Influence of vermiculite on the formation of porous cordierites

Applied Clay Science 46 (2009) 196–201

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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y

Influence of vermiculite on the formation of porous cordierites M. Valášková ⁎, G. Simha Martynková, B. Smetana, S. Študentová VŠB-Technical University of Ostrava, 17. listopadu 15/2172, 708 33 Ostrava-Poruba, Czech Republic

a r t i c l e

i n f o

Article history: Received 12 May 2009 Received in revised form 3 August 2009 Accepted 4 August 2009 Available online 11 August 2009 Keywords: Talc Kaolinite Vermiculite Cordierite Phase analysis Porosity

a b s t r a c t Porous cordierites were prepared from mixtures of talc, kaolinite and vermiculite with alumina or aluminum hydroxide by sintering in an argon atmosphere at 1300 °C for 1 h (porous cordierites of Group I) and 2 h (porous cordierites of Group II). Group I and Group II were different in pore size distribution, quantitative content of mineral phases and cordierite structure parameters. The crystalline phases identified in porous cordierites prepared without vermiculite were cordierite (69–88 vol.%), protoenstatite (25–13 vol.%) and corundum (3–10 vol.%). Vermiculite increased the content of cordierite (84–90 vol.%) and Mg–Al spinel (4–15 vol.%) at the expense of protoenstatite. The calculated unit cell parameters of cordierites were between those of orthogonal and hexagonal cordierites (Group I) and corresponded to disordered orthorhombic rather than hexagonal cordierites (Group II). Group I cordierites had pore size maxima at about ~35 µm or ~20 µm, while Group II had maxima at about 35 µm, 25 µm and 18 µm. Porous cordierites prepared from alumina admixture had slightly lower pore volumes than those prepared from gibbsite. Cordierites prepared from mixtures rich in vermiculite and gibbsite showed the highest porosity (63% and 57%). © 2009 Elsevier B.V. All rights reserved.

1. Introduction Ceramic cordierites have low thermal expansion coefficient, excellent high temperature properties, and good surface properties. The most commonly prepared cordierite in the system MgO–Al2O3–SiO2 is often accompanied by spinel, cristobalite, mullite, forsterite, periclase, and corundum (Smart and Glasser, 1976). The conventional methods for the synthesis of cordierite ceramics include solid-state sintering of magnesium, aluminum and silicon oxide corresponding to chemical composition of cordierite (2MgO·2Al2O3·5SiO2). Porous cordierites consisted of α-cordierite and a small amount of spinel and enstatite and were synthesized by sintering of kaolinite and talc (Nakahara et al., 1995; Trumbulovic et al., 2003; Goren et al., 2006a,b) and also Mg– vermiculite (Valášková and Simha Martynková, 2009). Other cordierites were synthesized from the mixtures of clay, talc, alumina, and silica sand (Alves et al., 1998), kaolin, quartz, technical silica, or from talc, kaolin, silica, feldspar (Acimovic et al., 2003), from talc, fly ash, fused silica, and alumina (Kumar et al., 2000). Kaolinite and magnesium hydroxide were sintered to compact cordierites (Kobayshi et al., 2000). Cordierite forms by exothermic reaction at ~1300 °C (Lamar and Warner, 1954). Mg–cordierites crystallize in two polymorphs. Hexagonal α-cordierite is stable above 1450 °C (Schreyer and Schairer, 1961; Putnis, 1980b). An orthorhombic β-cordierite is stable between 1450 °C and 1460 °C (Meagher and Gibbs, 1977; Cohen et al., 1977). Miyashiro (1957) uses the term “indialite” for samples isostructural with beryl of

⁎ Corresponding author. Tel.: +420 596991573; fax: +420 596991640. E-mail address: [email protected] (M. Valášková). 0169-1317/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2009.08.003

hexagonal symmetry and the term “cordierites” for orthorhombic forms. Mg–cordierite when rapidly crystallized from a glass of cordierite stoichiometry is well crystalline, hexagonal, defect-free and homogeneous (Putnis and Bish, 1983). The crystal structure of cordierite was investigated by Rankin and Merwin (1918), Takane and Takeuchi (1936), Byström (1942), Gibbs (1966), Cohen et al. (1977), Meagher and Gibbs (1977), Schwartz et al. (1994). Mg–cordierites have a tetrahedral silicates framework with the simplified structure formula Mg2Al4Si5O18. The Al and Si atoms are distributed over tetrahedral (T) sites: three T1 and six T2 sites per formula unit; Mg atoms are located in slightly flattened octahedra. Many studies reported changes of the tetrahedral Si/Al ordering and transition to the ordered form (Schreyer and Schairer, 1961; Putnis, 1980a,b; Putnis and Bish, 1983; Carpenter et al., 1983; Putnis et al., 1987; Güttler et al., 1989; Schwartz et al., 1994). Hochella et al. (1979), Wallace and Wenk (1980) and Armbruster (1985) studied substitution of structural iron in Fe–cordierites. The presence of smaller size cations causes flattening of the Mg octahedra, the cell parameter c becomes smaller and a and b becomes larger in comparison with Mg–cordierite. Evans et al. (1980) prepared cordierite by different techniques and studied the thermal expansion using high temperature X-ray diffraction. The unit cell volume increased with the amount of Al3+ substituted for Si4+. The thermal expansion of the unit cell of hexagonal cordierite was investigated by Predecki et al. (1987) using neutron powder diffraction. The decrease of the c axis resulted from the distortion of the T2 tetrahedra and the coupled distortion of the edge shearing T1 tetrahedra and M octahedra. Thermal decomposition above 700 °C of vermiculite results in removal of interlayer water and formation of a highly porous material. Okada et al. (2008) observed that mixing allophane with vermiculite

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2. Materials and methods

Table 1 Composition of the mixtures for cordierite ceramics.

2.1. Specimens and sintering

Mineral content, (mass %) Samples

Talc

Kaolinite

Vermiculite

Alumina/Gibbsite

ZA/ZG ZVA/ZVG XVA/XVG

40 30 30

47 45 20

– 12 30

13 13 20

could control various pore sizes, providing a simple way for enhancing the water-retention properties. Vermiculite was sporadically used for cordierite synthesis. The subject of our study is the preparation of porous cordierites from vermiculite, their morphology, porosity characteristics, quantitative content of mineral phases and structure parameters.

Kaolinite and vermiculite from Czech Republic and talc from Egypt were ground separately in a planetary ball mill for 20 min, and then sieved b40 µm. Powders Al2O3 (A, alumina) and Al(OH)3 (G, gibbsite) were purchased from Sigma-Aldrich, Co. The elemental analysis was obtained by X-ray fluorescence spectroscopy (SPECTRO XEPOS newenergy dispersive X-ray fluorescence spectrometer) and atom absorption spectrometry (UNICAM 989 QZ). The structural formulae were approximately Si2Al2O5(OH)4 for kaolinite, Mg3Si4O10(OH)2 for talc and VI (Si3.13Al0.86Ti0.02)IV(Mg2.53Fe3+ 0.45Al0.02) O10(OH)2(Mg0.19K0.01Ca 0.02) for vermiculite. Mixtures were prepared in the stoichiometric ratio close to cordierite (2MgO·2Al2O3·5SiO2) (Table 1). Kaolinite and talc with alumina (sample named ZA) or aluminum hydroxide (sample named

Fig. 1. Scanning electron micrographs of cordierite ceramics. Corundum in ZA is indicated.

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ZG) were prepared as standard mixtures usually used in scientific research and industry. Vermiculite was added to the mixtures ZA and ZG (termed ZVA and ZVG). The mass ratios of the clay mineral phases in ZA and ZG were changed to attain the stoichiometric composition of cordierites. The mixtures XVA and XVG contained higher vermiculite and lower kaolinite content in comparison with ZA and ZG. The mixtures were homogenized in a rotary mixer and milled in planetary mill for 20 min, then sintered in a flow of argon at 1300 °C for 1 h in a tube furnace. The samples marked ZAi, ZGi, ZVAi, ZVGi, XVAi and XVGi were prepared using sintering at the same conditions for 2 h. 2.2. Methods and sample characterization The X-ray powder diffraction (XRD) patterns were recorded with the diffractometer INEL (a curved position-sensitive detector CPSD120, CuKα1 radiation, reflection mode, Ge-monochromator). Each sample was four times repeatedly pressed in the sample holder and the XRD patterns were measured in ambient atmosphere (2000 s, 35 kV, 20 mA). The overlapping (200) and (110) reflexions of cordierites were subjected to profile fitting and decomposition using the program DIFPATAN (Kužel, 1991) to determine d(200) and d(110) values accurately. The quantitative XRD phase analysis of porous cordierite ceramics was performed using the program XQPA (Smrčok and Weiss, 1993). Morphology was examined by the scanning electron microscope Philips XL 30. The sample was coated with Au/Pd and SEM-images were obtained using a back-scattered electron detector. Thermogravimetric analysis (TGA) was carried out with the DTA system Setaram SETSYS 18TM with an “S” type measuring rod. Samples (mass of each was 12 mg in platinum crucible) were analysed in an inert atmosphere (Ar purityN 99.9999%, flow 2 l h− 1) with a heating rate of 10 °C min− 1 from 25 to 1300 °C. The mercury intrusion porosimeter AutoPore IV 9500 was used to measure porosity. 3. Results and discussion Table 1 shows the content of talc, kaolinite and alumina or gibbsite in the starting mixtures prepared with the stoichiometric ratio of cordierite 2MgO·2Al2O3·5SiO2. 3.1. Morphology The composition of mixtures influenced the morphology of the porous cordierite ceramics (Fig. 1). Sintered ZA and ZG showed similar microstructures exposing fine irregular open cells and dense domains. ZVA and ZVG appeared more compact than ZA and ZG. Samples XVA and XVG showed very fine and open irregular pores.

Fig. 2. Calculated powder XRD pattern (Calc) of cordierite No. 5 (Hochella et al., 1979) and observed patterns of the cordierite ceramics containing cordierite and C – corundum, E – protoenstatite and S – spinel.

(Smrčok and Weiss, 1993). Structure Nos. 1–7 showed linear relations between d(200) and d(110) depending on the substitution or deficit of Mg per formula unit (Fig. 3). The values of d(200) and d(110) determined for cordierites in porous ceramics justify for separation of samples into two groups: Group I. Cordierites sintered for 1 h at 1300 °C showed d(200) and d (110) values close to those of refined orthorhombic cordierites with Mg–Fe substitution from Mg1.79Fe0.19 (No. 3) to Mg0.60Fe1.40 (No. 7). Group II. Cordierites sintered for 2 h at 1300 °C showed extended d (200) and d(110) values outside of Group I. The 2θ values of individual cordierite reflections in the XRD patterns and indices in orthorhombic symmetry were input to the calculation of the unit cell parameters. Fig. 4 shows the discrepancy between the calculated unit cell parameters a, c in the studied cordierites and refined orthorhombic cordierites Nos. 1, 3, 5 and 7 and hexagonal (Chex) α-cordierite (Schwartz et al., 1994).

3.2. X-ray powder diffraction The porous cordierite samples contained cordierite as a main phase and protoenstatite (E), MgSiO4, spinel (S), MgO∙Al2O3, and corundum (C), α-Al2O3 (Fig. 2). The measured XRD patterns corresponded to the calculated pattern of cordierite (Calc) according to the published data (Hochella et al., 1979). The structure of cordierite in the porous ceramics was compared with the refined cordierite structure. Seven cordierite structures named from No. 1 to No. 7 were chosen from literature with respect to the variable structural content of Mg and Fe in the structural formula: No. 1: Mg2 Al4 Si5O18 (Schwartz et al., 1994); No.2: Mg1.86Fe0.14 Al4 Si5O18 (Wallace and Wenk, 1980); No. 3: Mg1.79Fe0.19 Al4Si5O18 (Wallace and Wenk, 1980); No. 4: Mg1.72Fe0.27 Al4 Si5O18 (Wallace and Wenk, 1980); No. 5: Mg1.91Fe0.08Mn0.01Na0.05K0.02Ca0.02Al4Si5O18 (Hochella et al., 1979); No. 6: Mg 1.17Fe0.86Mn0.02 Al4 Si5 O18 (Armbruster, 1985); No. 7: Mg0.60Fe1.36Mn0.03Al4 Si5O18 (Armbruster, 1985). The XRD patterns of these seven cordierites were calculated from coordinates and cell parameters using the program DIFK91

Fig. 3. Relation between d(200) and d(110) values. Refined cordierite structures Nos. 1–7 (⁎), and cordierites in samples of Group I (■) and Group II (□).

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Fig. 4. The unit cell parameters a and c of the refined orthorhombic cordierites Nos. 1, 3, 5, 7 (⁎), hexagonal cordierite (Chex) (Schwartz et al., 1994), and calculated a and c of cordierites of Group I (■) and Group II (□).

The parameters a calculated for Group I were lying between aort and ahex. Octahedral substitution of iron in cordierites (due to the presence of vermiculite in the mixtures) was assumed when a expanded in parallel to the vermiculite content. The slightly shortened c axis and an expansion of a in comparison with Mg–vermiculite (No. 1) can be caused by flattening of octahedra that occupied not only with Mg but also by Fe. Expanded c may be caused by cations of different sizes in the silicate framework (Hochella et al., 1979; Miyake, 2005). Group II corresponded to disordered orthorhombic rather than hexagonal cordierites. 3.2.1. Quantitative X-ray phase analysis The computer program DIFK91 (Smrčok and Weiss, 1993) subsumes program XQPA91 for calculation of individual crystalline phases in the mixture by a last-square procedure. The mineral standards with the calculated absolute intensity and d values were cordierite (Hochella et al., 1979), protoenstatite (Murakami et al., 1984), MgAl2O4 spinel (Yamanaka et al., 1984) and corundum (Ishizawa et al., 1980). The observed d values and absolute intensities in the XRD patterns were used as input (Fig. 5).

Fig. 6. Pore size distribution in the porous cordierites of the Group I (a) and Group II (b).

The porous cordierites ZA and ZG contained cordierite (69 and 72 vol.%, resp.), protoenstatite (22 and 25 vol.% resp.), corundum in ZA (9 vol.%) and MgAl-spinel in ZG (3 vol.%). ZAi and ZGi had a higher content of cordierite (77 and 88 vol.%, resp.) and 13 and 12 vol.% protoenstatite. Cordierite (85 vol.%) was also the dominant phase over protoenstatite in ZVA and ZVG. Prolonged time of sintering increased the cordierite (up to 90 vol.%) and spinel content and decreased the amount of protoenstatite. 3.3. Mercury porosimetry The porous cordierites of Group I revealed uniform pore diameters of about 35 µm (ZVA, ZVG) or 15–20 µm (XVA, XVG) (Fig. 6a). Three Table 2 Median pore diameter (MPD) and porosity of porous cordierites.

Fig. 5. Triangular diagram representing quantitative abundance of mineral phases (vol.%) in the Group I (●) and Group II (○) of the cordierite ceramics; ⁎ corundum in ZA and ZAi.

Samples

ZG

ZGi

ZA

ZAi

MPD (µm) Porosity (%)

35 55

32 55

34 47

28 48

Samples

ZVG

ZVGi

ZVA

ZVAi

MPD (µm) Porosity (%)

41 54

41 55

30 45

36 38

Samples

XVG

XVGi

XVA

XVAi

MPD (µm) Porosity (%)

21 57

20 63

34 55

28 55

200

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(IV) The mass loss between 592 and 703 °C in ZVA and ZVG and between 563 and 705 °C in XVA and XVG corresponded to the further dehydroxylation of vermiculite (Suquet et al., 1991). (V) The mass loss between 705 and 1120 °C indicated transformation of talc into enstatite and protoenstatite (Aglietti and Porto Lopez, 1992). 4. Conclusions Vermiculite influenced porosity design of porous cordierites. Prolonged sintering limited protoenstatite formation, slightly increased the content of cordierite, spinel and corundum. Alumina in comparison with gibbsite reduced the total mass loss during sintering and the pore volume. The cell parameters a, c classified the cordierites into disordered orthorhombic rather than hexagonal structure. Acknowledgements Financial support of the Ministry of Education of the Czech Republic (project MSM 6198910016 and ME08040) and the Czech Grant Agency (projects GA ČR 05/08/0869) are gratefully acknowledged. The authors thank M. Heliova for SEM micrographs. Fig. 7. DTA of porous cordierites Group I.

maxima of pore size distribution were observed in Group II (Fig. 6b): about 35 µm (ZVGi, ZVAi), about 25 µm (ZAi, ZGi), and about 18 µm (XVAi, XVGi). The median pore diameter (MPD) was maximal for ZVG and ZVGi and smallest for XVG and XVGi with the highest porosity (Table 2). The cordierites prepared from alumina showed slightly lower porosity than those prepared from gibbsite. 3.4. Thermogravimetric analysis Group I cordierites (Fig. 7). A single endothermic peaks was related to the dehydration of vermiculite at 200 and 240 °C (Suquet et al., 1991), at 285 °C to the decomposition of gibbsite, 515 °C dehydration of kaolinite, talc and vermiculite, 687 °C to dehydroxylation of vermiculite, and at 1232 and 1265 °C to the decomposition of protoenstatite (Goren et al., 2006a,b). Cordierite was formed by an endothermic reaction between 1242 and 1294 °C. Exothermic effects were observed at 835 °C for XVA and XVG and at 865 °C for ZVA and ZVG due to crystallization of enstatite Mg3Si4O10(OH)2 (Aglietti and Porto Lopez, 1992). The exothermic effect at 993 °C corresponded to the formation of oxides (Mackenzie, 1970), MgAl2O4 spinel and µcordierite (Naskar and Chatterjee, 2004; Goren et al., 2006b). The maximum at 1265 °C (XVG, XVA) and 1242 and 1294 °C (ZA, ZG, ZVA, and ZVG) resulted from the exothermic solid-state formation of cordierite (Sorrell, 1960; Kobayshi et al., 2000; Goren et al., 2006a,b). The total mass loss was10.0 (±0.7) % and 13.8 (±0.7) % for ZA, ZVA, XVA and ZG, ZVG, XVG, respectively. The TG curves (not shown here) can be divided into five stages: (I) Mass loss due to desorption of intermolecular water (from 200 to the 230 °C). (II) Dehydroxylation of hydroxides (from 230 to the 300 °C). (III) The temperature interval (Δ) was different for alumina and gibbsite containing samples: ZA: ZVA: XVA:

Δ 310 °C (375–685 °C); Δ 200 °C (387–592 °C); Δ 153 °C (410–563 °C);

ZG: ZVG: XVG:

Δ 292 °C (415–707 °C). Δ 184 °C (410–594 °C). Δ 143 °C (421–564 °C).

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