- Email: [email protected]
Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag
Available online at www.sciencedirect.com
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag Chuanbei Liua, Laibao Liua,b,n, Kefeng Tana, Lihua Zhanga,b, Kaijing Tanga, Xianpan Shia a
School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, Sichuan 621010, China b School of Materials Science and Engineering, Southeast University, Nanjing 211189, China Received 14 July 2015; received in revised form 31 August 2015; accepted 31 August 2015
Abstract Porous cordierite ceramics were successfully fabricated using ferrochromium slag, commercial alumina and silica powder as raw materials, without any additional pore forming agents, by sintering at temperatures ranging from 1100 to 1350 1C. The phase evolution, porosity, microstructure, mechanical properties, thermal expansion property and chromium leachability were systematically investigated. The results showed that the internal oxidation of forsterite in the slag contributed to the massive glassy phases formed in the prepared porous ceramics as well as the low-temperature sintering synthesis of cordierite, which started to form at 1100 1C and at 1350 1C, comprising up to ∼87.1 wt% of the ceramic. Furthermore, the iron oxides in the ferrochromium slag acted as pore forming agents at high temperatures, thereby resulting in significant volume expansion of the sample. The SEM images showed that the prepared cordierite ceramics had a porous microstructure composed of massive glassy phases embedded in polyhedral spinel and prismatic cordierite. In addition, the ceramic had a flexural strength of 47.2671.01 MPa, coefficient of thermal expansion of 3.5 10-6/1C, and chromium leachability that was only half as large as that of the green sample. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Ferrochromium slag; Porous cordierite; Phase evolution; Chromium leachability.
1. Introduction Porous ceramics materials have many interesting properties including low density, high permeability and good heat insulation. These characteristics make them widely used in various industrial fields such as liquid or gas fillers, catalysis supports, selective membranes, thermal insulation and pollution control [1]. Among those materials, porous cordierite is a very good candidate for these applications due to its low thermal expansion coefficient and high resistance to thermal shock [2,3]. Porous cordierite is synthesized primarily via a solid-state sintering method, using talc, kaolin, magnesite and other natural minerals or the pure chemical oxide as the raw materials. Sintering temperatures of up to ∼1400 1C are typically used n Corresponding author at: School of Materials Science and Engineering, Southwest University of Science and Technology, No. 59 Qinglong Road, Mianyang, Sichuan 621010, China. Tel.: þ86 13689691135. E-mail address: [email protected] (L. Liu).
[4–6]. Attempts to lower the sintering temperature have included the use of sintering additives [7,8] and the development of novel synthesis methods [9–11]. However, these attempts incur high fabrication costs and require a complex preparation process. Finding a meaningful way to use inexpensive and rich-sourced raw materials, especially the industrial byproducts, to synthesize porous cordierite ceramics is therefore essential. Some studies have focused on the preparation of porous cordierite ceramics using sepiolite, rectorite and vermiculite as raw materials [12– 14]; industrial byproducts such as rice husk, and fly ash have also been used [15,16]. There are, however, no published reports regarding porous cordierite ceramics prepared mainly from ferrochromium slag. Ferrochromium slag is a waste material produced during the manufacturing process of ferrochrome that is an essential component in stainless steel. The massive ferrochromium slag produced every year occupies significant land space and contaminates the environment owing to the presence of leachable heavy metals, especially chromium [17,18]. Effective use
http://dx.doi.org/10.1016/j.ceramint.2015.08.174 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
C. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
2
of the slag is therefore essential. Many approaches for reusing the slag have been reported. In fact, most of the slag is used for traditional construction materials, such as concrete [19,20] and brick [21]. Unfortunately, with these approaches, the slag is simply moved from one place to another and the potential hazard of the slag remains. Ferrochromium slag has a similar chemical composition to cordierite. Both contain mainly silicon (Si), manganese (Mg), and aluminum (Al), which together constitute ∼83% of the slag [19]. The slag also contains metallic elements such as iron (Fe), titanium (Ti), and heavy metals such as chromium (Cr), cobalt (Co) and nickel (Ni). These components are expected to act as sintering additives and have a positive effect on the phase evolution as well as the crystallization of cordierite [22–24]. In addition, the high-temperature reduction of iron oxides in the slag generates a large amount of oxygen, which acts as a pore forming agent [25]. Therefore, in this study, in order to carry out an innocuous treatment of ferrochromium slag, porous cordierite ceramics were prepared from a mixture of the slag, commercial alumina (Al2O3), and silica powder (SiO2) without additional pore forming agents. The phase evolution, volume expansion, porosity, microstructure, mechanical properties, thermal expansion behavior, and Cr leachability, of the prepared ceramics were evaluated. Furthermore, the phase evolution during sintering is discussed from the viewpoint of chemical reactions of the various MgO (FeO)–Al2O3–SiO2 compositions. 2. Experimental procedure 2.1. Sample preparation The porous cordierite ceramics were prepared from raw materials of ferrochromium slag (Leshan, Sichuan Province, China), commercial alumina and silica powder (both from Zibo Jiezhong New Material Co., Ltd., China). Cordierite having the composition 2MgO 2Al2O3 5SiO2, was prepared by weighing and mixing ferrochromium slag, alumina and silica powder for 6 min at room temperature in a vibration mill rotating at a constant speed of 710 rpm. The milled powder mixture was then mixed with organic binder PVA-1750 (3 wt% solution) and uniaxially pressed under 45 MPa into 25 mm(diameter) 5 mm (thickness) pellets (weighing ∼5.5 g each). The green pellets were subsequently dried at 10572 1C in a baking oven in order to remove the free water, and then sintered for 3 h in a muffle furnace at temperatures ranging from 1100 to 1400 1C. In addition, the green pellets were sintered at 1350 1C for 0.5, 1.0, 2.0 and 4.0 h, respectively, in order to investigate the effect of holding time on the properties of the cordierite. The heating rate was fixed at 5 1C/min, and the added organic binder was removed by holding the sample at 600 1C for 20 min. The sintered pellets were furnace-cooled to room temperature. 2.2. Characterization The chemical compositions of the raw materials were determined via quantitative X-ray fluorescence spectrum analysis (XRF; Axios-Advanced, PANalytical Corporation, The Netherlands). The
corresponding particle size distributions were determined by a laser particle size analyzer (Mastersizer 2000, Malvern Instruments Ltd., UK) using water as a dispersing medium. Moreover, X-ray diffraction (XRD; X'Pert Pro, PANalytical Corporation, The Netherlands) was used to determine the phase compositions of the raw materials and the sintered samples. The actual phase fractions were obtained via quantitative Rietveld analysis using the HighScore Plus software [26,27]. Furthermore, the morphology of the slag and the fracture surfaces of the porous cordierite ceramics were examined by using a field emission scanning electron microscope (FESEM; Ultra 55, Carl Zeiss Ltd., Germany); electron energy disperse spectroscopy (EDS) was performed with equipment attached to the microscope. The coefficient of thermal expansion (CTE) of the sintered samples was evaluated at temperatures between room temperature and 1000 1C by using a thermal dilatometer (DIL402PC, NETZSCH- Gerätebau GmbH, Germany). The CTE was calculated from: α = ΔL / (L0 × ΔT )
(1 )
where α, ΔL/L0, and ΔT are the CTE, linear expansion ratio, and applied temperature difference, respectively. The change in the diameter (i.e., linear shrinkage or expansion) of the sintered pellets was measured by using a vernier caliper. In addition, the open porosity and bulk density were measured in accordance with Archimedes' principle using distilled water as an immersion liquid. The distribution of pore sizes of the prepared porous cordierite ceramics was determined by using an automatic mercury porosimeter (AutoPore IV9500, Micromeritics Instrument Corp., America). This porosimeter allows the analysis of pores with sizes ranging from 0.005 to 630 μm, and the pore radius, rp, can then be determined from Washbum's equation that is given as: rp =
−4γ cos θ PHg
(2 )
where γ is the surface tension (0.485 N m-1) of Hg, θ is the contact angle between the Hg and the solid phase (equal to1401) and PHg is the pressure of Hg. Three-point flexural strength tests of the sintered samples (length: 45 mm, width: 4 mm and height: 3 mm) were performed in accordance with ISO 14704:2000(E), using a universal material testing machine (ReGeR-3010, Reger Instrument Co., Ltd., China). A crosshead speed of 0.5 mm/min was used in the testing of at least five specimens. The pH-dependent leaching behavior [28,29] of Cr was investigated in the green samples and the pellets sintered at 1350 1C for 3 h. The sintered pellets were initially ground and then sieved through a 200-mesh sieve for leach testing. A series of 5 g sub-samples was mixed with 100 mL of leaching solutions, of varying acidity, ranging from distilled water to 0.03 M HNO3 solution (with pH value of ∼1.5). The samples and leachates were mixed in sealed containers for 24 h on an oscillating shaking table, before being centrifuged. The leachates were then extracted by filtering through a 0.45 μm membrane filter and their pH was measured before being analyzed via inductively coupled plasma
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
C. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
3
Table 1 Chemical composition (wt%) of raw materials measured by quantitative XRF. Materials
Chemical composition (wt%)
Ferrochromium slag Commercial alumina Silica powder The mixture
SiO2
CaO
Al2O3
Fe2O3
MgO
Cr2O3
TiO2
SO3
MnO
Na2OþK2O
Others
35.12 0.04 99.9 47.69
1.57 / / 0.73
22.21 99.6 0.01 32.45
4.21 0.03 0.04 1.95
27.77 / / 12.83
7.37 / / 3.40
0.73 / / 0.33
0.46 / / 0.21
0.23 / / 0.11
0.15 0.26 / 0.07
0.18 0.07 0.05 0.08
Fig. 1. XRD patterns of the ferrochromium slag used. Fig. 2. Particle size distributions of the raw materials.
mass spectrometry (ICP-MS; Agilent 7700x, Agilent Technologies Inc., America). 3. Results and discussion 3.1. Characterization of raw materials Table 1 lists the chemical compositions of the ferrochromium slag, commercial alumina and silica powder, as determined by quantitative XRF analysis. The slag consists primarily of SiO2, Al2O3 and MgO, which together account for 85.1 wt% of the slag. Moreover, stoichiometric cordierite was synthesized from 46.12 wt%, 22.30 wt% and 35.10 wt% of the slag, alumina and silica powder, respectively; the chemical composition of the mixture is also shown in Table 1. The XRD pattern of the slag is shown in Fig. 1. The result confirmed the presence of the solid solution of forsterite, (Mg,Fe)2SiO4, in which the Mg was partially replaced by Fe. Hercynite (FeAl2O4), chrome-spinel ((Mg,Fe)(Cr,Al)2O4), and a small amount of enstatite (MgSiO3) were also detected. In general, the phase composition of the used slag is consistent with those reported in previous studies [19]. Fig. 2 shows the particle size distributions of the raw materials used. As it shows, the slag, alumina, silica and the corresponding vibration-milled mixture have particle sizes of 1–80 μm, 1–30 μm, 3–110 μm and 1–80 μm, respectively. Furthermore, the average (D50) particle sizes of these respective materials are 11.85 μm, 4.45 μm, 33.49 μm and 12.13 μm.
3.2. Phase evolution The samples all consisted of eight crystalline phases (Fig. 3); these were indialite (PDF#00-012-0235), spinel (PDF#00-0210540), forsterite (PDF#00-004-0796), magnesium aluminum chromium oxide labeled as Cr-spinel (PDF#00-023-1222), hercynite (PDF# 00-007-0068), ringwoodite (PDF#00-021-1258), quartz (PDF#00-046-1045), and corundum (PDF#00-010-0173). Indialite (also known as α-cordiarite) first appears at a temperature of 1100 1C, and the intensities of the corresponding peaks increase with increasing temperatures of 1100–1400 1C. Rietveld quantitative phase analysis results (Fig. 4) reveal that indialite constitutes up to 87.1 wt% of the samples sintered for 3 h at the optimal temperature of 1350 1C. In addition, the ringwoodite phase first appears at 1150 1C whereas forsterite and hercynite are not observed at this temperature. Ringwoodite as well as quartz and corundum were, however, intermediate phases and hence were not observed at 1350 1C. The spinel phase was observed at 1300 1C and seemed to transform into magnesium aluminum chromium oxide (Mg(Al,Cr)2O4) when the sintering temperature increased to 1400 1C. Similar tendencies were observed in the phase evolution of the sample sintered at 1350 1C for various holding times (Fig. 3 (b)). For example, the 3-h-heated sample consisted of only the indialite and spinel; intermediate phases such as quartz, corundum and hercynite were not observed after 2.0 h of heating. The ferrochromium slag consists predominantly of forsterite, whose chemical formula is 2(Mgx,Fe1-x)O SiO2; the Fe2 þ and
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
C. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
4
Fig. 3. XRD patterns of samples sintered at (a) temperatures ranging from 1100 to 1400 1C for 3 h and (b) 1350 1C for holding times of 0.5–4.0 h.
Mg2 þ can replace each other and thereby form a continuous solid solution. In fact, when forsterite is heat-treated at temperatures of 800 1C or higher in an oxidizing atmosphere, the Fe is released from the crystal structure along the crystal boundary and cleavage planes, and the internal oxidation of forsterite results in the formation of 2MgO SiO2 forsterite and amorphous silica, as shown in the following equation [30,31]: 2 (Mg, Fe) O⋅SiO2
≥ 800 ° C
→
2MgO
⋅SiO2 + SiO2 (amorphous) + Fe2 O3
(1 )
The resulting 2MgO SiO2 forsterite and amorphous silica are both in a high free-energy and hence unstable state, and at 1100 1C, are expected to react with the alumina in the raw materials used, thereby forming indialite. This reaction proceeds as follows:
Fig. 4. Crystalline-phase content of samples sintered at (a) temperatures of 1100–1400 1C for 3 h and (b) 1350 1C for holding times of 0.5–4.0 h.
2MgO⋅SiO2 + SiO2 (amorphous) + Al2 O3
1100 ° C
→
2MgO
⋅2Al2 O3⋅5SiO2
(2 )
The ∼29 wt% of indialite present at 1100 1C (Fig. 4(a)) is attributed to this reaction. Moreover, with continuously increasing heating temperatures, the high-iron glass phase is generated through the reaction between massive nomadic iron oxides (Fe2O3) and the formed 2MgO SiO2 forsterite; intense recrystallization also occurs [31]. The reaction between forsterite and Fe2O3 proceeds as follows: 2MgO⋅SiO2 + Fe 2 O3
≥ 1100 ° C
→
MgO⋅SiO2 + MgO⋅Fe2 O3
(3 )
Simultaneously, some of the Fe2O3 and SiO2 is expected to participate in the recrystallization of the glassy phase. This process results in the formation of the intermediate ringwoodite ((Mg,Fe)2SiO4) phase, which in turn reacts with silica and alumina to form the indialite phase; the reaction proceeds as follows:
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
C. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
(Mg, Fe) 2⋅SiO4 + SiO2 + Al2 O3
≥ 1150 ° C
→
3.4. Pore size distribution
2MgO⋅2Al2 O3
⋅5SiO2 + Fe2 O3
5
(4 )
3.3. Expansion, bulk density and open porosity Fig. 5 shows the linear expansion percent, bulk density, and open porosity of the porous cordierite ceramics after sintering for 3 h at temperatures of 1100–13501C. As the figure shows, the substantial increase in volume expansion is accompanied by a gradual decrease (from 2.1270.01 to 1.8070.01 g/cm3) in bulk density. This suggests that the process of preparing cordierite ceramics using ferrochromium slag as a raw material, differs from the usual ceramic sintering process, where samples typically exhibit significant shrinkage; this shrinkage is driven by the reduction in interfacial free energy. However, during the preparation of porous cordierite ceramics from the ferrochrome slag, the samples are also affected by the expansion pressure of O2 that is generated from the reduction of iron oxides. The reduction reaction proceeds as follows [25]:
Fig. 6 compares the pore size distributions of the porous cordierite ceramics sintered at 1250 1C for 3 h and at 1350 1C for both 1 and 3 h. As the figure shows, when the sintering temperature is increased from 1250 to 1350 1C, the pore size distribution is shifted to large pore sizes, as evidenced by an increase in the average pore size from 7.65 to 34.78 μm. The average pore size also increases (i.e., from 14.57 to 34.78 μm) when the holding time is increased from 1 to 3 h. These results reveal that the sintering temperature and holding time have a significant effect on the pore size distributions of the prepared ceramics. The aforementioned variation in open porosity is also attributed to this effect.
3.5. Microstructures and mechanical properties
Furthermore, the cordierites (2.50 g/cm3) and glassy phases in the sintered samples have smaller densities than those (3.34, 2.65 and 3.99 g/cm3, respectively) of the crystalline phases such as forsterite, quartz and corundum in the raw materials used. Therefore, the preparation process of the cordierite ceramics is, essentially, a volume expansion process. The open porosity of the cordierite ceramics decreased slowly from 32.9770.21 to 30.7270.17% at temperatures of 1100–1250 1C, owing to the increasing amount of liquid phases that resulted in the closure of connected pores [32,33]. However, the open porosity increased rapidly from 30.8370.08% to 32.870.16% with increasing sintering temperatures of 1300– 1350 1C. This increase is very likely resulted from the increased volume fraction, and simultaneous decrease in the viscosity, of the liquid phases; i.e., small pores become connected, owing to the expansion pressure of O2, and consequently larger pores are formed.
The SEM images in Fig. 7 compare the fracture surfaces of the ceramics sintered at 1250 and 1350 1C for holding times of 1 and 3 h. The prepared cordierite ceramic has a porous microstructure with pore sizes ranging from a few to dozens of microns. Moreover, the pore size increases significantly when the sintering temperature and time are increased from 1250 (Fig. 7(a)) to 1350 1C (Fig. 7(e)) and 1 (Fig. 7(c)) to 3 h (Fig. 7(e)), respectively; this result is consistent with the previously mentioned pore size distribution and variation in the open porosity. In addition, the crystal size increased considerably with the aforementioned increases in temperature and holding time, as revealed by comparing Fig. 7(b) with (f) and (d) with (f). These highmagnification images also reveal that the prismatic cordierites are generated from the matrix of the glassy phase. This resulted from the crystal growth of massive low-viscosity liquid phases at high sintering temperatures and long holding times. In addition, a plot (Fig. 8) of the flexural strength of the ceramics sintered for 3 h at temperatures of 1150–1350 1C, reveals that the strength increases linearly with increasing sintering
Fig. 5. Linear expansion percent, bulk density and open porosity of samples sintered at temperatures of 1100–1350 1C for 3 h.
Fig. 6. Pore size distributions of the samples sintered at 1250 1C for 3 h (1250 1C/3 h) and 1350 1C for 1 and 3 h (1350 1C /1 h and 1350 1C /3 h, respectively).
6Fe2 O3 → 4Fe3O4 + O2 ↑
(5 )
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
C. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
6
Fig. 7. SEM images of fracture surfaces of samples sintered at (a) and (b) 1250 1C for 3 h, (c) and (d) 1350 1C for 1 h, and (e) and (f) 1 350 1C for 3 h.
temperature. The sample sintered at 1350 1C had the highest (47.2671.01 MPa) strength. The densification of ceramic bodies and the growth of crystals lead to an increase in the flexural strength of cordierite ceramics [34]. In this work, the increase in flexural strength at temperatures of 1150–1250 1C results from growth of the cordierite crystals and densification of the samples. The continuous increase in strength after 1250 1C is attributed to further growth of the crystals; this is especially true in the case of the spinel crystals, as evidenced by the results of the XRD analysis.
(b). As the figure shows, the CTE decreases from 5.7 10-6/1C to 3.5 10-6/1C with increasing sintering temperatures of 1150–1350 1C. The CTE of cordierite ceramics stems mainly from the glassy and cordierite phases. In the current study, the high fraction of glassy phases result in the relatively higher CTE values of the samples compared to those (1.0–2.0 10-6/1C) of their conventional counterparts. However, the decrease in the CTE with increasing sintering temperature stems mainly from the gradual growth of the cordierite crystals.
3.6. Linear thermal expansion properties
3.7. Chromium leachability
Fig. 9(a) shows the linear thermal expansion behavior (from room temperature to 1000 1C) of the ceramics heat-treated for 3 h at various temperatures. The CTE of the samples is calculated from the linear thermal expansion ratio (dL/L0) and is shown in Fig. 9
The effect of leaching under acid leachate conditions (pH¼ 7.0–1.5) on the release of Cr is shown in Fig. 10. The Cr content of the leachates is expressed as mg/kg of dry sample. As the figure shows, the Cr leachability of the green samples and sintered
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
C. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
7
Fig. 8. Flexural strength of samples sintered for 3 h at temperatures ranging from 1150 to 1350 1C.
ceramics is lower than 35.0 mg/kg, which meet the US EPA limits of 100 mg/kg. This result is in agreement with the results reported by Huang [17] and Lind [18] for various soil samples near the ferrochrome slag. Furthermore, the Cr leachability of the sintered ceramics was approximately half as large as that of the green samples. Previous work [29,35] showed that high temperatures and long holding times were needed to reduce metal leaching from the sintered solid-waste samples. The incorporation of heavy metals into the amorphous and crystalline phases was also expected and therefore these metals should not be leached from the samples. With ionic radii of 0.073 and 0.062 nm, Cr2 þ and Cr3 þ have very similar ionic radii to those (0.072 and 0.054 nm) of Mg2 þ and Al3 þ , respectively. As such, a portion of the nomadic Cr in the ferrochromium slag can easily replace the Mg or Al in the crystal structures of spinel and cordierite phases, and thereby form a stable structure. Fig. 11 shows another high-magnification SEM image of the porous cordierite ceramics sintered for 3 h at 1350 1C and the corresponding EDS analysis of the crystal phases. As the figure shows, massive glassy phases are embedded in the polyhedral spinel and prismatic cordierite. The EDS analysis revealed the high Cr content of the spinel (Fig. 11(a)) and cordierite (Fig. 11(b)) phases, as well as the main elements, i.e., Si, Al and Mg. These results indicate that a stable phase formed owing to the Cr-replacement of Mg or Al in the crystalline structures of cordierite and spinel.
Fig. 9. (a) Thermal expansion behavior and (b) CTE of samples sintered at temperatures ranging from 1150 to 1350 1C.
4. Conclusions The properties of porous cordierite ceramics prepared from ferrochromium slag, commercial alumina and silica powder, via the reaction sintering technique and without additional pore forming agents, were investigated. The conclusions from this study are summarized as follows: (1) Phase evolution analysis of the samples revealed that the internal oxidation of forsterite in the slag contributed to the
Fig. 10. Dependence of the leaching of Cr from the green samples (labeled as green) and the porous cordierite ceramics sintered at 1350 1C for 3 h (labeled as sintered) on the pH of the leachate.
formation of massive glassy phases in the prepared porous ceramics; this oxidation also contributed to the lowtemperature sintering synthesis of cordierite, which formed
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
8
C. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
sintered at 1350 1C for 3 h exhibited an open porosity of up to 32.8 7 0.16%. (3) The prepared cordierite ceramics had a flexural strength of 47.267 1.01 MPa and a CTE of 3.5 10-6/1C. Moreover, some of the nomadic Cr in the slag was incorporated into the crystal structure of the as-synthesized cordierite and spinel phases. The Cr leachability of the ceramics was therefore only approximately half as high as that of the green samples. As such, this study provides a new ideal for innocuous treatments and resource use of ferrochromium slag. Acknowledgments The authors gratefully acknowledge the analytical support of the SWUST Analysis and Test Center, Mianyang, China. We are also grateful to the Leshan Xinhe Electric Power Comprehensive Opening Co., Ltd., Leshan, China, for providing the ferrochromium slag. References
Fig. 11. SEM image and EDS spectrum of the crystal phases in samples sintered for 3 h at 1350 1C.
first at a temperature of 1100 1C and at 1350 1C, constituted a phase fraction of ∼87.1 wt% of the ceramics. (2) The iron oxides in the slag act as pore forming agents during the preparation process of the ceramics. During this process, the samples exhibited substantial volume expansion (rather than the typical shrinkage) and a decrease in bulk density with increasing sintering temperature. Furthermore, the sintering temperature and holding time had a significant effect on the pore size; in fact, samples
[1] T. Ohji, M. Fukushima, Macro-porous ceramics: processing and properties, Int. Mater. Rev. 57 (2012) 115–131. [2] A. Shyam, E. Lara-Curzio, A. Pandey, T.R. Watkins, K.L. More, The thermal expansion, elastic and fracture properties of porous cordierite at elevated temperatures, J. Am. Ceram. Soc. 95 (2012) 1682–1691. [3] Y. Dong, X. Feng, D. Dong, S. Wang, J. Yang, J. Gao, X. Liu, G. Meng, Elaboration and chemical corrosion resistance of tubular macro-porous cordierite ceramic membrane supports, J. Membr. Sci. 304 (2007) 65–75. [4] M.A. Camerucci, G. Urretavizcaya, A.L. Cavalieri, Sintering of cordierite based materials, Ceram. Int. 29 (2003) 159–168. [5] R. Johnson, I. Ganesh, B.P. Saha, G.V.N. Rao, Y.R. Mahajan, Solid state reactions of cordierite precursor oxides and effect of CaO doping on the thermal expansion behaviour of cordierite honeycomb structures, J. Mater. Sci. 38 (2003) 2953–2961. [6] J.B. Rodrigues Neto, R. Moreno, Rheological behaviour of kaolin/talc/ alumina suspensions for manufacturing cordierite foams, Appl. Clay Sci. 37 (2007) 157–166. [7] Z.M. Shi, X. Bai, X.F. Wang, Ce4 þ -modified cordierite ceramics, Ceram. Int. 32 (2006) 723–726. [8] D. Zou, X. Chu, F. Wu, Sol–gel synthesis and infrared radiation property of Li-substituted cordierite, Ceram. Int. 39 (2013) 3585–3589. [9] J. Banjuraizah, H. Mohamad, Z.A. Ahmad, Crystal structure of single phase and low sintering temperature of α-cordierite synthesized from talc and kaolin, J. Alloy. Compd. 482 (2009) 429–436. [10] A. Yamuna, S. Honda, K. Sumita, M. Yanagihara, S. Hashimoto, H. Awaji, Synthesis, sintering and thermal shock resistance estimation of porous cordierite by IR heating technique, Microporous Mesoporous Mater. 85 (2005) 169–175. [11] M. Fukushima, M. Nakata, Y.-i Yoshizawa, Fabrication and properties of ultra highly porous cordierite with oriented micrometer-sized cylindrical pores by gelation and freezing method, J. Ceram. Soc. Jpn. 116 (2008) 1322–1325. [12] J.-e Zhou, Y. Dong, S. Hampshire, G. Meng, Utilization of sepiolite in the synthesis of porous cordierite ceramics, Appl. Clay Sci. 52 (2011) 328–332. [13] F. Zhang, C.-X. Qi, S. Wang, J.-h Liu, H. Cao, A study on preparation of cordierite gradient pores porous ceramics from rectorite, Solid State Sci. 13 (2011) 929–933. [14] M. Valášková, G.S. Martynková, B. Smetana, S. Študentová, Influence of vermiculite on the formation of porous cordierites, Appl. Clay Sci. 46 (2009) 196–201.
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
C. Liu et al. / Ceramics International ] (]]]]) ]]]–]]] [15] W. Guo, H. Lu, C. Feng, Influence of La2O3 on preparation and performance of porous cordierite from rice husk, J. Rare Earths 28 (2010) 614–617. [16] X. Zhang, X. Ren, S. Wang, X. Liu, G. Meng, Preparation and characterization of porous cordierite ceramics from fly ash, J. Chin. Ceram. Soc. 34 (2006) 247–250. [17] S.-h Huang, B. Peng, Z.-h Yang, L.-y Chai, Y.-z Xu, C.-q Su, Spatial distribution of chromium in soils contaminated by chromium-containing slag, Trans. Nonferr. Metals Soc. China 19 (2009) 756–764. [18] B.B. Lind, A.M. Fällman, L.B. Larsson, Environmental impact of ferrochrome slag in road construction, Waste Manag. 21 (2001) 255–264. [19] J. Zelić, Properties of concrete pavements prepared with ferrochromium slag as concrete aggregate, Cement Concrete Res. 35 (2005) 2340–2349. [20] O. Gencel, F. Koksal, C. Ozel, W. Brostow, Combined effects of fly ash and waste ferrochromium on properties of concrete, Constr. Build. Mater. 29 (2012) 633–640. [21] O. Gencel, M. Sutcu, E. Erdogmus, V. Koc, V.V. Cay, M.S. Gok, Properties of bricks with waste ferrochromium slag and zeolite, J. Clean. Prod. 59 (2013) 111–119. [22] T.I. Barry, J.M. Cox, R. Morrell, Cordierite glass-ceramics-effect of TiO2 and ZrO2 content on phase sequence during heat treatment, J. Mater. Sci. 13 (1978) 594–610. [23] A. Azhari, F. Golestani-Fard, H. Sarpoolaky, Effect of nano iron oxide as an additive on phase and microstructural evolution of Mag-Chrome refractory matrix, J. Eur. Ceram. Soc. 29 (2009) 2679–2684. [24] H.R. Zargar, C. Oprea, G. Oprea, T. Troczynski, The effect of nano-Cr2O3 on solid-solution assisted sintering of MgO refractories, Ceram. Int. 38 (2012) 6235–6241. [25] S. Li, J. Gao, P. Chen, P. Shen, Y. Zhang, R. Liu, Y. Wang, Experimental research of new technology on the precise reduction of micron-sized iron oxide, Adv. Mater. Res. 201–203 (2011) 1763–1769. [26] J. Banjuraizah, H. Mohamad, Z.A. Ahmad, Effect of excess MgO mole ratio in a stoichiometric cordierite (2MgO 2Al2O3 5SiO2) composition
[27]
[28]
[29] [30] [31]
[32]
[33]
[34]
[35]
9
on the phase transformation and crystallization behavior of magnesium aluminum silicate phases, Int. J. Appl. Ceram. Technol. 8 (2011) 637–645. J. Banjuraizaha, H. Mohamad, Z.A. Ahmad, Effect of impurities content from minerals on phase transformation, densification, J. Alloy. Compd. 509 (2011) 7645–7651. C.R. Cheeseman, A. Makinde, S. Bethanis, Properties of lightweight aggregate produced by rapid sintering of incinerator bottom ash, Resources, Conserv. Recycling 43 (2005) 147–162. S. Bethanis, Effect of sintering temperature on the properties and leaching of incinerator bottom ash, Waste Manag. Res. 22 (2004) 255–264. W. Luecke, D.L. Kohlstedt, Kinetics of the internal oxidation of (Mg, Fe) O Solid solutions, J. Am. Ceram. Soc. 71 (1988) 189–196. T. Wu, D.L. Kohlstedt, Rutherford backscattering spectroscopy study of the kinetics of oxidation of (Mg, Fe)2SiO4, J. Am. Ceram. Soc. 71 (1988) 540–545. Q. Lü, X. Dong, Z. Zhu, Y. Dong, Environment-oriented low-cost porous mullite ceramic membrane supports fabricated from coal gangue and bauxite, J. Hazardous Mater. 273 (2014) 136–145. A. Benhammou, Y. El Hafiane, L. Nibou, A. Yaacoubi, J. Soro, A. Smith, J.P. Bonnet, B. Tanouti, Mechanical behavior and ultrasonic non-destructive characterization of elastic properties of cordierite-based ceramics, Ceram. Int. 39 (2013) 21–27. A. Benhammou, Y. El Hafiane, A. Abourriche, Y. Abouliatim, L. Nibou, A. Yaacoubi, N. Tessier-Doyen, A. Smith, B. Tanouti, Effects of oil shale addition and sintering cycle on the microstructure and mechanical properties of porous cordierite-ceramic, Ceram. Int. 40 (2014) 8937–8944. K.-S. Wang, K.-Y. Chiang, J.-K. Perng, C.-J. Sun, The characteristics study on sintering of municipal solid waste incinerator ashes, J. Hazardous Mater. 59 (1998) 201–210.
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag Chuanbei Liua, Laibao Liua,b,n, Kefeng Tana, Lihua Zhanga,b, Kaijing Tanga, Xianpan Shia a
School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, Sichuan 621010, China b School of Materials Science and Engineering, Southeast University, Nanjing 211189, China Received 14 July 2015; received in revised form 31 August 2015; accepted 31 August 2015
Abstract Porous cordierite ceramics were successfully fabricated using ferrochromium slag, commercial alumina and silica powder as raw materials, without any additional pore forming agents, by sintering at temperatures ranging from 1100 to 1350 1C. The phase evolution, porosity, microstructure, mechanical properties, thermal expansion property and chromium leachability were systematically investigated. The results showed that the internal oxidation of forsterite in the slag contributed to the massive glassy phases formed in the prepared porous ceramics as well as the low-temperature sintering synthesis of cordierite, which started to form at 1100 1C and at 1350 1C, comprising up to ∼87.1 wt% of the ceramic. Furthermore, the iron oxides in the ferrochromium slag acted as pore forming agents at high temperatures, thereby resulting in significant volume expansion of the sample. The SEM images showed that the prepared cordierite ceramics had a porous microstructure composed of massive glassy phases embedded in polyhedral spinel and prismatic cordierite. In addition, the ceramic had a flexural strength of 47.2671.01 MPa, coefficient of thermal expansion of 3.5 10-6/1C, and chromium leachability that was only half as large as that of the green sample. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Ferrochromium slag; Porous cordierite; Phase evolution; Chromium leachability.
1. Introduction Porous ceramics materials have many interesting properties including low density, high permeability and good heat insulation. These characteristics make them widely used in various industrial fields such as liquid or gas fillers, catalysis supports, selective membranes, thermal insulation and pollution control [1]. Among those materials, porous cordierite is a very good candidate for these applications due to its low thermal expansion coefficient and high resistance to thermal shock [2,3]. Porous cordierite is synthesized primarily via a solid-state sintering method, using talc, kaolin, magnesite and other natural minerals or the pure chemical oxide as the raw materials. Sintering temperatures of up to ∼1400 1C are typically used n Corresponding author at: School of Materials Science and Engineering, Southwest University of Science and Technology, No. 59 Qinglong Road, Mianyang, Sichuan 621010, China. Tel.: þ86 13689691135. E-mail address: [email protected] (L. Liu).
[4–6]. Attempts to lower the sintering temperature have included the use of sintering additives [7,8] and the development of novel synthesis methods [9–11]. However, these attempts incur high fabrication costs and require a complex preparation process. Finding a meaningful way to use inexpensive and rich-sourced raw materials, especially the industrial byproducts, to synthesize porous cordierite ceramics is therefore essential. Some studies have focused on the preparation of porous cordierite ceramics using sepiolite, rectorite and vermiculite as raw materials [12– 14]; industrial byproducts such as rice husk, and fly ash have also been used [15,16]. There are, however, no published reports regarding porous cordierite ceramics prepared mainly from ferrochromium slag. Ferrochromium slag is a waste material produced during the manufacturing process of ferrochrome that is an essential component in stainless steel. The massive ferrochromium slag produced every year occupies significant land space and contaminates the environment owing to the presence of leachable heavy metals, especially chromium [17,18]. Effective use
http://dx.doi.org/10.1016/j.ceramint.2015.08.174 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
C. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
2
of the slag is therefore essential. Many approaches for reusing the slag have been reported. In fact, most of the slag is used for traditional construction materials, such as concrete [19,20] and brick [21]. Unfortunately, with these approaches, the slag is simply moved from one place to another and the potential hazard of the slag remains. Ferrochromium slag has a similar chemical composition to cordierite. Both contain mainly silicon (Si), manganese (Mg), and aluminum (Al), which together constitute ∼83% of the slag [19]. The slag also contains metallic elements such as iron (Fe), titanium (Ti), and heavy metals such as chromium (Cr), cobalt (Co) and nickel (Ni). These components are expected to act as sintering additives and have a positive effect on the phase evolution as well as the crystallization of cordierite [22–24]. In addition, the high-temperature reduction of iron oxides in the slag generates a large amount of oxygen, which acts as a pore forming agent [25]. Therefore, in this study, in order to carry out an innocuous treatment of ferrochromium slag, porous cordierite ceramics were prepared from a mixture of the slag, commercial alumina (Al2O3), and silica powder (SiO2) without additional pore forming agents. The phase evolution, volume expansion, porosity, microstructure, mechanical properties, thermal expansion behavior, and Cr leachability, of the prepared ceramics were evaluated. Furthermore, the phase evolution during sintering is discussed from the viewpoint of chemical reactions of the various MgO (FeO)–Al2O3–SiO2 compositions. 2. Experimental procedure 2.1. Sample preparation The porous cordierite ceramics were prepared from raw materials of ferrochromium slag (Leshan, Sichuan Province, China), commercial alumina and silica powder (both from Zibo Jiezhong New Material Co., Ltd., China). Cordierite having the composition 2MgO 2Al2O3 5SiO2, was prepared by weighing and mixing ferrochromium slag, alumina and silica powder for 6 min at room temperature in a vibration mill rotating at a constant speed of 710 rpm. The milled powder mixture was then mixed with organic binder PVA-1750 (3 wt% solution) and uniaxially pressed under 45 MPa into 25 mm(diameter) 5 mm (thickness) pellets (weighing ∼5.5 g each). The green pellets were subsequently dried at 10572 1C in a baking oven in order to remove the free water, and then sintered for 3 h in a muffle furnace at temperatures ranging from 1100 to 1400 1C. In addition, the green pellets were sintered at 1350 1C for 0.5, 1.0, 2.0 and 4.0 h, respectively, in order to investigate the effect of holding time on the properties of the cordierite. The heating rate was fixed at 5 1C/min, and the added organic binder was removed by holding the sample at 600 1C for 20 min. The sintered pellets were furnace-cooled to room temperature. 2.2. Characterization The chemical compositions of the raw materials were determined via quantitative X-ray fluorescence spectrum analysis (XRF; Axios-Advanced, PANalytical Corporation, The Netherlands). The
corresponding particle size distributions were determined by a laser particle size analyzer (Mastersizer 2000, Malvern Instruments Ltd., UK) using water as a dispersing medium. Moreover, X-ray diffraction (XRD; X'Pert Pro, PANalytical Corporation, The Netherlands) was used to determine the phase compositions of the raw materials and the sintered samples. The actual phase fractions were obtained via quantitative Rietveld analysis using the HighScore Plus software [26,27]. Furthermore, the morphology of the slag and the fracture surfaces of the porous cordierite ceramics were examined by using a field emission scanning electron microscope (FESEM; Ultra 55, Carl Zeiss Ltd., Germany); electron energy disperse spectroscopy (EDS) was performed with equipment attached to the microscope. The coefficient of thermal expansion (CTE) of the sintered samples was evaluated at temperatures between room temperature and 1000 1C by using a thermal dilatometer (DIL402PC, NETZSCH- Gerätebau GmbH, Germany). The CTE was calculated from: α = ΔL / (L0 × ΔT )
(1 )
where α, ΔL/L0, and ΔT are the CTE, linear expansion ratio, and applied temperature difference, respectively. The change in the diameter (i.e., linear shrinkage or expansion) of the sintered pellets was measured by using a vernier caliper. In addition, the open porosity and bulk density were measured in accordance with Archimedes' principle using distilled water as an immersion liquid. The distribution of pore sizes of the prepared porous cordierite ceramics was determined by using an automatic mercury porosimeter (AutoPore IV9500, Micromeritics Instrument Corp., America). This porosimeter allows the analysis of pores with sizes ranging from 0.005 to 630 μm, and the pore radius, rp, can then be determined from Washbum's equation that is given as: rp =
−4γ cos θ PHg
(2 )
where γ is the surface tension (0.485 N m-1) of Hg, θ is the contact angle between the Hg and the solid phase (equal to1401) and PHg is the pressure of Hg. Three-point flexural strength tests of the sintered samples (length: 45 mm, width: 4 mm and height: 3 mm) were performed in accordance with ISO 14704:2000(E), using a universal material testing machine (ReGeR-3010, Reger Instrument Co., Ltd., China). A crosshead speed of 0.5 mm/min was used in the testing of at least five specimens. The pH-dependent leaching behavior [28,29] of Cr was investigated in the green samples and the pellets sintered at 1350 1C for 3 h. The sintered pellets were initially ground and then sieved through a 200-mesh sieve for leach testing. A series of 5 g sub-samples was mixed with 100 mL of leaching solutions, of varying acidity, ranging from distilled water to 0.03 M HNO3 solution (with pH value of ∼1.5). The samples and leachates were mixed in sealed containers for 24 h on an oscillating shaking table, before being centrifuged. The leachates were then extracted by filtering through a 0.45 μm membrane filter and their pH was measured before being analyzed via inductively coupled plasma
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
C. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
3
Table 1 Chemical composition (wt%) of raw materials measured by quantitative XRF. Materials
Chemical composition (wt%)
Ferrochromium slag Commercial alumina Silica powder The mixture
SiO2
CaO
Al2O3
Fe2O3
MgO
Cr2O3
TiO2
SO3
MnO
Na2OþK2O
Others
35.12 0.04 99.9 47.69
1.57 / / 0.73
22.21 99.6 0.01 32.45
4.21 0.03 0.04 1.95
27.77 / / 12.83
7.37 / / 3.40
0.73 / / 0.33
0.46 / / 0.21
0.23 / / 0.11
0.15 0.26 / 0.07
0.18 0.07 0.05 0.08
Fig. 1. XRD patterns of the ferrochromium slag used. Fig. 2. Particle size distributions of the raw materials.
mass spectrometry (ICP-MS; Agilent 7700x, Agilent Technologies Inc., America). 3. Results and discussion 3.1. Characterization of raw materials Table 1 lists the chemical compositions of the ferrochromium slag, commercial alumina and silica powder, as determined by quantitative XRF analysis. The slag consists primarily of SiO2, Al2O3 and MgO, which together account for 85.1 wt% of the slag. Moreover, stoichiometric cordierite was synthesized from 46.12 wt%, 22.30 wt% and 35.10 wt% of the slag, alumina and silica powder, respectively; the chemical composition of the mixture is also shown in Table 1. The XRD pattern of the slag is shown in Fig. 1. The result confirmed the presence of the solid solution of forsterite, (Mg,Fe)2SiO4, in which the Mg was partially replaced by Fe. Hercynite (FeAl2O4), chrome-spinel ((Mg,Fe)(Cr,Al)2O4), and a small amount of enstatite (MgSiO3) were also detected. In general, the phase composition of the used slag is consistent with those reported in previous studies [19]. Fig. 2 shows the particle size distributions of the raw materials used. As it shows, the slag, alumina, silica and the corresponding vibration-milled mixture have particle sizes of 1–80 μm, 1–30 μm, 3–110 μm and 1–80 μm, respectively. Furthermore, the average (D50) particle sizes of these respective materials are 11.85 μm, 4.45 μm, 33.49 μm and 12.13 μm.
3.2. Phase evolution The samples all consisted of eight crystalline phases (Fig. 3); these were indialite (PDF#00-012-0235), spinel (PDF#00-0210540), forsterite (PDF#00-004-0796), magnesium aluminum chromium oxide labeled as Cr-spinel (PDF#00-023-1222), hercynite (PDF# 00-007-0068), ringwoodite (PDF#00-021-1258), quartz (PDF#00-046-1045), and corundum (PDF#00-010-0173). Indialite (also known as α-cordiarite) first appears at a temperature of 1100 1C, and the intensities of the corresponding peaks increase with increasing temperatures of 1100–1400 1C. Rietveld quantitative phase analysis results (Fig. 4) reveal that indialite constitutes up to 87.1 wt% of the samples sintered for 3 h at the optimal temperature of 1350 1C. In addition, the ringwoodite phase first appears at 1150 1C whereas forsterite and hercynite are not observed at this temperature. Ringwoodite as well as quartz and corundum were, however, intermediate phases and hence were not observed at 1350 1C. The spinel phase was observed at 1300 1C and seemed to transform into magnesium aluminum chromium oxide (Mg(Al,Cr)2O4) when the sintering temperature increased to 1400 1C. Similar tendencies were observed in the phase evolution of the sample sintered at 1350 1C for various holding times (Fig. 3 (b)). For example, the 3-h-heated sample consisted of only the indialite and spinel; intermediate phases such as quartz, corundum and hercynite were not observed after 2.0 h of heating. The ferrochromium slag consists predominantly of forsterite, whose chemical formula is 2(Mgx,Fe1-x)O SiO2; the Fe2 þ and
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
C. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
4
Fig. 3. XRD patterns of samples sintered at (a) temperatures ranging from 1100 to 1400 1C for 3 h and (b) 1350 1C for holding times of 0.5–4.0 h.
Mg2 þ can replace each other and thereby form a continuous solid solution. In fact, when forsterite is heat-treated at temperatures of 800 1C or higher in an oxidizing atmosphere, the Fe is released from the crystal structure along the crystal boundary and cleavage planes, and the internal oxidation of forsterite results in the formation of 2MgO SiO2 forsterite and amorphous silica, as shown in the following equation [30,31]: 2 (Mg, Fe) O⋅SiO2
≥ 800 ° C
→
2MgO
⋅SiO2 + SiO2 (amorphous) + Fe2 O3
(1 )
The resulting 2MgO SiO2 forsterite and amorphous silica are both in a high free-energy and hence unstable state, and at 1100 1C, are expected to react with the alumina in the raw materials used, thereby forming indialite. This reaction proceeds as follows:
Fig. 4. Crystalline-phase content of samples sintered at (a) temperatures of 1100–1400 1C for 3 h and (b) 1350 1C for holding times of 0.5–4.0 h.
2MgO⋅SiO2 + SiO2 (amorphous) + Al2 O3
1100 ° C
→
2MgO
⋅2Al2 O3⋅5SiO2
(2 )
The ∼29 wt% of indialite present at 1100 1C (Fig. 4(a)) is attributed to this reaction. Moreover, with continuously increasing heating temperatures, the high-iron glass phase is generated through the reaction between massive nomadic iron oxides (Fe2O3) and the formed 2MgO SiO2 forsterite; intense recrystallization also occurs [31]. The reaction between forsterite and Fe2O3 proceeds as follows: 2MgO⋅SiO2 + Fe 2 O3
≥ 1100 ° C
→
MgO⋅SiO2 + MgO⋅Fe2 O3
(3 )
Simultaneously, some of the Fe2O3 and SiO2 is expected to participate in the recrystallization of the glassy phase. This process results in the formation of the intermediate ringwoodite ((Mg,Fe)2SiO4) phase, which in turn reacts with silica and alumina to form the indialite phase; the reaction proceeds as follows:
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
C. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
(Mg, Fe) 2⋅SiO4 + SiO2 + Al2 O3
≥ 1150 ° C
→
3.4. Pore size distribution
2MgO⋅2Al2 O3
⋅5SiO2 + Fe2 O3
5
(4 )
3.3. Expansion, bulk density and open porosity Fig. 5 shows the linear expansion percent, bulk density, and open porosity of the porous cordierite ceramics after sintering for 3 h at temperatures of 1100–13501C. As the figure shows, the substantial increase in volume expansion is accompanied by a gradual decrease (from 2.1270.01 to 1.8070.01 g/cm3) in bulk density. This suggests that the process of preparing cordierite ceramics using ferrochromium slag as a raw material, differs from the usual ceramic sintering process, where samples typically exhibit significant shrinkage; this shrinkage is driven by the reduction in interfacial free energy. However, during the preparation of porous cordierite ceramics from the ferrochrome slag, the samples are also affected by the expansion pressure of O2 that is generated from the reduction of iron oxides. The reduction reaction proceeds as follows [25]:
Fig. 6 compares the pore size distributions of the porous cordierite ceramics sintered at 1250 1C for 3 h and at 1350 1C for both 1 and 3 h. As the figure shows, when the sintering temperature is increased from 1250 to 1350 1C, the pore size distribution is shifted to large pore sizes, as evidenced by an increase in the average pore size from 7.65 to 34.78 μm. The average pore size also increases (i.e., from 14.57 to 34.78 μm) when the holding time is increased from 1 to 3 h. These results reveal that the sintering temperature and holding time have a significant effect on the pore size distributions of the prepared ceramics. The aforementioned variation in open porosity is also attributed to this effect.
3.5. Microstructures and mechanical properties
Furthermore, the cordierites (2.50 g/cm3) and glassy phases in the sintered samples have smaller densities than those (3.34, 2.65 and 3.99 g/cm3, respectively) of the crystalline phases such as forsterite, quartz and corundum in the raw materials used. Therefore, the preparation process of the cordierite ceramics is, essentially, a volume expansion process. The open porosity of the cordierite ceramics decreased slowly from 32.9770.21 to 30.7270.17% at temperatures of 1100–1250 1C, owing to the increasing amount of liquid phases that resulted in the closure of connected pores [32,33]. However, the open porosity increased rapidly from 30.8370.08% to 32.870.16% with increasing sintering temperatures of 1300– 1350 1C. This increase is very likely resulted from the increased volume fraction, and simultaneous decrease in the viscosity, of the liquid phases; i.e., small pores become connected, owing to the expansion pressure of O2, and consequently larger pores are formed.
The SEM images in Fig. 7 compare the fracture surfaces of the ceramics sintered at 1250 and 1350 1C for holding times of 1 and 3 h. The prepared cordierite ceramic has a porous microstructure with pore sizes ranging from a few to dozens of microns. Moreover, the pore size increases significantly when the sintering temperature and time are increased from 1250 (Fig. 7(a)) to 1350 1C (Fig. 7(e)) and 1 (Fig. 7(c)) to 3 h (Fig. 7(e)), respectively; this result is consistent with the previously mentioned pore size distribution and variation in the open porosity. In addition, the crystal size increased considerably with the aforementioned increases in temperature and holding time, as revealed by comparing Fig. 7(b) with (f) and (d) with (f). These highmagnification images also reveal that the prismatic cordierites are generated from the matrix of the glassy phase. This resulted from the crystal growth of massive low-viscosity liquid phases at high sintering temperatures and long holding times. In addition, a plot (Fig. 8) of the flexural strength of the ceramics sintered for 3 h at temperatures of 1150–1350 1C, reveals that the strength increases linearly with increasing sintering
Fig. 5. Linear expansion percent, bulk density and open porosity of samples sintered at temperatures of 1100–1350 1C for 3 h.
Fig. 6. Pore size distributions of the samples sintered at 1250 1C for 3 h (1250 1C/3 h) and 1350 1C for 1 and 3 h (1350 1C /1 h and 1350 1C /3 h, respectively).
6Fe2 O3 → 4Fe3O4 + O2 ↑
(5 )
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
C. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
6
Fig. 7. SEM images of fracture surfaces of samples sintered at (a) and (b) 1250 1C for 3 h, (c) and (d) 1350 1C for 1 h, and (e) and (f) 1 350 1C for 3 h.
temperature. The sample sintered at 1350 1C had the highest (47.2671.01 MPa) strength. The densification of ceramic bodies and the growth of crystals lead to an increase in the flexural strength of cordierite ceramics [34]. In this work, the increase in flexural strength at temperatures of 1150–1250 1C results from growth of the cordierite crystals and densification of the samples. The continuous increase in strength after 1250 1C is attributed to further growth of the crystals; this is especially true in the case of the spinel crystals, as evidenced by the results of the XRD analysis.
(b). As the figure shows, the CTE decreases from 5.7 10-6/1C to 3.5 10-6/1C with increasing sintering temperatures of 1150–1350 1C. The CTE of cordierite ceramics stems mainly from the glassy and cordierite phases. In the current study, the high fraction of glassy phases result in the relatively higher CTE values of the samples compared to those (1.0–2.0 10-6/1C) of their conventional counterparts. However, the decrease in the CTE with increasing sintering temperature stems mainly from the gradual growth of the cordierite crystals.
3.6. Linear thermal expansion properties
3.7. Chromium leachability
Fig. 9(a) shows the linear thermal expansion behavior (from room temperature to 1000 1C) of the ceramics heat-treated for 3 h at various temperatures. The CTE of the samples is calculated from the linear thermal expansion ratio (dL/L0) and is shown in Fig. 9
The effect of leaching under acid leachate conditions (pH¼ 7.0–1.5) on the release of Cr is shown in Fig. 10. The Cr content of the leachates is expressed as mg/kg of dry sample. As the figure shows, the Cr leachability of the green samples and sintered
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
C. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
7
Fig. 8. Flexural strength of samples sintered for 3 h at temperatures ranging from 1150 to 1350 1C.
ceramics is lower than 35.0 mg/kg, which meet the US EPA limits of 100 mg/kg. This result is in agreement with the results reported by Huang [17] and Lind [18] for various soil samples near the ferrochrome slag. Furthermore, the Cr leachability of the sintered ceramics was approximately half as large as that of the green samples. Previous work [29,35] showed that high temperatures and long holding times were needed to reduce metal leaching from the sintered solid-waste samples. The incorporation of heavy metals into the amorphous and crystalline phases was also expected and therefore these metals should not be leached from the samples. With ionic radii of 0.073 and 0.062 nm, Cr2 þ and Cr3 þ have very similar ionic radii to those (0.072 and 0.054 nm) of Mg2 þ and Al3 þ , respectively. As such, a portion of the nomadic Cr in the ferrochromium slag can easily replace the Mg or Al in the crystal structures of spinel and cordierite phases, and thereby form a stable structure. Fig. 11 shows another high-magnification SEM image of the porous cordierite ceramics sintered for 3 h at 1350 1C and the corresponding EDS analysis of the crystal phases. As the figure shows, massive glassy phases are embedded in the polyhedral spinel and prismatic cordierite. The EDS analysis revealed the high Cr content of the spinel (Fig. 11(a)) and cordierite (Fig. 11(b)) phases, as well as the main elements, i.e., Si, Al and Mg. These results indicate that a stable phase formed owing to the Cr-replacement of Mg or Al in the crystalline structures of cordierite and spinel.
Fig. 9. (a) Thermal expansion behavior and (b) CTE of samples sintered at temperatures ranging from 1150 to 1350 1C.
4. Conclusions The properties of porous cordierite ceramics prepared from ferrochromium slag, commercial alumina and silica powder, via the reaction sintering technique and without additional pore forming agents, were investigated. The conclusions from this study are summarized as follows: (1) Phase evolution analysis of the samples revealed that the internal oxidation of forsterite in the slag contributed to the
Fig. 10. Dependence of the leaching of Cr from the green samples (labeled as green) and the porous cordierite ceramics sintered at 1350 1C for 3 h (labeled as sintered) on the pH of the leachate.
formation of massive glassy phases in the prepared porous ceramics; this oxidation also contributed to the lowtemperature sintering synthesis of cordierite, which formed
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
8
C. Liu et al. / Ceramics International ] (]]]]) ]]]–]]]
sintered at 1350 1C for 3 h exhibited an open porosity of up to 32.8 7 0.16%. (3) The prepared cordierite ceramics had a flexural strength of 47.267 1.01 MPa and a CTE of 3.5 10-6/1C. Moreover, some of the nomadic Cr in the slag was incorporated into the crystal structure of the as-synthesized cordierite and spinel phases. The Cr leachability of the ceramics was therefore only approximately half as high as that of the green samples. As such, this study provides a new ideal for innocuous treatments and resource use of ferrochromium slag. Acknowledgments The authors gratefully acknowledge the analytical support of the SWUST Analysis and Test Center, Mianyang, China. We are also grateful to the Leshan Xinhe Electric Power Comprehensive Opening Co., Ltd., Leshan, China, for providing the ferrochromium slag. References
Fig. 11. SEM image and EDS spectrum of the crystal phases in samples sintered for 3 h at 1350 1C.
first at a temperature of 1100 1C and at 1350 1C, constituted a phase fraction of ∼87.1 wt% of the ceramics. (2) The iron oxides in the slag act as pore forming agents during the preparation process of the ceramics. During this process, the samples exhibited substantial volume expansion (rather than the typical shrinkage) and a decrease in bulk density with increasing sintering temperature. Furthermore, the sintering temperature and holding time had a significant effect on the pore size; in fact, samples
[1] T. Ohji, M. Fukushima, Macro-porous ceramics: processing and properties, Int. Mater. Rev. 57 (2012) 115–131. [2] A. Shyam, E. Lara-Curzio, A. Pandey, T.R. Watkins, K.L. More, The thermal expansion, elastic and fracture properties of porous cordierite at elevated temperatures, J. Am. Ceram. Soc. 95 (2012) 1682–1691. [3] Y. Dong, X. Feng, D. Dong, S. Wang, J. Yang, J. Gao, X. Liu, G. Meng, Elaboration and chemical corrosion resistance of tubular macro-porous cordierite ceramic membrane supports, J. Membr. Sci. 304 (2007) 65–75. [4] M.A. Camerucci, G. Urretavizcaya, A.L. Cavalieri, Sintering of cordierite based materials, Ceram. Int. 29 (2003) 159–168. [5] R. Johnson, I. Ganesh, B.P. Saha, G.V.N. Rao, Y.R. Mahajan, Solid state reactions of cordierite precursor oxides and effect of CaO doping on the thermal expansion behaviour of cordierite honeycomb structures, J. Mater. Sci. 38 (2003) 2953–2961. [6] J.B. Rodrigues Neto, R. Moreno, Rheological behaviour of kaolin/talc/ alumina suspensions for manufacturing cordierite foams, Appl. Clay Sci. 37 (2007) 157–166. [7] Z.M. Shi, X. Bai, X.F. Wang, Ce4 þ -modified cordierite ceramics, Ceram. Int. 32 (2006) 723–726. [8] D. Zou, X. Chu, F. Wu, Sol–gel synthesis and infrared radiation property of Li-substituted cordierite, Ceram. Int. 39 (2013) 3585–3589. [9] J. Banjuraizah, H. Mohamad, Z.A. Ahmad, Crystal structure of single phase and low sintering temperature of α-cordierite synthesized from talc and kaolin, J. Alloy. Compd. 482 (2009) 429–436. [10] A. Yamuna, S. Honda, K. Sumita, M. Yanagihara, S. Hashimoto, H. Awaji, Synthesis, sintering and thermal shock resistance estimation of porous cordierite by IR heating technique, Microporous Mesoporous Mater. 85 (2005) 169–175. [11] M. Fukushima, M. Nakata, Y.-i Yoshizawa, Fabrication and properties of ultra highly porous cordierite with oriented micrometer-sized cylindrical pores by gelation and freezing method, J. Ceram. Soc. Jpn. 116 (2008) 1322–1325. [12] J.-e Zhou, Y. Dong, S. Hampshire, G. Meng, Utilization of sepiolite in the synthesis of porous cordierite ceramics, Appl. Clay Sci. 52 (2011) 328–332. [13] F. Zhang, C.-X. Qi, S. Wang, J.-h Liu, H. Cao, A study on preparation of cordierite gradient pores porous ceramics from rectorite, Solid State Sci. 13 (2011) 929–933. [14] M. Valášková, G.S. Martynková, B. Smetana, S. Študentová, Influence of vermiculite on the formation of porous cordierites, Appl. Clay Sci. 46 (2009) 196–201.
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174
C. Liu et al. / Ceramics International ] (]]]]) ]]]–]]] [15] W. Guo, H. Lu, C. Feng, Influence of La2O3 on preparation and performance of porous cordierite from rice husk, J. Rare Earths 28 (2010) 614–617. [16] X. Zhang, X. Ren, S. Wang, X. Liu, G. Meng, Preparation and characterization of porous cordierite ceramics from fly ash, J. Chin. Ceram. Soc. 34 (2006) 247–250. [17] S.-h Huang, B. Peng, Z.-h Yang, L.-y Chai, Y.-z Xu, C.-q Su, Spatial distribution of chromium in soils contaminated by chromium-containing slag, Trans. Nonferr. Metals Soc. China 19 (2009) 756–764. [18] B.B. Lind, A.M. Fällman, L.B. Larsson, Environmental impact of ferrochrome slag in road construction, Waste Manag. 21 (2001) 255–264. [19] J. Zelić, Properties of concrete pavements prepared with ferrochromium slag as concrete aggregate, Cement Concrete Res. 35 (2005) 2340–2349. [20] O. Gencel, F. Koksal, C. Ozel, W. Brostow, Combined effects of fly ash and waste ferrochromium on properties of concrete, Constr. Build. Mater. 29 (2012) 633–640. [21] O. Gencel, M. Sutcu, E. Erdogmus, V. Koc, V.V. Cay, M.S. Gok, Properties of bricks with waste ferrochromium slag and zeolite, J. Clean. Prod. 59 (2013) 111–119. [22] T.I. Barry, J.M. Cox, R. Morrell, Cordierite glass-ceramics-effect of TiO2 and ZrO2 content on phase sequence during heat treatment, J. Mater. Sci. 13 (1978) 594–610. [23] A. Azhari, F. Golestani-Fard, H. Sarpoolaky, Effect of nano iron oxide as an additive on phase and microstructural evolution of Mag-Chrome refractory matrix, J. Eur. Ceram. Soc. 29 (2009) 2679–2684. [24] H.R. Zargar, C. Oprea, G. Oprea, T. Troczynski, The effect of nano-Cr2O3 on solid-solution assisted sintering of MgO refractories, Ceram. Int. 38 (2012) 6235–6241. [25] S. Li, J. Gao, P. Chen, P. Shen, Y. Zhang, R. Liu, Y. Wang, Experimental research of new technology on the precise reduction of micron-sized iron oxide, Adv. Mater. Res. 201–203 (2011) 1763–1769. [26] J. Banjuraizah, H. Mohamad, Z.A. Ahmad, Effect of excess MgO mole ratio in a stoichiometric cordierite (2MgO 2Al2O3 5SiO2) composition
[27]
[28]
[29] [30] [31]
[32]
[33]
[34]
[35]
9
on the phase transformation and crystallization behavior of magnesium aluminum silicate phases, Int. J. Appl. Ceram. Technol. 8 (2011) 637–645. J. Banjuraizaha, H. Mohamad, Z.A. Ahmad, Effect of impurities content from minerals on phase transformation, densification, J. Alloy. Compd. 509 (2011) 7645–7651. C.R. Cheeseman, A. Makinde, S. Bethanis, Properties of lightweight aggregate produced by rapid sintering of incinerator bottom ash, Resources, Conserv. Recycling 43 (2005) 147–162. S. Bethanis, Effect of sintering temperature on the properties and leaching of incinerator bottom ash, Waste Manag. Res. 22 (2004) 255–264. W. Luecke, D.L. Kohlstedt, Kinetics of the internal oxidation of (Mg, Fe) O Solid solutions, J. Am. Ceram. Soc. 71 (1988) 189–196. T. Wu, D.L. Kohlstedt, Rutherford backscattering spectroscopy study of the kinetics of oxidation of (Mg, Fe)2SiO4, J. Am. Ceram. Soc. 71 (1988) 540–545. Q. Lü, X. Dong, Z. Zhu, Y. Dong, Environment-oriented low-cost porous mullite ceramic membrane supports fabricated from coal gangue and bauxite, J. Hazardous Mater. 273 (2014) 136–145. A. Benhammou, Y. El Hafiane, L. Nibou, A. Yaacoubi, J. Soro, A. Smith, J.P. Bonnet, B. Tanouti, Mechanical behavior and ultrasonic non-destructive characterization of elastic properties of cordierite-based ceramics, Ceram. Int. 39 (2013) 21–27. A. Benhammou, Y. El Hafiane, A. Abourriche, Y. Abouliatim, L. Nibou, A. Yaacoubi, N. Tessier-Doyen, A. Smith, B. Tanouti, Effects of oil shale addition and sintering cycle on the microstructure and mechanical properties of porous cordierite-ceramic, Ceram. Int. 40 (2014) 8937–8944. K.-S. Wang, K.-Y. Chiang, J.-K. Perng, C.-J. Sun, The characteristics study on sintering of municipal solid waste incinerator ashes, J. Hazardous Mater. 59 (1998) 201–210.
Please cite this article as: C. Liu, et al., Fabrication and characterization of porous cordierite ceramics prepared from ferrochromium slag, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.08.174