Preparation of mullite based ceramics from clay
–kaolin waste mixtures
Hugo P.A. Alves
a, Jaquelígia B. Silva
b, Liszandra F.A. Campos
a, Sandro M. Torres
a,
Ricardo P.S. Dutra
a, Daniel A. Macedo
a,n aMaterials Science and Engineering Postgraduate Program, Rapid Solidification Laboratory, UFPB, 58051-900 João Pessoa, Brazil
b
Department of Civil Engineering, UFRN, 59078-970 Natal, Brazil
a r t i c l e i n f o
Article history: Received 6 May 2016 Received in revised form 8 September 2016 Accepted 9 September 2016
Keywords:
A. Powders: solid state reaction A. Sintering
D. Mullite Kaolin waste
a b s t r a c t
Mullite based ceramics were prepared by reaction sintering of mixtures containing kaolin clay and kaolin waste. Phase composition, apparent density, apparent porosity, and modulus of rupture were in-vestigated as a function of sintering temperature and kaolin waste content (varied from zero to 50 wt%). The sintering shrinkage behavior was monitored by dilatometry. The results showed that samples de-rived from formulations containing kaolin waste were composed of acicular mullite and glass phases at 1500°C. Ceramic bodies prepared with kaolin waste (25 or 50 wt%) showed better physico-mechanical properties (for sintering up to 1400°C) than the ones of pure kaolin clay. The evidence seems to suggest that this behavior can be due to a liquid phase assisted sintering mechanism associated with a content of mica ranging from 13.3 to 22.4 wt%.
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1. Introduction
Mullite (3Al2O3 2SiO2) is an aluminosilicate of great
techno-logical importance. Usually, it can be synthesized by solid state reaction (reactive sintering) of kaolinite (Al2Si2O5(OH)4) and
alu-mina (Al2O3) at temperatures above 1300°C [1–3]. Mullite has
been qualified as one of the most important refractory ceramic materials due to its special set of properties such as low density (3.17 g/cm3), high melting point (1830°C), good chemical stability,
high modulus of rupture, and low coefficient of thermal expansion (4.5 106K1) [4,5]. The production of mullite by reactive
sintering of silico-aluminous sources occurs by ionic diffusion of aluminum ions (Alþ 3) to the interior of SiO2layers formed during
the decomposition of kaolinite (4500 °C). Hence, mullitization can be characterized as a slow and thermally activated process[6–
9]. Various synthetic and raw materials have been extensively used to produce mullite based ceramics. The most popular are clays, alumina, aluminum isopropoxide, and kaolinite[2,10,11].
During the mining processes, large amount of kaolin waste is produced, consisting mostly of mica and quartz alongside residual kaolinite as well as other minor constituents. The kaolin waste generally discarded in the open air causes significant environ-mental problems; affecting fauna,flora and people's health. Nu-merous studies have been dedicated to the recycling of kaolin
processing waste aimed at the production of bricks, tiles and ceramic coatings[12–16]. However, recycling and reuse of wastes should not only be seen from an environmental point of view but also in terms of economic viability. In this sense, the development of applied research towards higher added value technical solutions encourages optimized ways for enhancing the culture of recycling and reuse.
In this scenario, this study aims to investigate the potential for reuse of kaolin waste to obtain mullite based ceramics. Con-siderations are made with regards to the effect of contaminants on the mullitization process.
2. Materials and methods
Mullite based ceramics were prepared by reactive sintering of mixtures containing a kaolin clay and kaolin waste. The kaolin clay and kaolin waste are from the State of Paraíba (Brazil). Ceramic formulations were prepared by replacing kaolin clay by 0, 25 and 50 wt% kaolin waste. The proportion of powder, alumina ball and distilled water was kept constant at a 1:3:2 mass ratio. Following milling, the mixes were dried at 110°C for 24 h. Dry powders were deagglomerated, humidified (by mixing with 6 wt% water in a porcelain mortar) and granulated through a 35 mesh sieve. In order to prepare the ceramic samples (61 21 7.6 mm), 13 g of powder was uniaxially pressed at 40 MPa. The samples were dried at 110°C for 24 h. The sintering process was conducted in an electric furnace at 1300–1550 °C for 3 h (in air atmosphere) using a heating/cooling rate of 3°C/min.
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Ceramics International
http://dx.doi.org/10.1016/j.ceramint.2016.09.068
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nCorresponding author.
E-mail addresses:hugo_plinio@hotmail.com(H.P.A. Alves),
The chemical compositions of both raw materials were de-termined using X-ray fluorescence spectroscopy (Shimadzu, EDX-700). They showed similar contents of Al2O3(33–36 wt%) and SiO2
(46–52 wt%). The kaolin waste contained significantly larger content of K2O (4.8 wt%) than that of kaolin clay (0.5 wt%). The sintering
shrinkage behaviors of kaolin clay and kaolin clayþ50 wt% kaolin waste were evaluated using a horizontal pushrod Netzsch DIL 402 PC dilatometer in air from 30 to 1500°C with an Al2O3Netzsch standard
as reference. The apparent porosity, apparent density (determined using the Archimedes method), and modulus of rupture (MOR) measured at room temperature were evaluated as a function of sintering temperature and kaolin waste content. MOR was obtained by the three points bend test following ASTM C674-77[17].
The identification of crystalline phases of sintered samples was carried out by X-ray diffraction (Shimadzu, XRD 7000, using Cu-K
α
radiation). The samples were ground and sieved through a 200 mesh sieve prior to testing. Microstructural characterization was made using scanning electron microscopy of fractured surfaces (Quanta 450, FEI).3. Results and discussion
Fig. 1shows X-ray diffraction patterns of raw kaolin clay and kaolin waste. Both raw materials contained the crystalline phases
kaolinite (Al2Si2O5(OH)4), mica muscovite (KAl2Si3AlO10(OH)2) and
quartz (SiO2). Kaolinite is an excellent structure forming in a wide
range offiring temperatures, with mica acting as a fluxing agent at high sintering temperatures. However, mica in large quantity may
Fig. 1. X-ray diffraction patterns of (a) kaolin clay and (b) kaolin waste.
Fig. 2. X-ray diffraction patterns of (a) kaolin clay, (b) clayþ25 wt% kaolin waste and (c) clayþ50 wt% kaolin waste sintered at 1300–1550 °C.
cause excessive dimensional change [18]. The mineral quartz causes an increase in the mechanical strength and a decrease in thefiring shrinkage, acting as a “skeleton” during the formation of liquid phase. According to the methodology of rational analysis proposed by Varela et al.[18], the kaolin clay used in this study is mainly composed of kaolinite (86.7 wt%) and quartz (9 wt%). The amount of mica can be considered small (4.3 wt%). According to the same methodology, the main minerals present in the kaolin waste are kaolinite (49.65 wt%) and mica (40.5 wt%). The amount of quartz was found to be 9.85 wt%. These mineralogical results suggest that the herein studied raw materials have potential to be used in the manufacturing of mullite based ceramics.
The structural characterization of samples sintered between 1300 and 1550°C is shown inFig. 2. The crystalline phases present in the sample of clay (Fig. 2a) are mullite (M, JCPDS 15-0776), quartz (Q, hexagonal silica, JCPDS 46-1045) and cristobalite (C, tetragonal silica, JCPDS 03-0270). Since the amount of silica (SiO2)
in the kaolin clay is much higher than that in mullite (50 wt% vs. 28 wt%), it can be expected that the excess amount of silica will experience a series of transformations with increasing tempera-ture. According to the literature[19,20], excess silica (in the form of quartz above 980°C) transforms to amorphous silica after heat treatment above 1200°C and crystallizes to form cristobalite as temperature increases. A final transformation of cristobalite to amorphous phase is also reported to occur above 1500°C [19]. Based on these literature observations, we assume that the ther-mal transformation of the excess silica taking place in our kaolin clay (at 1300–1550 °C) can be expressed by Eq.(1).
3Al2O3 2SiO2þSiO2 (quartzþamorphous)-3Al2O3 2SiO2þSiO2
(quartzþamorphousþcristobalite) (1)
According to Eq. (1), quartz and amorphous silica (most likely formed below 1300°C) crystallize into cristobalite with increasing temperature. Part of the cristobalite returns to the amorphous phase, with orthorhombic mullite as the main crystalline phase at 1550°C. As far as samples derived from formulations containing kaolin waste are concerned, acicular mullite and glass phase are observed at 1500°C, as can be seen in Figs. 2(b–c) and 3 (SEM image). Yamuna et al.[21]also reported the synthesis of SiO2-free
mullite using K2CO3as a mineralizer. In the present work, a similar
effect is observed by incorporating kaolin waste (a mica-rich material) to the formulations. As in K2CO3systems, kaolin waste
(often reported to containing significant amounts of potassium) increased the amount of liquid phase, promoting the dissolution of silica into it, and as a result, crystalline silica phases (quartz and cristobalite) were eliminated after sintering at 1500°C. The pre-sence of glassy phase (mainly from mica) is clearly observed by the occurrence of a halo in the region between 15 and 30° 2
θ
(Fig. 2). The two reflections of mullite in 2θ
26°, planes (120) and (210), are classically attributed to the presence of orthorhombic mullite (also known as secondary mullite, the mullite formed from the amorphous aluminosilicate phase)[22].The sintering shrinkage behavior of green rectangular samples made of kaolin clay (continuous line) and a mixture clayþ50 wt% kaolin waste (dashed line) are shown inFig. 4. The small linear shrinkage (lower than 1%) observed between 450 and 600°C is associated with the transformation of kaolinite to metakaolinite. The sample containing kaolin waste is less sensitive to this struc-tural transformation (dehydroxylation of kaolinite) because its kaolinite content is lower than that of the clay, as reported else-where in the text (see section on X-ray diffraction). The clay sample shrinkage was 2% at 900–1000 °C due to the metakaolinite decomposition to form spinel or mullite[23]. The corresponding shrinkage in the sample with kaolin waste is partially mitigated by the volume increase of 2% during the dehydroxylation of the mica
[24]. The ceramic powders studied in this work had mica contents of 4.3 wt% (kaolin clay), 13.3 wt% (clayþ25 wt% kaolin waste) and 22.4 wt% (clayþ25 wt% kaolin waste); which means that increas-ing the kaolin waste in the ceramic mixture causes the thermal shrinkage to become more sensitive to kaolinite–mica interactions. Irrespective of the mica content, the sintering densification starts around 1100°C, but ends by 1400 °C for the sample with 22.4 wt% mica and persists up to 1500°C for the kaolin clay (with lower mica content). These results are explained by the occurrence of a peritectic liquid in the mica phase at 41140 °C, as predicted by the SiO2–Al2O3–K2O phase diagram[25]. As reported by Lecomte
et al.[26], such a liquid phase accelerates the densification process through a viscousflux mechanism that is partially inhibited by the crystallization of new phases.
Fig. 5 shows the apparent density and apparent porosity as a function of the kaolin waste content and sintering temperature. Considering that orthorhombic mullite has a density of 3.17 g/cm3,
it is observed that kaolin clay samples achieved relative densities of 92–95% after sintering at 1400–1550 °C. It may be noted that the samples containing kaolin waste exhibited greater densification at lower sintering temperatures (1300 and 1400°C). This observation
Fig. 3. Acicular mullite and glass phase formed in the formulation clay–50 wt% kaolin waste sintered at 1500°C (m: mullite, g: glass).
Fig. 5. Apparent density (a) and apparent porosity (b) of samples sintered at 1300– 1550°C.
Fig. 6. Modulus of rupture (MOR) measured at room temperature afterfiring at a series of higher temperatures. MOR is indicated as a function offiring temperature and kaolin waste content: (1) kaolin clay, (2) clayþ 25 wt% kaolin waste and
(3) clayþ 50 wt% kaolin waste. Fig. 7. Fracture surfaces of samples derived from mixtures clayþ50 wt% kaolin waste sintered at (a) 1400, (b) 1500 and (c) 1550°C.
can be explained by the higher glassy phase formation and con-sequentfilling of the open porosity. The decrease in density above 1400°C may be due to air expansion in closed pores and the de-crease of the viscosity of the glass phase with the inde-crease of sintering temperature. Fig. 5b shows kaolin waste processed ceramics with apparent porosities lower than 5% (nearly zero after sintering at 1300°C), regardless of the sintering temperature evaluated. These results are consistent with the existence of a large fraction of liquid phase in the samples containing kaolin waste, especially up to 1400°C, compared with waste-free samples.
Fig. 6shows the modulus of rupture (MOR) as a function of sintering temperature and kaolin waste content. As can be seen, samples of kaolin clay show a gradual increase of MOR with in-creasing sintering temperature between 1300 and 1500°C. This event is followed by a MOR drop after heat treatment at 1550°C. In samples derived from formulations containing kaolin waste, the maximum MOR value was observed after sintering at 1400°C. In these samples, the liquid phase sintering accelerates the densi fi-cation (and mechanical strength) up to 1400°C. This explains why samples containing 50 wt% kaolin waste showed the highest me-chanical strength among samples sintered at 1300°C. For this particular formulation, the MOR value is twice as high when compared to the one observed for kaolin clay samples (afterfiring at 1300°C). This result highlights the potential of the kaolin waste (occupying up to 50 wt% of the formulation) in the manufacture of higher added value products, such as the herein obtained mullite based ceramics. The decrease in mechanical strength with in-creasing the sintering temperature between 1400 and 1550°C can be related to the porosity increase, resulting from air expansion in the closed pores (as a consequence of the lower viscosity of the glass phase), as shown inFig. 7(SEM image).
4. Conclusions
Mullite based ceramics were prepared byfiring a kaolin clay and by reactive sintering of kaolin clay and kaolin waste mixtures at 1300–1500 °C. X-ray diffraction results combined with micro-structure evaluation suggest that single acicular mullite and glass phases were obtained in kaolin waste processed ceramics sintered at 1500–1550 °C. Compared with the samples prepared from pure kaolin clay, the mixed ceramics derived from formulations con-taining kaolin waste showed better physico-mechanical properties when sintered up to 1400°C. A liquid phase assisted sintering mechanism associated with a mica content ranging from 13.3 to 22.4 wt% led to the formation of high quality ceramics with good strength from kaolin waste. The evaluation of high temperature strength properties of mullite based ceramics as a function of the kaolin waste content (varied from 0 to 50 wt%) is a subject of forthcoming work.
Acknowledgements
Hugo P.A. Alves thanks CAPES (Coordenação de Aperfeiçoa-mento de Nível Superior, Brazil) for thefinancial support during his master course.
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