Phase‐Pure Mullite from Kaolinite
TL;DR: In this paper, the influence of carbonate mineralizers at the above temperatures was evaluated using XRD and SEM techniques, and a comparative study of phase formation of the above compositions showed that the sodium-and calcium-fluxed samples give rise to multiphase systems, while K 2 CO 3 incorporated samples give phase-pure mullite.
Abstract: Sintering of kaolinite in the presence of certain carbonate mineralizers, viz., CaCO 3 , Na 2 CO 3 , and K 2 CO 3 , has been conducted at 950°-1350°C. The influence of these mineralizers at the above temperatures is evaluated using XRD and SEM techniques. A comparative study of phase formation of the above compositions shows that the sodium- and calcium-fluxed samples give rise to multiphase systems, while K 2 CO 3 -incorporated samples give phase-pure mullite. The observation that kaolinite in the presence of K 2 CO 3 can act as a precursor material for phase-pure mullite is of great industrial significance.
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TL;DR: X-ray diffraction and thermal analysis data showed that, above this temperature, the ash participates in the sintering process and in the formation of new important phases and the reuse of SCBA in the ceramic industry is feasible.
101 citations
TL;DR: In this paper, a low cost macroporous support for ceramic membranes was prepared by in situ reaction sintering from local natural mineral kaolin with dolomite as an inhibitor.
Abstract: A low cost macroporous support for ceramic membranes was prepared by in situ reaction sintering from local natural mineral kaolin with dolomite as sintering inhibitor. The characterization focused on the phase evolution, microstructure, pore structure, mechanical strength and water permeability at various compositions and sintering temperatures. The sintering of kaolin was improved with 5 wt% dolomite, but clearly inhibited with ≥10 wt% dolomite. For the 20 wt% dolomite samples, the crystalline phases were mainly composed of mullite, cordierite and anorthite after sintering between 1,150 and 1,300 °C. Moreover, both mean pore size and mechanical strength increased with increasing sintering temperature from 1,100 to 1,300 °C, but the water permeability and porosity decreased. The 1,250 °C sintered macroporous support with 20 wt% dolomite exhibited good performances such as porosity 44.6%, mean pore size 4.7 μm, bending strength 47.6 MPa, water permeability 10.76 m3 m−2 h−1 bar−1, as well as good chemical resistance. This work provides opportunities to develop cost-effective ceramic supports with controllable pore size, porosity, and high strength for high performance membranes.
55 citations
TL;DR: In this paper, a new processing route for manufacturing partially interconnected open-cell, microcellular mullite ceramics has been developed, which consists of fabricating a formed body from combining polysiloxane, Al 2 O 3 (a reactive filler), polymer microbeads (used as sacrificial templates), and Y 2 O3 (a sintering additive).
Abstract: A new processing route for manufacturing partially interconnected open-cell, microcellular mullite ceramics has been developed. The strategy adopted for making microcellular mullite ceramics entailed the following steps: (i) fabricating a formed body from combining polysiloxane, Al 2 O 3 (a reactive filler), polymer microbeads (used as sacrificial templates), and Y 2 O 3 (a sintering additive); (ii) cross-linking the polysiloxane in the formed body; (iii) transforming the polysiloxane by pyrolysis into SiO 2 ; and (iv) synthesizing mullite by reacting SiO 2 and Al 2 O 3 . By controlling the sintering temperature and the micro-bead and additive contents, it was possible to adjust the porosity so that it ranged from 38% to 85%. The compressive strengths of the microcellular ceramics with ∼40% and ∼70% porosities were ∼90 and ∼10 MPa, respectively. The superior compressive strengths were attributed to the homogeneous distribution of small (≤ 20 μm), spherical cells with dense struts in the microcellular ceramics.
54 citations
TL;DR: In this article, the effect of crystallization promoters, additions and sintering temperatures on the crystallizing and densifying behavior, microstructures and mechanical properties of glass-ceramics was investigated.
Abstract: Diopside–albite glass–ceramics were fabricated by sintering the powder mixtures of crystallization promoters and waste glass. Two kinds of promoters were synthesized using kaolin clay, talc and chemical reagents. The crystalline phases were formed by a reactive crystallization between promoters and glass during sintering. The effect of promoter components, additions and sintering temperatures on the crystallizing and densifying behavior, microstructures and mechanical properties of glass–ceramics was investigated. The results showed that the higher densities and better mechanical properties were obtained for the glass–ceramics with 12–15% crystallization promoters sintered at 950 °C for 2 h.
51 citations
TL;DR: In this paper, the sintering shrinkage behavior was monitored by dilatometry and the results showed that samples derived from formulations containing kaolin waste were composed of acicular mullite and glass phases at 1500°C.
47 citations
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TL;DR: In this paper, a powder compact was prepared by applying the die-pressing technique and the powder compact showed anisotropic shrinkage after firing, and the size and aspect ratio of the mullite grains increased with the increase of firing temperature.
243 citations
TL;DR: In this paper, the authors investigated the decomposition reaction of kaolinite as a function of the defectivity of the starting material and the temperature of reaction, and the results of the analysis of the kinetic data indicate that the starting reaction mechanism is controlled by diffusion in the particle.
Abstract: The decomposition reaction of kaolinite has been investigated as a function of the defectivity of the starting material and the temperature of reaction. Time resolved energy-dispersive powder diffraction patterns have been measured using synchrotron radiation, both under a constant heating rate (heating rates from 10 to 100° C/min) and in isothermal conditions (in the temperature range 500 to 700° C). The apparent activation energy of the dehydroxylation process is different for kaolinites exhibiting a different degree of stacking fault density. The results of the analysis of the kinetic data indicate that the starting reaction mechanism is controlled by diffusion in the kaolinite particle. The diffusion process is dependent on the defective nature of both kaolinite and metakaolinite. At high temperatures, and at higher heating rates, the reaction mechanism changes and the resistance in the boundary layer outside the crystallites becomes the rate-limiting factor, and nucleation begins within the reacting particle. During the final stage of the dehydroxylation process the reaction is limited by heat or mass transfer, and this might be interpreted by the limited diffusion between the unreacted kaolinite domains and the metakaolinite matrix.
206 citations
TL;DR: In this paper, a follow-up of the investigation on the decomposition reaction of kaolinite as a function of the defectivity of the starting material and the temperature of reaction is presented, showing that the initial reaction mechanism is controlled by mullite nucleation, while as the reaction proceeds it shifts towards a grain growth-limited process which is intermediate between phase boundary and diffusion controlled.
Abstract: The present work is a follow-up of the investigation on the decomposition reaction of kaolinite as a function of the defectivity of the starting material and the temperature of reaction. In the present work we study the high temperature reaction of mullite synthesis from kaolinite, from the starting point of the results obtained in the first part. Time resolved energy-dispersive powder diffraction patterns have been measured using synchrotron radiation in isothermal conditions. The apparent activation energy for mullite nucleation and growth is found to be related to the defective structure of the starting kaolinite, which thus must have an influence on the chemical homogeneity of the amorphous intermediate phase. The analysis of the kinetic data indicate that the initial reaction mechanism is controlled by mullite nucleation, while as the reaction proceeds it shifts towards a grain growth-limited process which is intermediate between phase boundary and diffusion controlled. The order of the reaction obtained from standard analysis of the isothermal kinetic data is lower in the case of the ordered kaolinite KGa-1, in agreement with a rate limiting process more strongly limited by diffusion. For each sample there is a small but significant decrease in the order of the reaction at higher temperature: we interpret the change as related to the variation of the diffusion process in the amorphous phase due to the growing grains of mullite and cristobalite. The values of the activation energies and induction times are comparable neither to a model of mullite formation from a monophasic gel, nor mullite formation from a diphasic gel, being intermediate between the two. We can infer that the amorphous precursors from natural kaolinites can be considered pseudo-monophasic gel-like phases, approaching the monophasic gel-like behaviour as the defectivity of the initial kaolinite increases.
147 citations
TL;DR: In this paper, critical analyses of published reaction studies on kaolinite, and sol-gel and powder mixtures equivalent to 3Al2O3·2SiO2 mullite, were made.
Abstract: Critical analyses of published reaction studies on kaolinite, and sol-gel and powder mixtures equivalent to 3Al2O3·2SiO2 mullite, were made. Supplementary experiments on reactions of kaolinite at different rates of heating were analyzed. Complete atomic homogeneity of precursors results in formation of tetragonal mullite. Colloidal precursors initially form spinel which reacts with the remaining silica by a diffusion-nucleation mechanism to form orthorhombic mullite. Precursors with intermediate atomic homogeneity follow both reaction paths. Kaolinite with a layer lattice structure has two-dimensional atomic homogeneity. Both reaction routes are followed and their degree of interaction is dependent on the crystallinity and impurities of the kaolinite.
143 citations
TL;DR: In this article, the nature of the intermediate spinel-type phase is discussed in relation to the crystal chemistry of spinels, and the X-ray data show variation in the mullite parameters between 1200° and 1400°C; the composition probably approximates 3A12O3·2SiO2.
Abstract: X-ray diffraction data for the high-temperature phases show that a spinel-type structure develops with marked orientation at about 925°C. This phase is considered to be an aluminum silicon spinel with vacant cation sites. Mullite is thought to be formed by the decomposition of the spinel. Silica is eliminated progressively as metakaolin transforms to the spinel phase and thence to mullite. The X-ray data show variation in the mullite parameters between 1200° and 1400°C.; at 1400°C., the composition probably approximates 3A12O3·2SiO2. The nature of the intermediate spinel-type phase is discussed in relation to the crystal chemistry of spinels.
140 citations