase for the sample corroded in the alkali solution (Fig. 10c). And no new phase was formed. This supports the EDS result. 4. Conclusions In this work, tubular porous cordierite supports for ceramic membranes were prepared by extrusion followed by sintering from low cost industrial grade powder. It is demonstrated that the supports with the optimized properties and good alkali resistance have been obtained. The main properties of porous cordierite supports such as pore size, gas and water fluxes, porosity and fracture strength could be adjusted by selecting sintering temperature between 1350 and 1400 ◦ C. But the effect of sintering temperature on the thermal expansion coefficient is little in spite of the significant reduction in total porosity. This endows us with the probability of fabricating multi-layer cordierite membranes with similar thermal expansion performance in the future. Compared with the alumina supports, the fabricated cordierite supports exhibited good resistance to the thermal strong alkali solutions. Residual silica-rich glassy phase first dissolved from the supports, and therefore cordierite of higher purity whose composition is close to stoichiometric cordierite was produced. However, the porous cordierite supports are not suitable for the application of strong acid environment due to the quick dissolution of Mg and Al elements and the subsequent generation of free amorphous silica. Low cost of starting materials, low sintering temperature and good alkali resistance are important for the commercialization of ceramic membrane supports. Thus, this is expected to be of great economic significance and applied value. Utilizing such ceramic membrane support not only enhances the already existing applications but also offers perspectives for some new potential applications such as the pre-purification of industrial strong alkaline wastewater and dust-containing hot gas. Acknowledgements The authors would like to thank the Ministry of Science and Technology of China for the financial supports (contract no. 2003CB615700). The editor and reviewers are gratefully acknowledged for their good advice. We also thank Dr. Ling Li (Rochester University, USA) for reading through and correcting the manuscript of this paper.

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