Investigation of the sintering mechanisms of kaolin–muscovite

doi:10.1016/j.clay.2011.01.007

Applied Clay Science

Volume 51, Issue 4, March 2011, Pages 445-451

https://doi.org/10.1016/j.clay.2011.01.007 Get rights and content

Abstract

Kaolin Kga-1b and a muscovite from Bihar (India) and their mixtures were used to investigate the densifying mechanisms during the sintering. The kaolin–muscovite mixtures containing up to 25 mass% muscovite were studied by isothermal and non-isothermal methods such as the step-wise isothermal Dorn method and the constant heating rate developed by Bannister (1968) and Woolfrey and Bannister (1972).

The densification of the reference proceeded through a viscous flow sintering due to the amorphous phases and diffusion at the grain boundaries at high temperatures (> 1200 °C). The activation energy was between 115 kJ/mol and 250 kJ/mol, with a peak value of 650 kJ/mol around 1250 °C due to the formation of mullite. The same trend was observed when 10 mass% muscovite were added. However, the densification started at 1000 °C and the related activation energy was 500 to 700 kJ/mol because of the formation of high-temperature phases (leucite and mullite).

At muscovite contents > 10 mass%, the linear shrinkage was more sensitive to the muscovite dehydroxylaton. The densification was also controlled by grain boundary diffusion and viscous flow up to 1300 °C. Beyond this temperature, a dissolution-limited liquid sintering mechanism proceeded up to the end of sintering. The presence of crystallites, despite a larger amount of the viscous melt, led to a lower densification rate. The activation energy was below 250 kJ/mol.

Research Highlights

► The sintering of pure kaolin is governed by viscous flow and diffusion at grain boundaries. ► Related activation energies are increased from 250 to 600 kJ/mol around 1250 °C. ► Up to 10 mass%, muscovite enables a densification at lower temperature (1000 °C). ► Above 10 mass muscovite, a dissolution-limited liquid sintering prevails from 1300 °C. ► The formation of crystallised phases tends to slow down the densification rate.

Introduction

Most of common structural ceramic materials were produced by sintering of clay-based raw materials, in most cases kaolinitic–illitic clays. The ceramic process involved a sintering step to yield the required properties. It was obvious that these properties were monitored by the thermal transformations of the materials and the kinetics of consolidating reactions during sintering. Many studies (discussed below) had investigated the main interactions occurring in kaolinite–muscovite system, but the mechanisms were not clearly described.

The numerous investigations performed on the sintering behaviour of kaolinite and/or muscovite (including illite and sericite) based materials were conducted in a different approach than that proposed in the present study. Some authors had studied the effect of the amount of each constituent on the ceramic properties of the products (Brindley and Maroney, 1960, Brindley and Udagawa, 1960, Gridi-Bennadji et al., 2009, Khalfaoui et al., 2006, Pilipchatin, 1999, Sedmale and Ya Sedmalis, 1999, Wattanasiriwech et al., 2009, Wyszomirski and Galos, 2010). They showed that by adjusting the content of these clay minerals in the ceramic body, the final properties can be improved. Other authors directed their investigation to the thermal phase transformations and the relevant effect on the final microstructure (Anseau et al., 1981, Aras, 2004, Balkyavichus et al., 2003, Castelein et al., 2001a, Castelein et al., 2001b, Chen et al., 2000, Gualtieri, 2007, Guggenheims et al., 1987, Ivankovic et al., 2003, Kara and Little, 1996, Udagawa et al., 1974). These studies showed that the properties of the starting materials and the heating cycle play a determinant role on the microstructure and phase distribution of the products. In some cases, with increasing the heating rate for example, the thermal transformations within the system follow a non equilibrium path, different from the prediction of standard equilibrium phase diagrams. In most studies, using natural, thus complex mixtures, it was very difficult to get a fundamental description of the governing mechanisms during sintering. This was due to the presence of small amount of various associated species (impurities or not), the variability of crystallinity degree and the different interactions or reactions between the starting constituents. Clay based ceramics (mainly traditional ceramics) are widely used for domestic as well as for industrial purposes. The fundamental understanding of their sintering process may lower the production costs and should help to develop innovative and easily tunable products. Since the phase transformations of most of clay minerals had been already well described in literature, the study of the sintering of model clay mixtures appears as the first step in developing sintering models of the currently complex clay materials used in ceramic industry.

Therefore, the aim of this study was to investigate the densification process of raw clay materials using kaolinite and muscovite instead of raw clays, with more complex and variable compositions.

Section snippets

Experimental

The reference kaolin KGa-1b supplied by the Mineralogical Society and a muscovite from the Bihar region in India labelled MB were used. Their chemical and mineralogical compositions were reported previously (Lecomte et al., 2007) (Table 1). The raw muscovite contained up to 99 mass% muscovite, Kga-1b kaolin 95 mass% kaolinite associated with minor phases such as quartz, hematite and gibbsite. Kga-1b kaolin was used as received, muscovite was ground and sieved to 63 μm. The different mixtures of

Results and discussion

The thermal shrinkage of some typical samples is shown in Fig. 2, Fig. 4, Fig. 6, and the corresponding derivative curves are shown in Fig. 3, Fig. 5, Fig. 7.

For Kga-1b, three main phenomena were observed:

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between 450 and 650 °C, the size change of the sample (shrinkage of 2%) due to the dehydroxylation of kaolinite;
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at 1000 °C, 2% shrinkage due to the reorganisation processes involving the decomposition of metakaolinite; and
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from 1200 °C to 1400 °C, shrinkage of 10% and a reduced shrinkage rate at

Conclusion

The addition of muscovite to kaolin up to 25 mass% had a significant effect on the densification during sintering.

The densification of the reference kaolin Kga-1b, proceeded through a viscous flux sintering, due to the amorphous phase, accompanied by a diffusion mechanism at the grain boundaries. The activation energy varied from 115 kJ/mol to 250 kJ/mol, with a peak value of 650 kJ/mol around 1250 °C due to mullite formation.

When adding muscovite < 10 mass%, the total linear shrinkage was governed by

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Citation Excerpt :
The second endothermic peak takes place at 471 °C corresponding to the second weight loss of TG curve, which is probably caused by the dehydroxylation of kaolinite [33]. The third endothermic peak appears at 860 °C, which is related to the dehydroxylation of muscovite [34]. Compared with Fig. 5a, the TG curve of Fig. 5b shows four weight loss regions, and two new exothermic peaks appear and one endothermic peak disappears in range of 200 to 600 °C as seen from the DSC curve.
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Research paperInvestigation of the sintering mechanisms of kaolin–muscovite

Abstract

Research Highlights

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Applied Clay Science

Journal of the European Ceramic Society

Ceramics International

Ceramics International

Journal of the European Ceramic Society

Acta Metallurgica

Journal of the European Ceramic Society

Journal of the European Ceramic Society

Journal of the European Ceramic Society

Materials Research Bulletin

Applied Clay Science

The separation of sintering mechanisms for clay-based ceramics

Transactions of the Journal of British Ceramic Society

Sinterability of low-melting illite-bearing clays

Glass and Ceramics

Shape sensitivity of initial sintering equations

Journal of the American Ceramic Society

High temperature reactions of clay mineral mixtures and their ceramic properties: II. Reactions of kaolinite–mica–quartz mixtures compared with the K2O–Al2O3–SiO2 equilibrium diagram

Journal of the American Ceramic Society

Research paper
Investigation of the sintering mechanisms of kaolin–muscovite