Kieselguhr–Mullite ceramic membrane substrate fabricated by line compounding technique

Journal of Membrane Science 204 (2002) 89–95

Kieselguhr–Mullite ceramic membrane substrate fabricated by line compounding technique Chuan-Feng Li, Shun-He Zhong∗ College of Chemical Engineering and Technology, State Key Laboratory of C1 Chemistry and Technology, Tianjin University, Tianjin 300072, PR China Accepted 5 January 2002

Abstract The properties of ceramic membrane substrate mainly affect the selective layer membrane on several aspects, such as operating condition and permeability. In this paper, a new type of Kieselguhr–Mullite (K–M) ceramic membrane substrate was fabricated by line compounding technique, where skeleton pores of Kieselguhr particles as the passage and dense Mullite as the linking substance. The substrate was proved by TGA, SEM, XRD, IR, mercury porsimetry and permeability measurement to be an ideal one with a narrow pore-size distribution of average pore-size 2.01 ␮m, porosity of 0.42 by volume and high permeability. There exist silica phase and Mullite phase stimulatingly, which demonstrates that it is successful in incorporating Mullite component by the precursor of silica–alumina sol. The front compressive strength of substrate is above 4.5 MPa and side one above 1.1 MPa when the content of Mullite phase exceeded to 10% (wt.%), which complete accorded to industrial applications. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Ceramic membrane substrate; Kieselguhr; Mullite; Fabrication; Line compounding technique

1. Introduction Industrial applications of inorganic membranes, particularly ceramic membranes, for separation activities like ultrafiltration, nanofiltration, wastewater treatment and gas separation are now well established and the preparation and characteristic of ceramic membrane substrate or support play an important role in asymmetric ceramic membrane that deposited on it [1]. In recent years, special emphasis has been placed on attaining the substrate with heat-resistant, chemical stability, fine pore structure, high mechanical strength and excellent gas permeability. It not only can be directly used for industry filtration (need ∗ Corresponding author. Tel.: +86-22-87893574. E-mail address: [email protected] (S.-H. Zhong).

pores <5–10 ␮m), but also can be used to provide high strength and permeability for top selective layer membrane in the manner of reduce its thickness and sustain the pressure diffusion necessary to obtain a sufficient flux [2]. A macroporous substrate can be produced by dry-pressing, paste processing, colloidal processing, centrifugal casting [3,4], doctor-blade [5] or gelcasting [6] and subsequent sintering of a ceramic powder. Nowadays, the ceramic membrane substrate is generally made of alumina, porous glass (Vycor glass) or carbon, but only a few attempts have been made to using silica or Kieselguhr as such ones. In the present work, a new type of membrane substrate “Kieselguhr–Mullite (K–M)” is fabricated by line compounding technology and shaped by slip-casting method. Its fabricating process and properties have also been investigated.

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2. Experimental 2.1. Materials The Kieselguhr is the one with the content of SiO2 85 wt.% and the industrial silica sol is a commercial product (Changhong Chemical Factory, Beijing, China) containing 25–26 wt.% of SiO2 in aqueous solution stabilized at PH 9. The reagents of aluminium powder (AR) and crystalline AlCl3 ·6H2 O (AR) both with 97% mayor components are the products of Tianjin Reagent Corporation, China. 2.2. Preparation of silica–alumina sol Excess quantity of aluminium powder was added into AlCl3 aqueous solution at room temperature, in turn heated slowly to 80 ◦ C and maintained for reaction up to no H2 and HCl emitting (checked by wet pH test paper). After filtration, amount of industrial silica sol was dropped into filtrate. Then the Mullite precursor silica–alumina sol was obtained after concentrated by heat to proper viscosity of 1–2 poise and pH 3.1–3.5 adjusted by acids, which the proportion of alumina to silica accorded with the Mullite component (3Al2 O3 ·2SiO2 ). 2.3. Preparation of ceramic membrane substrate The ultrathin Kieselguhr particles with grain size 40–45 ␮m and low degree of agglomeration were dried at 110 ◦ C for 6 h, and a quantity of distilled deionized water was added and dispersed uniform in them. The mobility homogeneous slurry with proper viscosity of 1–2 poises was obtained by mixture and vibrated ultrasonically (CQ-50 ultrasonic regenerator) when

Fig. 1. Sintering curve of K–M ceramic membrane substrate.

silica–alumina sol, water and Na2 CO3 or Na2 SiO3 were added. The slurry with the pH 5 adjusted by acids could be used only after aged for several hours, then was poured into plaster mold with contact time strict controlled in the range of 40–50 s. The wet tube was obtained after spilling out unconcreted slurry and dried at ambient temperature with 60% relative humidity for several days, in turn dried under 60 ◦ C temperature for 2 days. Finally, the green tube was sintered under a strict thermal processing schedule with different heat rate at different sintering stages, which was shown in Fig. 1. The ceramic membrane substrate available was in tubular configuration with a diameter of 18 mm a thickness of 3 mm and rough surface, it can be used as membrane substrate after polished on the outside using diamond polishing paper and cleaned by ultrasonic vibration in a bath of ethanol or acetone. The entire sintering process was illustrated in Fig. 2.

Fig. 2. Preparing diagram of ceramic membrane substrate.

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2.4. Characterization of the ceramic membrane substrate

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3. Results and discussion 3.1. Procedure model

Thermogravimetric analysis (TGA) on the substrate powder without sintering was used to analyzing their thermal behaviors with the heat rates of 10 ◦ C/min to 1250 ◦ C in flowing dry air by TG instrument (WCT-1, Beijing, China) and the results were processed by RSZ analysis systems. The surface morphology of the substrate was observed by enhanced scanning electron microscopy (ESEM, Philips XL30). The ceramic substrate materials crystalline phase structures were detected by using X-ray diffraction device (2038X, Rigaku) with Cu K␣ target. Infrared spectra were performed on the substrate by IR spectrometer (HITACHI 270-30 Spectrometer) to determine chemical groups. Pore-size distribution of the ceramic membrane substrate was characterized to determine its pore volume and pore-size distribution by mercury porsimetry (Micromeritics Autopore II 9220). Front and side compressive strength of the material by cut into 20 mm × 6 mm × 3 mm slices were determined by strength test measurement (QCY-620). Single component gas permeability of the ceramic membrane substrate was measured at room temperature by using the following gases: N2 , H2 , C2 H6 and CO2 . The tube was mounted into a stainless-steel membrane reactor and sealed by silicone rubber. The pressure dropped across the tube could be adjusted by changing valve on downstream and measured by pressure gauges.

The whole preparation procedure can be divided into four stages [7] and the model was shown in Fig. 3. (1) Pre-treatment of Kieselguhr by pore-protect agent. (2) Crosslinking of Kieselguhr particles with silica–alumina sol. (3) Shaping by slip-casting and drying with water evaporation. (4) Sintering of the ceramic membrane substrate. The skeleton pores of Kieselguhr particles need to be pore-protected at first in order to prevent the entry of silica–alumina sol and the content of pore-protect agent decided the finial pore volume and porosity of the substrate. Volatility liquid such as alcohol or acetone and low heat-resistant organic group can be used as pore-protect agent. In this work, distilled deionized water was used. Na2 CO3 or NaSiO3 used in this work as additive (deflocclant) to stabilized dispersal of slurry by absorbed on the ceramic particles and to adjust the interaction, cluster structure and viscosity properties among particles in virtue of surface charge action and space steric effect. Suitable dosage of them may improve the mobility of slurry remarkably, too low dosage will be suffered more difficulties in slip-casting process, but too high one may lead to precipitation and reduce the mobility of slurry. The degree of mixing affects the transforming temperature of Mullite phase. The more mixture uniformly done, the low temperature of Mullite crystallizes exothermally

Fig. 3. The whole procedure model of the ceramic membrane substrate.

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formed [8]. The viscosity was also important factor to decide slurry properties, too high a viscosity can cause inhomogeneous and subject to crack during the thermal treatment and too low a one was not favorite to slip-casting process by delaying shaping time and no liable to separating the tube own from the mold. During the age time, colloidal particles were aggregated and cross-linked with Kieselguhr particles to form network structure. After slip-casting and drying, most of water was removed, the Kieselguhr particles were stack compactly due to the shrinkage and densification of silica– alumina micelle clusters. The heat rate was the most important parameter in the sintering process and the sintering mainly serves to obtain sufficient strength by the formation of necks without significant grain growth and shrinkage [9,10]. The TGA and DTA in Fig. 4 show that absorbed water and organic groups begin to be eliminated around 250 ◦ C and the crystallization of amorphous silica particles in cristobalite occurs in the range of 860–1000 ◦ C. This phenomenon can be explained by strongly catalyzed by Na or other mineral impurities, such as Mg, Al, and so on. Unfortunately, the Mullite phase transformation was in this temperature to the moment, so we cannot get the transition temperature in detail. The total weight losses of the materials at 1200 ◦ C are calculated to be 35% and there was no weight losses and no obvious thermal effects when the temperature above 1100 ◦ C. So it should be controlled strictly in sintering procedure and avoided significant grain growth or shrinkage. At first (before 300 ◦ C), low heat rate should be used to prevent rapid growth of grains,

Fig. 4. TGA and DTA curves of ceramic membrane substrate without sintering.

which directly result in cracking and reforming of the materials. In the temperature range of 300–800 ◦ C, the combine water and decompose organic groups were removed, the heat rate may be raised a little. Under the high sintering stage (800–1200 ◦ C), silica–alumina sol was turned into Mullite phase and cross-linking with Kieselguhr particles, which process will result in a new shrinkage. So low heat rate should be used. After cooled slowly to room temperature, the total linear contraction percentage is less than 6%, which must be considered when making a certain size ceramic membrane substrate. 3.2. Surface morphology and crystalline structure Fig. 5 was the SEM photograph of the substrate. It can be seen that Kieselguhr maintain their instinct morphologies (sheet shape) and pore structures, which particle voids were filled with Mullite phases. Fig. 6 shows the XRD spectra of the ceramic membrane substrate with 10 wt.% Mullite. It can be observed that two phase exist simultaneously. The primary phase is cristobalite phase and the other phase is Mullite phase. It should be noted that Kieselguhr used in this work and the silica–alumina sol prepared by mixing of colloid sols were non-crystalline or amorphous. So the formations of the SiO2 crystalline phase can be considered as in the process of sintering. IR spectra of Kieselguhr, green substrate materials and ceramic membrane substrate materials were

Fig. 5. SEM photograph of ceramic membrane substrate.

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Fig. 6. XRD pattern of K–M ceramic membrane substrate material. Fig. 8. Pore-size distribution of the ceramic membrane substrate.

shown in Fig. 7. There was a broad absorption band at 850–1350 cm−1 , mainly corresponded to Si–O bonds of SiO2 , 466 cm−1 in (a), 464 cm−1 in (b) and 470 cm−1 in (c) were assigned to Si–O vibrations of SiO2 [11], 792 cm−1 in (a), 798 cm−1 in (b) and (c) were contributed to the octahedral Al–O bonds [12]. Compared to (b), there was a shift in (c) in the peak from 1080 to 1094 cm−1 suggests formation, in a certain degree, of Si–O–Al bonds, appearance of band at 560 cm−1 corresponds to the octahedral Al–O vibra-

tions characteristic of Mullite [11–13]. The absorption band at around 1035 cm−1 corresponds to tetrahedral Si–O–Al bonds [14]. The bands at 620–640 cm−1 correspond to critobalite are observed [12], which also registered by XRD analysis. The IR results indicate clearly that SiO2 –Al2 O3 system transformed from silica–alumina sol are not mere mixtures of SiO2 and Al2 O3 , but have Si–O–Al chemical bonds in the structure. 3.3. Pore structure The pore distribution of the ceramic membrane substrate was showed in Fig. 8. It seem obvious that the pore distribution was concentrated on about 2.01 ␮m, which accorded with the skeleton pore diameter of Kieselguhr particle. Because for the packing action of dense Mullite phase to Kieselguhr particles, the pore channels were mainly supplied by the skeleton pore of Kieselguhr particles and there were no pore diameter more than 10 ␮m when comparing to Kieselguhr ceramic membrane substrate [15]. The pore volume is 0.664 ml/g and pile density is 0.6273 g/cm3 , from which we can calculate the porosity to be 41.65% and the skeleton density to be 1.0750 g/cm3 . 3.4. Strength measurement

Fig. 7. IR spectra of Kieselguhr, green substrate and ceramic membrane substrate: (a) Kieselguhr; (b) green substrate; (c) ceramic membrane substrate.

The general mechanical characteristics of ceramic materials are high intrinsic strength and a low

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Fig. 9. Compressive strength of the ceramic membrane substrate: (a) front compressive strength; (b) side compressive strength.

toughness, i.e. they are very brittle. Fig. 9 showed that the front and side compressive strength are all increasing with the increasing of Mullite content. The silica– alumina sol was turned into dense Mullite and adhere tightly to Kieselguhr particles, which modified the microstructure and reinforced the total mechanical strength, which pressure-resistant strength were more than 4.5 MPa when the Mullite content exceeded 10%, and standard the industry requirement. Mullite phase incorporated by the converting of silica–alumina sol in this work was proposed as linking substance to enhancing strength of the substrate. With the same content of Mullite, the increasing rate of front compressive strength is higher a few than that of side ones. These phenomena indicate that Mullite phase used as strength-enhance agent is anisotropy and it must be arranged during the transformation. 3.5. Gas permeability measurement The permeability results on H2 , N2 , C2 H6 and CO2 flux are given in Fig. 10. It shows that the substrate is the one with high permeability and low permselectvity. The gases permeability of the substrate versus the (P12 − P02 ) show straight lines, these suggest a model of Poiseuille flow (viscous flow) or laminar flow, and there is almost no selectivity to mixture gases. Therefore, it must be supported other membrane layer with ultrathin and nanoscale pore-size to promote its gas permselectivity.

Fig. 10. Permeability of the ceramic membrane substrate.

4. Conclusions This study describes the fabrication of a K–M ceramic membrane substrate with skeleton pores of the Kieselguhr particles as passage and dense Mullite as linking substance or tenacity-enhance agent by line compounding technique. It is proved to be an ideal low density and high strength one with average pore-size 2.01 ␮m, porosity 0.42 and high gas permeability. The properties of slurry and thermal treatment process play a critical role in determining the performance of the K–M ceramic membrane substrate. Inhomogeneous slurry with unsuitable viscosity or irrelevant thermal treatment process will be subjected to cracking and deforming of the ceramic membrane substrate. The Mullite phase is incorporated successfully by the precursor of silica–alumina sol, while amorphous silica in Kieselguhr is turned into cristobalte phase and the mechanical strength of the ceramic membrane substrate is enhanced obviously; even the strengthenhance action of Mullite phase is anisotropic.

Acknowledgements The authors would like to thank the Chinese National Science Foundation (20076035) for the financial supports.

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