Vitrification rate and estimation of the optimum firing conditions of ceramic materials from raw clays: A review

Journal Pre-proof Vitrification rate and estimation of the optimum firing conditions of ceramic materials from raw clays: A review Eduardo Garzón, Luis Pérez-Villarejo, Dolores Eliche-Quesada, Sergio MartínezMartínez, Pedro J. Sánchez-Soto PII: S0272-8842(22)00525-9 DOI: https://doi.org/10.1016/j.ceramint.2022.02.129 Reference: CERI 31761 To appear in: Ceramics International Received Date: 19 November 2021 Revised Date: 28 January 2022 Accepted Date: 12 February 2022 Please cite this article as: E. Garzón, L. Pérez-Villarejo, D. Eliche-Quesada, S. MartínezMartínez, P.J. Sánchez-Soto, Vitrification rate and estimation of the optimum firing conditions of ceramic materials from raw clays: A review, Ceramics International (2022), doi: https://doi.org/10.1016/j.ceramint.2022.02.129. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2022 Published by Elsevier Ltd. 1 Vitrification rate and estimation of the optimum firing conditions of ceramic materials from raw clays: A review Eduardo Garzóna*, Luis Pérez-Villarejob, Dolores Eliche-Quesadac, Sergio MartínezMartínezb,d, Pedro J. Sánchez-Sotod aDepartment of Engineering, University of Almería, 04120-Almería, Spain bDepartment of Chemical, Environmental and Materials Enginering, Higher Polytechnic School of Linares, University of Jaén, 23700-Linares, Jaén, Spain cDepartment of Chemical, Environmental and Materials Enginering, Higher Polytechnic School of Jaén, University of Jaén, 23071-Jaén, Spain dInstitute of Materials Science of Sevilla, Joint Center of CSIC (Spanish National Research Council) and University of Sevilla, 41092-Sevilla, Spain *Corresponding author: E-mail address: [email protected] (Prof. E. Garzón) Journal Pre-proof 2 A B S T R A C T The present work is a review concerning the previous investigations on the vitrification behaviour of clays containing kaolinite, feldspars, muscovite (illite/sericite) and pyrophyllite. These clays are silico-aluminous and have interesting properties as raw materials for structural ceramics. The mineralogical and chemical composition were determined. Then, the vitrification in these clay samples using pressed bodies was investigated by few researchers in the temperature range 800 - 1350 ºC with 0.5 - 5.5 h of soaking times. The effect of heat treatments on the degree of vitrification in these clays was characterized by bulk densities of the ceramic bodies at the fired stage. It was found some variations of bulk density values for all these clays fired in the range 10001150 ºC, with marked decreases of the values obtained at 1200 ºC and 1300 ºC. A first order reaction kinetics was applied to the analysis of vitrification of the ceramic bodies under isothermal heating. The method is based on experimental data of bulk densities, being proposed for the estimation of the relative degree of vitrification resulting from different firing schedules. The analysis considered the temperature dependence of the rate of vitrification following Arrhenius behaviour. Thus, the vitrification activation energy can be obtained. The activation energies for the physical process of vitrification in these clays ranged from 45 to 151 kJ/mol. The relative rates of vitrification or degree of vitrification attained during heating and soaking were calculated. The results suggested that the contribution of vitrification due to heating in all these clays was relatively small compared to the vitrification during soaking. However, it was evidenced that the influence of the particle sizes in the thermal behaviour of these clays cannot be neglected. The vitrification rate equations, as deduced in these previous studies, can be useful tools to estimate the optimum firing conditions of these clays, allowing the extension of this method to other clay types. Keywords: clays, vitrification, activation energy, vitreous phase, ceramics, kaolinite, illite, pyrophyllite Journal Pre-proof 3 1. Introduction Vitrification is considered the result of heat treatment and fusion during which a glassy or non-crystalline phase is produced with a progressive reduction in the porosity [1-3]. The vitrification range is the temperature interval between the temperature at which a ceramic material begins to fuse and the temperature at which the ceramic begins to deform by melting [3]. Vitrification is considered complete when maximum density or zero porosity are achieved without deformation. The knowledge of the vitrification process using common kaolinite clays, as ceramic raw materials, is fundamental in ceramic processing of such as clays according to the great magnitude of consumption by the industry for tile and whiteware manufacture, in particular to achieve the optimum firing conditions. The effect of heat treatment on the degree of vitrification of ceramic bodies, in particular obtained by processing raw clays, can be characterized by important variations in physical properties, such as linear firing shrinkage (LFS, %), water absorption capacity (WAC, %), bulk density (BD, g/cm3), apparent (open) porosity (AP, %) and mechanical strength. There are some characteristic changes in these properties beyond the point of complete vitrification, as pointed out in previous works [1-7]: (1) WAC, AP and LFS increase and bloating effects of the ceramic bodies can be observed; (2) volume shrinkage ceases and swelling begins, as assumed by Bogahawatta and Poole [2], and (3) BD decreases and deformation begins. Norris et al. [1] reported a method for the study of vitreous pottery bodies. These authors proposed the determination of the range curves starting from the raw materials. Then, with this method, it was possible to estimate the temperature of vitrification (Tv) or temperature where porosity becomes almost zero. As proposed by Bogahawatta and Poole [2], the point of complete vitrification was assumed to lie midway between incipient vitrification and fusion where critical changes in physical properties occur, as mentioned above. This was ascertained by the optima of physical properties when BD and volume contraction were at maxima values and AP was minimum. The optimum heat treatment was identified as coincident with the state of optimum properties. The evolution observed in experimental results of AP and BD were the most useful in identifying the optima precisely. Thus, a semiempirical approach to study the vitrification process has been reported [2]. The purpose of this approach was to establish optimum firing conditions for ceramic raw materials based on clays, in particular kaolinitic brick clays, based on kinetic features applied to the reactions Journal Pre-proof 4 leading to the vitrification of these clays under isothermal heating. It is based on experimental observations of BD as a function of firing temperatures. Considering the temperature dependence of the vitrification rate, an Arrhenius analysis is involved in this approach. From these calculations, the vitrification activation energy can be obtained and the optimum firing conditions can be calculated. The optimum firing, according to Bohagawatta and Poole [2], refers to the extent to which a clay body must be fired in order to have an acceptable range of properties, such as strength and durability. These properties improve with the degree of heat treatment. The degree of vitrification, as the criterion for determining the optimum state of firing under isothermal heating, was considered in this previous investigation [2]. Monteiro and Vieira [4] proposed a model of close packed discs (diameter < 2 μm) and nanoscale thickness (< 0.1 μm) to explain a much more efficient the sintering process for clay minerals than the classical spherical particle model [5]. These authors studied the nanoscale solid-state sintering of kaolinite. Other authors, such as Khalfaoui et al. [6] studied the sintering of clay samples. They investigated the sintering mechanism and ceramic phases of an illitic-chloritic raw clay. Freyburg and Schwarz [7] studied structural ceramics showing the influence of the clay type on the pore structure and microstructural development. These authors considered a group of clayey raw materials (51 samples) of different geology. They distinguished four clay types: kaolinitic, illitic-kaolinitic, mixed layer clays containing expandable minerals and carbonate-containing clays. These authors found significant differences of sintering behaviour in these four clay types. Concerning vitrification, Wattanasiriwech et al. [8] investigated the vitrification of an illitic clay (~ 23 wt.% illite, ~ 41 % kaolinite, ~ 30 wt.% quartz) via phase and microstructural changes in the temperature range 800-1250 ºC. The densification should start around 900 ºC, being achieved at 1200 ºC. Illite in this clay sample is considered that acted as superb melting agent in the mixture upon elimination of pores in the matrix. The determination of the vitrification curve of stoneware tiles was studied by Melnick et al. [9] using a dilatometric method, with dwell time 1 h and 1100 ᵒC as the final temperature. The samples were composed by a mixture of 50 mass % of claysiltstone shale and capping siltstone. Kaolinite was predominant (49.8 mass %), with quartz (30.3 mass %), illite (10 mass %) and hematite (7.3 mass %). Lecomte-Nana et al. [10] investigated the sintering mechanisms of kaolin-muscovite mixtures (0-25 mass % muscovite). These authors used reference kaolin (sample KGa-1 from The Clay Journal Pre-proof 5 Mineralogical Society, 96 mass % kaolinite) and a muscovite sample (Bihar, India, 99 mass % muscovite) as raw materials under non-isothermal conditions. They showed that the densification resulted of the same mechanism as the kaolin up to 1300 ºC, and above this temperature up to 1500 ºC, densification was also the consequence of dissolution limited liquid sintering, with an activation energy of less than 250 kJ/mol. Other clay types, such as pyrophyllite and pyrophyllite clays with sericite and kaolinite, have been investigated several years as ceramic raw materials [11-17]. The sintering capacity of pyrophyllite and pyrophyllite raw materials was determined for quartz-pyrophyllite and quartz-sericite-pyrophyllite clays [11]. It has been investigated the influence of mechanical, by dry grinding, and thermal treatments on raw materials containing pyrophyllite and pyrophyllite clays, which contain kaolinite and sericite or illite [14]. Mukhopadhyay et al. [16, 17] investigated for the first time the effect of a raw pyrophyllite (with sericite/muscovite) on vitrification, mullitization and on physical properties of triaxial porcelain. Pyrophyllite was added as a replacement of china clay in a conventional whiteware ceramic body. It resulted in lowering its vitrification temperature. Several authors reported on the application of pyrophyllite clays, for instance in the processing of tubular ceramic for microfiltration membrane [18], diatomite-pyrophyllite porous membranes [19] and porous composites based on alumina-coated pyrophyllite [20]. The sintering temperatures were 1200 ºC, 1300 ºC and 1400 ºC for 1h. Palaeozoic weathered shales named as “Aluminum-clays” have also been investigated [21]. These clays contain kaolinite, muscovite mica (illite/sericite) and pyrophyllite, mixed with the marls traditionally used in Spanish structural ceramic production. The illite/sericite mica is very-fine grained (10-20 μm) and quartz is found in low relative proportion. Traditional ceramics used clays with high-illite content applied for the production of bricks, tiles, plates and stoneware tiles [22, 23]. However, vitrification analysis of such as clays has not been considered. In fact, sericite clays (without or with pyrophyllite) were proposed as new ceramic raw materials [24, 25]. According to their properties, these clays produce very high mullite content at relatively low firing temperatures, a very reactive glassy phase and an exceptional firing range [12, 13, 23-28]. Sericite has also been used to induce textural structures in the preparation of emulsion-templated high-porosity mullite ceramics, as demonstrated by Wang et al. [29]. According to this study, sericite acts both as a template and a sintering additive: it promotes the densification and mullitization of the matrix with formation of Journal Pre-proof 6 mullite crystals in these ceramics. Hence, the use of sericite as a natural flux is interesting because the raw materials with higher sericite content and finer particle size provide very high mullite content at a comparatively low temperature (1000 ºC or lower), with improvements in energy savings [24, 25]. Using the semi-empirical attempt to calculate heat treatment conditions using experimental observations of bulk density, as proposed by Bogahawatta and Poole [2], Faieta-Boada and McColm [3] performed a preliminary analysis of the thermal behaviour of an industrially used kaolinitic clay. Recently, the vitrification behaviour of pyrophyllite clays, containing sericite and kaolinite, has been studied using the same method [30]. It can be noted that all these clays have application as raw materials for structural ceramics. Thus, the aim of the present work is to review these previous investigations concerning the vitrification process of several kaolinitic clays, containing kaolinite, feldspars, muscovite (illite/sericite) and pyrophyllite. It is favoured that the same method was applied to the analysis of vitrification process in these clays. Then, all the results can be summarized, compared and discussed. From this analysis it can be obtained values of vitrification activation energy and equations of heat treatment with time, heating rate and temperature parameters for all these clays. They are established using a scientific method. These equations, as summarized in this work, are interesting because they are useful to calculate the optimum firing conditions when such as clay types are considered. 2. Materials and methods 2.1 Samples studied: mineralogical and chemical characteristics The clays included in this study are described as follows: (I) Two samples of kaolinitic clays of residual origin (Samples A and B), two samples of podzolic clays (Samples C and D) and a latosol (Sample E), as described by Bogahawatta and Poole [2]. According to these authors, these five clays represent the range of brick making clays in common use in Sri Lanka. (II) An industrial Ecuadorian kaolin clay (ore from Ecuador), named as C-1 clay, as described by Faieta-Boada and McColm [3]. This kaolin clay is applied in the manufacture of ceramic whitewares and tiles. Journal Pre-proof 7 (III) A sample of raw pyrophyllite clay (named as “Pizarrilla”, supplied by Alicún Prospecciones S.L.), as studied recently by Sánchez-Soto et al. [30]. This pyrophyllite clay is being studied with respect to its characteristics for applications in structural ceramics containing mullite. Table 1 summarizes the mineralogical (Table 1a) and chemical (Table 1b) data of all these clay samples [2, 3, 30]. Samples A-E are similar in mineralogical composition, with variations in the percentages of kaolinite and feldspars being the percentage of quartz the same [2]. The sample of Ecuadorian clay contains ~ 35 wt.% of kaolinite (disordered kaolinite), with quartz plus cristobalite and feldspars (albiteorthoclase) in similar proportion [3]. Finally, the pyrophyllite clay contains kaolinite, quartz, illite/sericite and pyrophyllite as the main mineral phases deduced by X-ray diffraction [30]. Table 1b includes the RO, R2O and the total flux content calculated from the chemical data for all these clay samples. However, chemical data of Ecuadorian clay sample C-1 were not reported [3]. It was relevant to indicate that, in sample A, if RO is the sum of CaO + MgO and R2O is the sum of Na2O to K2O, the values of these parameters are 2.86 and 0.73 and the total flux content is 21.43 %. The chemical analysis of the pyrophyllite clay was determined in a previous paper [30] using atomic absorption spectrometry taking into account the problems of pyrophyllite determination in mineral samples using acid mixtures [31, 32]. The total flux content for the pyrophyllite clay is 4.18 wt.%, as calculated from the chemical data [30]. From these results, the value found in this sample is the lower as compared to those reported for Samples A-E. The total flux content of all these clay samples ranges from 4.18 to 26.76 wt.%. 2.2 Preparation of samples for firing experiments As described by Bogahawatta and Poole [2], samples A-E were ground to pass a 125 μm sieve. They were dried to a pre-pressing moisture content of ~ 10 wt.%. The clay powders (~ 8 g each) were then isostatically pressed using a hydraulic press at 500 psi (i.e., 3.44 MPa). After oven-drying at 110 ºC/24 h, the pressed samples were fired in an electric furnace in air at temperatures from 1025 to 1100 ºC at a heating rate of 5.6 ºC/min (i.e., 336 ºC/h). The intervals between the different firings was a temperature of 25 ºC. Depending on the characteristics of each clay sample, a range of soaking times at peak temperatures from 0.5 to 5.5 h was selected. After the completion of firing trials, Journal Pre-proof 8 the properties which determine the degree of vitrification of fired specimens, in particular bulk density (in g/cm3), were measured in accordance with British Standards (or American Society for Testing of Materials, ASTM) specifications as mentioned by Bogahawatta and Poole [2]. For the analysis of vitrification behaviour of Ecuadorian clay [3], the clay was ground mechanically for one and a half hours. Samples of the ground clay (with 8 % moisture) were pressed in a hydraulic press at 283 MPa and dried at 110 ºC/24 h. The pressed samples were subjected to a range of heat treatments up to the desired temperature in the range 1025 to 1375 ºC for soaking periods of 1-4 h. The heating rate was 5 ºC/min (i.e., 300 ºC/h). Bulk densities of fired samples were determined following the ASTM method [3]. Finally, pressed samples of powdered pyrophyllite clay [30] were obtained at 150 MPa using dry pressing (~ 30 g of powder moisturized with ~ 5 wt.% of deionized water). First of all, these samples were air-dried (24 h) and secondly dried at 60 ºC/4 h. The dried samples were heated to several temperatures in the range 800-1500 ºC using an electric laboratory furnace in air. However, the heating rate was higher than in the case of precedent samples. It was 8 ºC/min (i.e., 480 ºC/h) with 0.5-5 h of soaking times at each temperature. Then, the samples were cooled under the furnace up to 200 ºC and stored in an oven at 110 ºC. The determination of bulk density (in g/cm3) was performed following the Spanish method proposed for this test, as previously described [24, 26, 33, 34]. 2.3 Vitrification analysis A first order kinetic model reported using kaolinite [35] was applied to the reactions leading to the vitrification of all these clays. The details of the procedure have been described [2]. A summary is as follows. The temperature dependence of the rate of vitrification can be expressed by the Arrhenius equation as: k = A exp (-Ea/RT) (1) where k is the rate of reaction (rate of vitrification in the present study), A is a constant (pre-exponential factor or frequency factor), Ea is the activation energy (vitrification Journal Pre-proof 15 The relative degree of vitrification resulting from soaking can be obtained multiplying the rate of vitrification at the maximum temperature of firing by the soaking time. The procedure for these calculations involves two steps: (a) calculation of the degree of vitrification resulting from heating (Vheating), by integration of the empirical rate equation in each clay sample (within the required limits and constant rate), and (b) calculation of the degree of vitrification resulting from soaking (Vsoaking), using the rate of vitrification at the maximum temperature of firing obtained by the exact rate equation when multiplied by the soaking period according to equation (6). As an example, in the case of sample A, according to [2], the relative degree of vitrification for each stage of firing (vitrification) is calculated from: Vsoaking (sample A) = {e16.22/[e18.214/(t + 273)]}· hs (23) and similarly for samples BE. In the case of pyrophyllite clay [30], it is calculated from: Vsoaking (pyrophyllite clay) = {e4.828/[e5.422/(t + 273)]}· hs (24) In the case of Ecuadorian clay C-1 [3], the relative degree of vitrification for each stage of firing (vitrification) is calculated from: Vsoaking (Ecuadorian clay) = {e13.23/[e17.171/(t + 273)]}· hs (25) Then, the sum of these two partial degrees of vitrification gave the total degree of vitrification or overall degree of vitrification calculated by equation (7). Table 2 includes the results of calculations of degree of vitrification for sample A, as an example [2], pyrophyllite clay [30] and Ecuadorian clay C-1 [3] subjected to various heat treatments. The total relative degree of vitrification is the sum of contribution of heating and soaking. Then, it can be calculated the percentage of contribution of heating to overall vitrification. From these results, it can be deduced that the contribution of vitrification due to the heating was relatively small compared to vitrification during soaking. Furthermore, it can be observed that sample A shows the higher degree of vitrification achieved by the heat treatment at 1050 ºC with a soaking Journal Pre-proof 16 period of 4 h. Then, the ceramic bodies obtained under these conditions had superior physical properties as compared to those subjected to other forms of heat treatments [2]. The highest degree of vitrification is achieved by heat treatment at 1100-1150 ºC with 3-3.5 h for pyrophyllite clay. Finally, the highest degree of vitrification is achieved by the 1325 ºC heat treatment with 2 h for Ecuadorian sample C-1. In this sample, the vitrification is considered that begins at 1025 ºC [3], as mentioned above. The degrees of vitrification calculated using the data for all the clays (samples A-E, pyrophyllite clay and Ecuadorian clay C-1) subjected to experimental firing schedules [2, 3, 30] is summarized in Table 3. The soaking periods at the maximum firing temperatures (Figures 2 and 3) are included. The temperatures of soaking of all these clay samples were in the range 1025-1100 ºC [2], 1050-1150 ºC [30] and 12251325 C [3]. It is remarked that, according to the investigation of Bogahawatta and Poole [2], the most favourable heat treatment for attaining the highest degree of vitrification and, therefore, the maximum mechanical strength for the sample clays B, C and E is accomplished at 1075 ºC. For sample clays A and D a heat treatment at 1050 ºC for 4 h seems adequate. The sample B attained near optima properties at 1025 ºC and, hence, firing above 1025 ºC is unproductive. Bogahawatta and Poole [2] indicated that since the contribution of heating to the overall degree of vitrification is small at all temperatures of firing (Table 2), the order or error involved in the assessment of the total degree of vitrification is relatively insignificant. The same consideration can be plausible for the other clays. It should be noted that, according to Faieta-Boada and McColm [3], there is a very large experimental effort to achieve data for a vitrification equation, being confirmed the usefulness of the method firstly proposed [2]. These authors concluded that the proposed equations allow the most favourable heat treatment regime to be determined for each clay to optimize vitrification. The equations may also be used to determine any two of the three variables: (a) maximum firing temperature, (b) soaking period and (c) rate of heating. Then, it can be achieved a required degree of vitrification and, hence, mechanical strength. For a more complete discussion, and as source of criticism, a factor of influence must be considered. It was not studied in previous investigations [2, 3]. It is the particle size, being reported only several data in the case of pyrophyllite clay sample [30]. For this sample, the values reported indicated 62 wt.% of 50 - 2 μm fraction and 21.8 wt.% of < 2 μm fraction, with the complete description of distribution of particle sizes. For Journal Pre-proof 17 the other samples, only a few indications were mentioned: (a) In reference [2], the clay samples were ground to pass a 125 m sieve; (b) in reference [3], the clay sample was ground mechanically for one and half hours. However, the influence of this factor in the vitrification behaviour cannot be neglected and further studies will be necessary. On the other hand, taking into account the mineralogical results of all these clay samples (Table 1a), there appears to be no direct relation between their mineralogical composition and their firing behaviour. However, it is clear that vitrification is produced by the increasing amount of liquid phase, which will produce the glassy or vitreous phase after cooling the ceramic bodies. The vitreous phase produced in these clays is liquid at high temperatures [6, 27, 28, 36-38] and almost all the oxides distinct of silica and alumina (Table 1b) are forming this phase. In the case of clay-based ceramics, such as triaxial whiteware bodies, the feldspars melt by progressive heating and dissolves fine quartz grains, increasing the liquid’s Si content and, hence, its viscosity [38]. The presence of fluxes (see Table 1) influences the thermal behaviour, producing a vitreous or glassy phase, which is liquid from 985 ºC (eutectic) [36, 37]. Thus, the incipient vitrification at 850 ºC is a good approach in the method proposed by Bogahawatta and Poole [2]. Furthermore, the predictions deduced from the K2O-Al2O3-SiO2 ternary phase diagram [24, 36-39] are, in general, in agreement with the observed thermal behaviour of these clays because they are silico-aluminous. It was already discussed in previous investigations on thermal behaviour of sericite clays as precursors of mullite materials [27] and studies on the effect of vitreous phase on mullite and mullite-based composites obtained from sericite clays and kaolinite [28]. A greater amount of liquid by heating, i.e. glassy or vitreous phase, will be produced if several components, such as Na2O, TiO2, Fe2O3, CaO and MgO, considered as RO and R2O, are also present besides SiO2 and Al2O3 as showed in Table 1b. In the other clay samples, except pyrophyllite clay [30], the contents of SiO2 and Al2O3 were not reported [2, 3]. Thus, in the present study there is a total flux content in the range 4.18 – 21.43 wt.%, being higher for sample A and the lowest for the pyrophyllite clay sample. Table 4 summarizes the results of activation energies for the physical process of vitrification in these clays and the total flux content (data form Table 1b). Vitrification activation energies for all these clay samples range from 45 to 151 kJ/mol. The value reported for the Ecuadorian clay C-1 (143 kJ/mol) is in the range of 74 – 151 kJ/mol reported by sample clays A-E [2]. However, the lower value of Ea (45 kJ/mol) was determined for the isothermal data of pyrophyllite clay sample [30], being the sample Journal Pre-proof 18 with the lowest value of total flux content (4.18 wt.%, Table 1b). It can be remarked that there is not any relation between Ea and the total flux content. As pointed out by FaietaBoada and McColm [3]: “the activation energy of vitrification may bear no relation to any specific kinetic chemical process. It is in fact a kinetic physical process and is related to capillary and liquid phases present, liquid viscosities and their changes, etc.”. In fact, these authors suggested, by comparison, that the controlling mechanism of vitrification is simply one of viscous flow [2]. Finally, as pointed out by the previous studies on this subject [2, 3], the vitrification rate equations, better as semi-empirical firing equations, can be useful tools for clay manufacturers to calculate the optimum firing cycles. 4. Summary and conclusions Clays containing kaolinite, feldspars, muscovite (illite or sericite) and pyrophyllite have interesting properties as raw materials for structural ceramics. The present work reviewed the previous investigations on the vitrification of such as clays, considering two kaolinitic clays of residual origin, two podzolic clays, one latosol, one industrial kaolinitic clay, besides a clay containing kaolinite, muscovite (illite or sericite) and pyrophyllite [2, 3, 30]. The mineralogical and chemical composition of these clays were summarized (Table 1). Thus, the kaolinitic nature of all these clays was evidenced and their silico-aluminous characteristics. The total flux content of these samples, using the chemical data, ranges from 4.18 to 26.76 wt.%. The vitrification process of all these clay samples, as a first approximation, was studied using a method previously described in the literature for kaolinitic clay samples [2]. The method considered an Arrhenius approach under isothermal conditions, based on the temperature dependence of the vitrification rate, and assuming a first order kinetic, as studied using pure kaolinite [35]. It was a semiempirical attempt to analyse vitrification in kaolinitic clays. The method started on experimental data of bulk densities obtained by firing of the clays under isothermal heating. The vitrification in the clay samples was investigated in the range 800 - 1350 ºC with 0.5 - 5.5 h of soaking times. It was found variations of bulk density values for all these clays fired in the range 1000-1350 ºC, with marked decreases of the values for the samples fired at 1200 ºC and 1300 ºC. Thus, the activation energies for the physical process of vitrification in these clays or vitrification activation energy (Ea) can be obtained assuming first order kinetic. Journal Pre-proof 19 The values obtained of Ea range from 45 to 151 kJ/mol with differences in the whole of these clays. The lower value (45 kJ/mol) can be associated to the lower value of total flux content (4.18 wt.%), being obtained for the pyrophyllite clay sample with ~ 15 % kaolinite and ~ 35 % of pyrophyllite [30]. The vitrification in these clay samples follows an Arrhenius behaviour suggesting, by comparison, a viscous flow as the controlling mechanism as proposed in a previous investigation on a kaolinitic clay sample [3]. The relative rates of vitrification or degree of vitrification attained during heating and soaking period were calculated using the theoretical equations or, alternatively, a simplified empirical rate equation. Thus, it was determined the overall degree of vitrification attained during heat treatment of all these clay samples according to a fixed firing schedule. The results allow conclude that the contribution of vitrification due to heating in all these clays was relatively small compared to the vitrification during soaking. Then, it is deduced from these investigations along time (1991, 1993 and 2021) a better knowledge of the characteristics of vitrification of kaolinitic and pyrophyllitic clays applied as ceramic raw materials. It can be concluded the utility of this scientific methodology to look for the optimum firing conditions. Thus, the vitrification rate equations, as deduced in these previous studies [2, 3, 30] and summarized in this paper, can be useful tools to estimate the optimum firing conditions of these kind of kaolinitic and pyrophyllitic clays applied as ceramic raw materials. It is important to remark the interest of this method, allowing its extension to other clay types applied in the ceramic industry, such as chloritickaolinitic clays and smectite-kaolinite clays. These will be matter for future studies. Acknowledgements This work was supported by Junta de Andalucía through Research Groups TEP 204 and AGR107. The authors acknowledge helpful discussions on kinetic models with Dr. F.J. Gotor, Research Scientists of CSIC. References [1] A.W. Norris, D. Taylor, I. Thorpe, Range curves: an experimental method for the study of vitreous pottery bodies, Br. Ceram. Trans. J. 78 (1979) 102-108. 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Soc. 28 (2008) 465-471. [39] K.C. Liu, G. Thomas, A. Caballero, J.S. Moya, S. de Aza, Time-temperaturetransformation curves for kaolinite-α-alumina, J. Am. Ceram. Soc. 77 (1994) 15451552. Journal Pre-proof 23 Figure captions Figure 1.- Schematic representation of the reaction rate – time of firing dependence for the clay samples A - E, as described by Bogahawatta and Poole [2]. Adapted from reference [2]. Figure 2.- Variation of bulk density with soaking period (0.5 – 5.5 h at different temperatures (1025 – 1100 ºC) for the clay samples A, B, C, D and E (adapted from reference [2]). Figure 3.- Variation of bulk density with soaking period (0.5 - 5 h) at different temperatures (1000 - 1300 ºC) for the pyrophyllite clay sample (adapted from reference [30]). The values of soaking period at the maximum value of bulk density, t (máx), obtained at each temperature following the model proposed in reference [2] are indicated in the insert. Figure 4.- Plot of lnt versus the reciprocal of firing temperature for isothermal heating of the clay samples A – E following the isothermal kinetic model (adapted from reference [2]). The correlation coefficients of the straight lines are in the range 0.9840.997. Figure 5.- Plot of lnt versus the reciprocal of firing temperature for isothermal heating of the pyrophyllite clay sample following the isothermal kinetic model (adapted from reference [30]). The correlation coefficient of the straight line is 0.9980. Figure 6.- Plot of lnt versus the reciprocal of firing temperature for isothermal heating of the Ecuadorian clay sample C-1 following the isothermal kinetic model (adapted from reference [3], but the correlation coefficient of the straight line was not provided). Journal Pre-proof Table 1 (a). Mineralogical data of the clay samples: K = Kaolinite; Q = Quartz; F = Feldspars; I = Illite; Py = Pyrophyllite. Sample % K % Q % F % I % Py A* >30 >30 15-30 - - B* >30 >30 15-30 - - C* 15-30 >30 5-15 - - D* 15-30 >30 5-15 - - E* 5-15 >30 5-15 - - Ecuadorian Clay C-1** 30-35 30-35+ 30-35x - - Pyrophyllite clay*** 15 20 <5 25 35 Table 1 (b). Chemical data of the clay samples: Total Flux Content = RO + R2O + Fe2O3 + TiO2; RO = CaO + MgO; R2O = K2O + Na2O; NR = Not Reported. Sample Total Flux Content (%) RO (%) R2O (%) A* 21.43 2.86 0.73 B* 20.17 1.72 0.40 C* 16.31 1.25 1.58 D* 15.37 0.96 1.49 E* 16.45 0.91 1.23 Ecuadorian Clay C-1** NR§ NR NR Pyrophyllite clay*** 4.18 0.24 1.75 *Bogahawatta and Poole [2] ** Faieta-Boada and McColm [3] + Quartz and Cristobalite, according to [3]. x Albite-Orthoclase Feldspar, according to [3]. *** Sánchez-Soto et al. [30], using the chemical data of calcined sample (in wt. %) with SiO2 = 63.17; Al2O3 = 32.45; Fe2O3 = 0.68; TiO2 = 1.51; MgO = 0.18; CaO = 0.06; Na2O = 0.25; K2O = 1.50, then RO = CaO + MgO = 0.06 + 0.18 = 0.24, R2O = Na2O + K2O = 0.25 + 1.50 = 1.75, and Total Flux Content = 4.18. § Taking into account the mineralogical content of samples A and B, which are very similar to the Ecuadorian Clay C-1, the Total Flux Content for this clay sample could be estimated in the range 21.43-20.17 %. Journal Pre-proof Figure 2.- Variation of bulk density with soaking period (0.5 – 5.5 h at different temperatures (1025 – 1100 ºC) for the clay samples A, B, C, D and E (adapted from reference [2]). Journal Pre-proof Figure 3.- Variation of bulk density with soaking period (0.5 - 5 h) at different temperatures (1000 - 1300 ºC) for the pyrophyllite clay sample (adapted from reference [30]). The values of soaking period at the maximum value of bulk density, t (máx), obtained at each temperature following the model proposed in reference [2] are indicated in the insert. Journal Pre-proof Figure 4.- Plot of lnt versus the reciprocal of firing temperature for isothermal heating of the clay samples A – E following the isothermal kinetic model (adapted from reference [2]). The correlation coefficients of the straight lines are in the range 0.984-0.997. Journal Pre-proof Figure 5.- Plot of lnt versus the reciprocal of firing temperature for isothermal heating of the pyrophyllite clay sample following the isothermal kinetic model (adapted from reference [30]). The correlation coefficient of the straight line is 0.9980. Journal Pre-proof Figure 6.- Plot of lnt versus the reciprocal of firing temperature for isothermal heating of the Ecuadorian clay sample C-1 following the isothermal kinetic model (adapted from reference [3], but the correlation coefficient of the straight line was not provided). Journal Pre-proof Declaration of competing interest The authors declare that they have no known competing financial interests or personal realtionships that could have appeared to influence the work reported in this paper Journal Pre-proof