Fabrication and characterization of macroporous flyash ceramic pellets

M A TE RI A L S CH A RACT ER IZ A TI O N 62 ( 20 1 1 ) 8 1 7 –8 2 4

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Fabrication and characterization of macroporous flyash ceramic pellets Yosep Han a , Hyunjung Kim b,⁎, Jaikoo Park a,⁎⁎ a

Department of Natural Resources and Environmental Engineering, Hanyang University, #17, Heangdang-dong, Seongdong-gu, Seoul 133-791, South Korea b Department of Mineral Resources and Energy Engineering, Chonbuk National University, 664-14 Duckjin-Dong 1Ga, Deokjin-Gu, Jeonju, Jeonbuk 561-756, South Korea

AR TIC LE D ATA

ABSTR ACT

Article history:

Macroporous ceramic pellets (MCPs) were prepared from industrial waste flyash via a

Received 18 January 2010

mechanical foaming and pseudo-double-emulsion (PDE) method. The MCPs with different

Received in revised form 30 March 2011

relative densities (ρr) were characterized in terms of total and open porosities (εt and εo),

Accepted 24 May 2011

average pore size (davg), specific surface area (Sp), surface roughness, and microstructure. The inner and outer pore structure of the MCPs was found to be composed of interconnected

Keywords:

circular pores. The average size of the MCPs was controlled over the range of 2.0–3.0 mm by

Macroporous ceramic pellet

varying the solid loading ratio of slurries. Increasing the slurry loading led to increasing

Pseudo-double-emulsion method

viscosity of the slurry and consequently greater size of the MCPs. The physical properties of

Physical property

the MCPs investigated herein (i.e., εt, εo, davg, Sp, and surface roughness) increased with

Heavy metal leaching

decreasing ρr (i.e., increasing foaming ratio). It was found that the degree of pore connectivity can be controlled by the ρr of the MCPs. Heavy metal leaching test results showed that the amount of heavy metals released from the MCPs was much lower than that from flyash powders. The results were attributed to melting and subsequent immobilization of the heavy metals as well as their partial sublimation during the sintering process. © 2011 Elsevier Inc. All rights reserved.

1.

Introduction

Macroporous ceramic materials are known to possess high permeability, high specific surface area, good insulating characteristic, high refractoriness, chemical resistance and long life in severe environments due to their high porosity and well-developed surface pores [1,2]. Those characteristics enable the macroporous ceramic materials to be widely used in environmental and biomedical industries [3–6]. Many applications for the macroporous ceramic materials include filters for contaminated hot gas purification [5,7,8], solid/liquid separation process [8], catalyst carriers [9,10], refractories [8,11], thermal insulators [8,11], and biomedical substitutes with skeletal and dental functions [2,3].

Two methods, a polymeric sponge method [8,12,13] and a foaming method [8,14–19] have been widely used to prepare macroporous materials. Among them, the foaming method has an outstanding advantage in that it can conveniently control the pore structure by artificially controlling the foaming ratio, which is defined as the ratio of volume of the foamed slurry to that of the original slurry. Two types of the foaming methods (physical (mechanical) versus chemical) exist depending on the way to generate bubbles in the inside of slurry [7,8,14,16–20]. Especially, the macroporous materials prepared by the mechanical foaming method normally exhibit a 3-dimensional network pore structure [7,8,14,20,21]. The pore structure (e.g., porosity, pore size, pore morphology) is known to be the key property of porous materials because it

⁎ Corresponding author. Tel.: + 82 63 270 2370; fax: +82 63 270 2236. ⁎⁎ Corresponding author. Tel.: + 82 2 2220 0416; fax: +82 2 2296 9724. E-mail addresses: [email protected] (H. Kim), [email protected] (J. Park). 1044-5803/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2011.05.014

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considerably influences secondary properties, such as specific surface area, water absorption, surface roughness, and permeability. Pores are generally classified into two types depending on the connectivity with an outer surface of materials; open or closed. The two pores have distinct differences in terms of functionality and application of the macroporous materials. For instance, the closed pores play a role in decreasing bulk density of the macroporous materials, so that the materials can be used as carriers in a fluidized bed system for drinking and/or wastewater treatment plants; however, fluid cannot penetrate the closed pores. Accordingly, if the surface of materials is not rough enough for microorganisms to attach onto, the bacterial density onto the carrier surface would be insufficient for biological treatment. The macroporous materials with well-developed open pores, on the other hand, normally exhibit high permeability, excellent water absorption capacity, and rough surface, which may be more suitable as microorganism-immobilized carriers than those with closed pores. Hence, precise characterization of the physical properties of the macroporous materials is required for us to selectively design optimum materials for industrial applications stated above by controlling the pore structure. Most studies related to macroporous materials focused on the fabrication of cubic-type of materials; however, the study regarding the preparation of spherical macroporous materials is scarce. The main reason is likely due to the limitation of current technology to prepare macroporous pellets with high porosity and specific surface area, and excellent water absorption. Especially, the spherical ceramic materials are preferable in a fluidized bed system in that pellets can be uniformly suspended and can minimize abrasion by collision. Granulation of ceramic slurry via vibration, pressing, or spray drying is known to be representative ways to prepare ceramic pellets [20]. However, the granulation method by the vibration and the pressing is unlikely suitable for preparing ceramic pellets with high porosity since bubbles generated in slurry are easily broken by external force during the fabrication process. The spray drying method is also limited to fabricate a few hundred micrometer-sized pellets, not a few millimeter-sized ones. Hence, new techniques to prepare sizable pellets are needed due to handling and packing density in a reactor. Lee and Park [22] have recently developed a novel PDE method to prepare MCPs. However, they only focused on the fabrication of MCPs from a starting material with high purity (i.e., mullite). Hence, this study was designed to test the feasibility of preparing sizable MCPs from industrial waste flyash, which has multi-components, and further to evaluate the optimized fabrication condition for utilizing the MCPs as microorganism-immobilized carriers. For this purpose, MCPs with four different foaming ratios (i.e., 1, 2, 3, and 3.5) were prepared from flyash via the combined process of the mechanical foaming and the PDE method, and further their physical properties were systematically characterized. Heavy metal leaching tests were also conducted to compare the amount of heavy metals (Cu, Cd, Pb, Cr, and Zn) released from the flyash powders and the MCPs. Furthermore, the effect of solid loading ratio of initial flyash slurry on the average size and size distribution of the MCPs was investigated.

2.

Experimental Procedure

2.1.

Preparation Process

Fig. 1 shows the overall process for preparing the MCPs. The process is composed of slurry preparation, foaming, pelletizing, drying, and sintering [7,20,22]. The starting material was flyash (Poryung Thermoelectric Power Plant, Korea) with a density (ρt) of 2.185 g/cm3. The chemical composition (wt.%) of the flyash powder was as follows: 36.4% SiO2, 24.9% Al2O3, 3.1% Fe2O3, 1.3% TiO2, 8.5% CaO, 0.2% MgO, 3.1% K2O, 0.2% P2O5, loss of ignition 15.8%. Slurries containing flyash powders, a 0.8 wt.% dispersant of polymethylmethaacrylate (PMMA, Sigma-Aldrich Korea Ltd., Korea), and distilled water were prepared by attrition milling (KMD-1B, Korea Material Development Co. Ltd., Korea) for 3 h. The solid loadings of the prepared slurries were 40, 45, and 50 vol.%. A surfactant, sodium lauryl sulfate (SLS, Samchun Pure Chemical Co., Korea), was used as a foaming agent and was added at the level of 0.5 wt.% to 100 cm3 of the flyash slurry. The slurries were stirred rapidly with an electric mixer at a revolution rate of 1000 rpm in a 500 cm3 plastic beaker in order to prepare the foamed slurries of 200, 300, and 350 cm3 which correspond to foaming ratio of 2, 3, and 3.5, respectively. The gelation agents, 3.5 wt.% polyethyleneimine (PEI, Lupasol-HF, BASF Co. Ltd., Germany) and 1.2 wt.% epoxy resin (DenacolEX614B, Nagase Chemicals Co. Ltd., Japan) were then carefully added to the foamed slurries with constant mixing at 600 rpm for 1 min. Fig. 2 shows a schematic representation of a reactor for preparing the MCPs using the PDE method. The reactor for the PDE was an acrylic cylinder (100 mm in diameter and 120 mm in height) fully filled with liquid paraffin. Foamed slurries were injected through an injection hole in the bottom. An inner cylindrical bar (40 mm in diameter and 120 mm in length) was disposed in the center of the reactor for giving a constant shear force during the process for preparing the MCPs. Rotation was performed at a constant revolution rate of 200 rpm for 2 h. The pellet greenbodies were separated by filtering from the liquid paraffin, which were immediately dried in air at 80 °C for 12 h. Pyrolysis of the dried greenbodies was carried out at 600 °C in air for 1 h, and they were sintered

Fig. 1 – Flow chart of the overall process preparing MCPs.

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ratios was determined using US standard sieves in the range, 0.297–4.760 mm.

2.3.

Fig. 2 – Schematic of the palletizing reactor.

with an electric furnace (51314, General Signal Co., UK) at 1200 °C for 2 h using heating rates of 100 °C/h up to 600 °C and then 200 °C/h up to the final temperature in air.

Heavy metal leaching tests were carried out based on a toxicity characteristic leaching procedure [25] to compare the amount of heavy metals leached from flyash powders and MCPs. The MCPs tested have four different relative densities (ρr = ρb/ρt): 0.65, 0.52, 0.37, and 0.28. The relative densities correspond to foaming ratios of 1, 2, 3, and 3.5, respectively. In order to rule out the effect of pellet size on heavy metal leaching the MCPs with a size range of 2–3 mm were only used for this test. 10 g of the samples were added into 200 ml extraction fluid in an extraction vessel, and gently mixed at a speed of 30 ± 2 rpm for 18 h. The experiments were conducted at pH 4.93 ± 0.05 and room temperature (22–25 °C). The concentration of heavy metals released from the samples (expressed as mg/L) was analyzed using a Perkin Elmer Elan 6000 Inductively Coupled Plasma Mass Spectrometry (Perkin Elmer, Norwalk, CT, USA). All experiments were conducted in triplicate.

2.4. 2.2.

Leaching Test

Statistical Analysis

Characterization of Macroporous Ceramic Pellets

MCPs for evaluating physical properties were the specimens with four foaming ratios of 1, 2, 3, and 3.5. In this case, the initial slurry loading was maintained as 50 vol.%. In order to calculate total porosity (εt), bulk density (ρb) of the MCPs was obtained by normalizing weight of the MCPs by their volume. For this measurement, cylindrical-shaped specimens (60 mm in diameter and 6 mm in thickness) were separately prepared according to the same procedure except for pelletizing described in Section 2.1 (see Fig. 1). For calculating the closed porosity (εc), the open porosity (εo) of the MCPs was measured by a water immersion method [23]. The average pore diameter (davg) of the MCPs was measured from the images (n ≥ 50) taken from a scanning electron microscopy (JSM-6300, JEOL Co., Japan) using an image analysis system [24] (Image-Pro Plus ver. 4.0, I&G Plus, USA). The specific surface area (Sp) of the MCPs was measured using the multipoint Brunauer–Emmett– Teller technique by the adsorption of nitrogen (ASAP 2020, Micrometrics, USA). The surface roughness of the MCPs was evaluated using a Coordinate Measuring Machine (ASTRO 543 C, Wegu Messtechnik GmbH, Germany) with a probe of a contact type. The measurable maximum range of the machine was 800 mm in length by 600 mm in width by 300 mm in thickness, and the accuracy of the measurement was ±0.5 μm. The measurement was carried out using the specimens with different foaming ratios at intervals of 10 μm in one direction. In order to investigate the influence of solid loading in slurry on average size and size distribution of the MCPs, slurries with different solid fraction were prepared. The solid loading ratios (as a percent) investigated here were 40, 45, and 50 vol.%. Foaming ratio was maintained for all cases as 3.5. The rheological behavior of the slurries was also characterized by measuring the slurry viscosity. The viscosity was measured at various shear rates (1–70/s) with a cylindrical viscometer (RV-DV II+, Brookfield Co., USA). The size distribution of the MCPs prepared from the slurries with various solid loading

In tables and figures mean data are presented along with error bars associated with one standard deviation. Statistical differences between mean values were analyzed using a student t-test.

3.

Results and Discussion

3.1.

Macroporous Ceramic Pellets

Fig. 3a shows the representative MCPs prepared from 50 vol.% slurry. The foaming ratio was 3.5. The average pellet size was determined to be approximately 3.0 mm and the appearance was almost spherical. Fig. 3b shows the external surface of the MCP. The image suggests that the MCPs have many open pores on their surfaces. The open pores were likely formed due to the removal of bubbles from a pellet greenbody during the pyrolysis process. Fig. 3c shows an image for the cross section of the MCP. The Fig. 3c indicates that many circular-shaped pores were well developed in the inside of the MCPs. Furthermore, it was observed that windows (i.e., new pores which formed by pore-pore connection) were also well developed. Similar type of pore structure was also observed from previous studies [26–28]. The results from the Figs. 3b and c indirectly suggest that the macropores in the inner and external surfaces of the MCPs were interconnected. Further discussion regarding the pore interconnectivity is given in Section 3.2.

3.2.

Effect of Foaming Ratio on Physical Properties

Four specimens with different ρr (i.e., foaming ratios) were prepared to investigate the influence of foaming ratio on the physical properties of the MCPs, and the results are presented in Table 1. The ρb of the MCPs were determined to be 1.43 ± 0.05, 1.14 ± 0.04, 0.83 ± 0.03, and 0.60 ± 0.03 g/cm3 depending on the foaming ratios. The ρr calculated from the ρt and ρb ranged

820

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Fig. 3 – MCPs prepared using the PDE method (a) appearance; (b) external surface; (c) inner structure. The solid loading ratio of the slurry for this test was 50% (v/v).

from 0.65 to 0.28. The corresponding εt of the MCPs varied from 34.6 to 72.5%. The experimentally determined εt values are smaller than theoretically estimated ones. The difference is

likely due to the pore shrinkage during drying and sintering processes. The ratio of the open and closed pores was also determined from the water absorption data for the MCPs. The water absorption data can give us the information regarding the fraction of open pores out of total pores. High fraction of the open pores reflects the excellent connectivity between external and inner pores. In Table 1, the fraction of the open pores increases with the decreasing ρr, and has a maximum value of 0.93 when ρr = 0.28. The result indicates that more than 90% of pores are interconnected and further supports the images presented in Fig. 3b and c. The data presented in Table 1 show that the davg increases with decreasing ρr. The trend is likely attributed to the increase of collision probability between bubbles as the ρr decreases. Selpulveda et al. [26] reported similar results and suggested that the increase in the number of bubbles in the slurry with constant volume could promote the coalescence of the bubbles. Here, it should be noted that the size of single bubble would not change since the surfactant concentration added in this study was greater than critical micelle concentration [29]. The Sp of the MCPs ranged from 2.0 to 12.0 m2/g and overall increased with decreasing ρr. The trend is due to the increase in pore connectivity with increasing foaming ratio. Fig. 4 shows the degree of roughness on the surface of the MCPs according to the ρr. The surface roughness data are especially important when the MCPs are used as microorganismimmobilized carriers due to the following reasons: First, the extent of surface roughness can influence the magnitude of attractive and repulsive forces acting between microorganisms' and MCP's surfaces, and consequently affect the attachment efficiency of microorganisms [30,31]. Second, the surface roughness determines the magnitude of hydrodynamic force which applies to the microorganisms attached to the surface of the MCPs. For example, when the surface roughness is greater than the radius of microorganisms, the applied hydrodynamic torque will be negligible [32], indicating that the roughness also affects the attachment/detachment of microorganisms from the MCP's surface. High value of surface roughness represents that surface pores are well developed. As expected, in the case of the MCPs with ρr of 0.65, the maximum surface roughness was approximately 16.1 μm; however, the MCPs with minimum ρr exhibited a maximum value of 365 μm. The results suggest that the MCPs with minimum ρr are expected to be more suitable as compared to those with greater ρr when they are used as microorganism-immobilized carries in wastewater treatment processes [33,34]. Fig. 5a, b, c, and d shows the inner pore structure of the MCP with ρr of 0.65, 0.52, 0.37, and 0.28, respectively. As shown in

Table 1 – Physical properties of MCPs with various ρr. The solid loading ratio of the slurry used for this test was 50% (v/v). Foaming ratio

Relative density (ρb/ρt, –)

Total porosity (εt, %)

Open porosity (εo, %)

Average pore size (davg, μm)

Specific surface area (Sp, m2/g)

Maximum surface roughness (–, μm)

1 2 3 3.5

0.65 ± 0.02 0.52 ± 0.02 0.37 ± 0.01 0.28 ± 0.01

34.6 ± 2.1 47.6 ± 2.1 61.9 ± 1.3 72.5 ± 1.2

24.4 ± 3.1 34.2 ± 1.9 55.0 ± 2.7 67.6 ± 3.1

ND a 66.2 ± 4.1 85.1 ± 5.2 96.4 ± 3.5

2.0 ± 0.2 3.5 ± 0.5 6.1 ± 0.9 12.0 ± 1.1

16.1 ± 5.2 61.7 ± 10.4 232.0 ± 17.5 363.3 ± 37.5

a

Not determined.

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neous systems to develop the pore interconnectivity (i.e., window); however, our results (Fig. 5b and c) are not consistent with the theoretical value. The discrepancy is likely due to the randomness of bubble size that may lead to the earlier formation of the windows.

3.3.

Fig. 4 – Representative roughness profiles for the surface of the MCPs with various ρr. The solid loading ratio of the slurry for this test was 50% (v/v).

Fig. 5a, no pores were observed in the cross-sectional area of the MCP (ρr = 0.65); however, the MCP with ρr of 0.28 exhibited well-developed macropores and windows (Fig. 5d). In addition, the windows were also observed for the MCPs that had ρr of 0.37 and 0.52. Peng et al. [28] previously reported that theoretically ρr should to be <0.24 in uniform and homoge-

Heavy Metal Leaching

The results for the physical properties of MCPs demonstrated that the MCPs are likely suitable for the application as microorganism-immobilized carriers. However, heavy metals might be released from the MCPs since the MCPs were prepared from industrial wastes. Thus, we have conducted heavy metal leaching tests using the MCPs, and the results were compared with those from flyash powders. Table 2 shows the leaching test results for flyash powders and MCPs. Overall, the amount of heavy metals released from the MCPs was much smaller than that from the flyash powders. Especially, substantially higher concentration of copper, chromium, and zinc was observed for the powder samples (Co = 0.215, Cr = 0.133, and Zn = 0.417) than the MCPs (Co = 0.015 − 0.017, Cr = 0.048 − 0.069, and Zn = 0.013 − 0.022). Notably, similar amount of heavy metals were released from the MCPs regardless of ρr, indicating that the effect of surface area exposed to leaching solution was negligible. Two plausible reasons can explain the trend for the decrease in the amount of heavy metals leached from the MCPs after sintering process. First, the heavy metals may be partially sublimated during sintering process. Second, the heavy metals included in

Fig. 5 – Inner pore structure of the MCPs with various ρr of (a) 0.65, (b) 0.52, (c) 0.37, and (d) 0.28. The solid loading ratio of the slurry for this test was 50% (v/v).

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Table 2 – Concentration of heavy metals released from flyash powders and MCPs. Leaching concentration (mg/L)

Flyash

Cu Cd Pb Cr Zn

0.215 0.021 0.011 0.133 0.417

a

MCPs (foaming ratio) 1

2

3

3.5

0.017 ND a 0.003 0.048 0.016

0.017 ND a ND a 0.067 0.013

0.015 ND a ND a 0.069 0.022

0.017 ND a ND a 0.058 0.019

Not determined (below the detection limit (0.0001 mg/L)).

flyash powders were likely melted and immobilized to the glassy Si–O matrix because the sintering temperature used in this study was 1200 °C [35]. It was previously reported that heat treatment of waste materials at 1300–1400 °C led to the immobilization of heavy metals via melting process and consequently inhibited the release of heavy metals from waste materials [36]. Here, it should be noted that the inhibition of heavy metal leaching could be achieved in more cost-effective way by adding some fluxing (sintering) agents into slurry as reported by previous studies [16,17]; however, it is uncertain, at current stage, how the fluxing agents would influence the physical properties of the final MCPs. Hence, further study is warranted.

3.4.

determined to be 2.01 ± 0.05, 2.51 ± 0.05, and 3.01 ± 0.08 mm, respectively, with a confidence level of 95% (p < 0.05). The results indicate that the size of the MCPs can be easily controlled by varying solid loading. Fig. 7a and b show the rheological behavior of flyash slurries with different solid loading ratios. It was observed that the flyash slurries followed a Bingham behavior [37] with various yield stresses. Similar behavior was also reported by Mao et al. [14] from the study using silica/starch composite slurry (solid loading = 45 vol.%). The viscosity of the slurry with 50 vol.% solid loading was greater than those for 40 and 45 vol.%. The trend (viscosity versus solid loading) is similar with that for the size distribution of the MCPs, indicating that more viscous slurry resulted in larger pellet size. Lee and Park [20] reported from the study conducted under constant solid loading ratio

Role of Solid Loading on the Size of MCPs

Fig. 6 shows the size distribution for MCPs prepared from slurries with different solid loading ratios. The size of the MCPs (as a diameter) was approximately in the range of 1 to 5 mm. Overall, the size of the MCPs generally increased with increasing solid loading ratio. Specifically, the average size of the MCPs for 40, 45, and 50 vol.% solid loadings was

Fig. 6 – Size distribution of the MCPs with various solid loading ratios (Error bars indicate one standard deviation). The MCPs with ρr of 0.28 was used for this test.

Fig. 7 – Representative rheological behaviors of flyash slurries with different solid loading ratios: (a) relationship between shear stress and shear rate; (b) relationship between viscosity and shear rate.

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that the size of a droplet (i.e., foamed slurry in organic solvent) was dependent on the balance between the viscosity of the droplet (viscous force) and the shear rate of an impeller (shear force). Considering that the pelletizing process was carried out under constant shear rate in this study, our results suggest that the viscosity of the initial slurry (i.e., foaming ratio= 0) can be a factor controlling the average size of the MCPs.

4.

Conclusions

This study was conducted to examine the feasibility of the reuse of industrial waste flyash as resources for fabricating MCPs. The MCPs were prepared using both a mechanical foaming and PDE method. The physical properties (i.e.,, εt, εo, davg, Sp, and surface roughness) were thoroughly evaluated and heavy metal leaching tests were also carried out to test the suitability of the MCPs as microorganism-immobilized carriers. The findings from this study are as follows: 1. The MCPs prepared were almost spherical and exhibited well-developed spherical macropores and windows (i.e., interconnected pores), which can be used for microorganisms' habitat, on the surface as well as in the inside. 2. Physical properties of the MCPs were successfully controlled as a function of the ρr of the MCPs. Overall, the physical properties investigated herein (i.e., εt, εo, davg, Sp, and surface roughness) increased with decreasing ρr, indicating that the degree of pore interconnectivity was considerably influenced by the ρr and that the MCPs with minimum ρ r may be more suitable for the use as microorganism-immobilized carriers. 3. The results for heavy metal leaching tests showed that the amount of heavy metals released from the MCPs was considerably lower than that for flyash powders. The trend was likely attributed to melting and subsequent immobilization of the heavy metals as well as their partial sublimation during the sintering process. 4. The average size of the MCPs increased with increasing solid loading ratio. The result was due to the difference in the viscous force of the slurry, suggesting that the solid loading ratio can be used as a controlling parameter for the average size of the MCPs. Findings from this study have an implication that the industrial waste flyash can be reused as raw materials for fabricating the MCPs. The good sphericity, excellent water absorption capacity (i.e., high fraction of open pores), and high degree of surface roughness (i.e., high specific surface area) further imply that the MCPs may be suitable for environmental (bio)catalysts, such as microorganism immobilized carriers or catalytic supports. Currently, the study on the role of inner pores of the MCPs on nitrification/denitrification in biological wastewater treatment is in progress.

Acknowledgments This paper was supported by research funds of Chonbuk National University in 2010.

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