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The non-isothermal kinetics analysis of the thermal decomposition of kaolinite by Effluent Gas Analysis technique
Powder Technology 203 (2010) 272–276
Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
The non-isothermal kinetics analysis of the thermal decomposition of kaolinite by Effluent Gas Analysis technique Petr Ptáček ⁎, František Šoukal, Tomáš Opravil, Magdaléna Nosková, Jaromír Havlica, Jiří Brandštetr Institute of Materials Chemistry, Faculty of Chemistry, Brno University of Technology Purkyňova 464/118, Brno, CZ-612 00, Czech Republic
a r t i c l e
i n f o
Article history: Received 11 February 2010 Received in revised form 15 April 2010 Accepted 14 May 2010 Available online 21 May 2010 Keywords: Kaolin Kaolinite Non-isothermal dehydroxylation Activation energy Heterogeneous kinetics Effluent Gas Analysis
a b s t r a c t The thermal decomposition of kaolin with high-content of the medium ordered kaolinite was studied by Effluent Gas Analysis (EGA) under non-isothermal conditions. This technique enables to distinguish two overlaying processes during the thermal decomposition of kaolin: oxidation of organic compounds and dehydroxylation. The kinetic of non-isothermal dehydroxylation of kaolinite is controlled by the rate of the third-order reaction. For the given reaction mechanism, the overall activation energy (EA) and preexponential (frequency) factor (A) values are 242 kJ mol−1 and 2.21 × 108 s−1, respectively. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Kaolin is an important raw material for the industrial production. Washed kaolin is mainly used for manufacturing porcelain and sanitary ceramics, whereas fireclay refractory products are fabricated using the crude kaolin. Beyond the ceramics, kaolin is utilized as filling agent for paper, plastics, rubber, cosmetics, etc. Metakaolin, produced by thermal decomposition of kaolin, has found applications in foodprocessing industry, oil shale processing and ceramics [1–4]. Furthermore, kaolin is very promising raw material for waste management [5,6] and preparation of geopolymers and geopolymer based composites [7–11], zeolites [12–14] and intercalates [15–17]. Kaolinite (Al2Si2O5(OH)4) is an essential component of kaolin. However, there will always be certain amount of other phyllosilicates such as smectite (e.g. montmorillonite), mica group minerals (e.g. illite) and tectosilicates (e.g. feldspars). The kind and amount of impurities depend on conditions of aluminosilicates (mainly feldspars) weathering and applied industrial purification (washing process). Furthermore, quartz (β-SiO2), rutile (TiO2), hematite (αFe2O3), ilmenite (Fe2O3∙TiO2), zircon (ZrO2∙SiO2), carbonates (e.g. FeCO3), sulphides (e.g. FeS2) and various kinds of ferrous and alumina hydroxides or oxo-hydroxides (goethite, gibbsite, boehmite…) are the most common accessory minerals of crude kaolins [1,18–20]. Kaolinite as well as other clay minerals can interact with organic
⁎ Corresponding author. Tel.: + 420 541 149 389; fax: + 420 541 149 361. E-mail address: [email protected] (P. Ptáček). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.05.018
compounds, e.g. humic and fulvic acids, consequently there is always certain content of an organic compounds in the kaolin [21,22]. The thermal treatment of kaolinite leads to formation of metakaolinite (Eq. (1)), Al–Si cubic spinel, amorphous silica phase (Eq. (2)) and thermodynamic stable phases and mullite and cristobalite, are formed above 1050 °C (Eqs. (3) and (4)). The whole process can be described by the following reactions [4,23–25]: 450–700 ˚C Al2 O3 ⋅ 2SiO2 ⋅ 2H2 O → Al2 O3 ⋅2SiO2 + 2H2 OðgÞ
ð1Þ
925–1050 ˚C 2ðAl2 O3 ⋅ 2SiO2 Þ → 2Al2 O3 ⋅3SiO2 + SiO2ðamorphousÞ
ð2Þ
≥1050 ˚C 3ð2Al2 O3 ⋅ 3SiO2 Þ → 2ð3Al2 O3 ⋅ 2SiO2 Þ + 5SiO2
ð3Þ
≥1200 ˚C SiO2ðamorphousÞ → SiO2ðcristobaliteÞ
ð4Þ
Simple Eqs. (1)–(4) are unable to describe the process complexity and non-stoichiometric composition of originating phases, predehydroxylation process [26,35], gradual disappearing of residual hydroxyl groups [35] and mullite formation processes (mullitization) [27]. Process is also influenced by many factors, much-frequently degree of disorder of the kaolinite structure [28], pressure and partial water vapour pressure [29,30], heating rate [23,30], mechanical treatments and ultrasound processing of sample [31,32] are mentioned.
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kaolinite guaranteed by producer is higher than 90 wt.% with equivalent grain diameter median in the range 1.2–1.4 µm. The main impurities are mica group minerals and quartz. The colorant oxide content—hematite (α-Fe2O3) and tetragonal TiO2 (rutile), is lower than 0.85 and 0.2 wt.%, respectively. The initial state, properties and chemical composition of applied kaolin is described elsewhere [37]. 2.2. Effluent Gas Analysis
Fig. 1. Effluent Gas Analysis of kaolinite using heating rate 10 °C min−1.
In published literature [28,29,32–36,41,42] an extensive attention is given to the mechanism and kinetics of thermal decomposition of kaolinite and clay mineral in general. The process was investigated by a broad-spectrum of methods, including molecular spectroscopy [28,29], electron microscopy [33] and thermal analysis techniques [32,34–36]. The aim of the present paper is the investigation of two overlapping processes which take place during thermal decomposition of kaolin, i.e. dehydroxylation of kaolinite and combustion of organic compounds. Furthermore, the mechanism of dehydroxilation and pertinent kinetic parameters (overall activation energy and preexponential factor) will be determined on the basis of EGA experiments and compared with TGA results. 2. Experimental procedure 2.1. Kaolin Washed kaolin Sedlec Ia from the region Carlsbad (Czech Republic) produced by the company Sedlecký kaolin a.s. was used for the study of thermal decomposition of kaolinite. This high quality kaolin, originally mined in open-cast mine near Sedlec, is commercially available since 1892 and it is often considered as be world's standard. The content of
The 40 µm undersize of kaolin with specific surface 20 m2 g−1 (Chembet 3000) was used for the EGA experiments. The 20 mg of sample was introduced into alumina cup and arranged in a uniform layer. The crucible was inserted into TG-DTA analyzer Q600 (TA Instruments) connected with measuring cell (TGA/FT-IR Interface, Thermo Scientific) of FT-IR spectrometer Nicolet iS10 (Thermo Scientific) through heated capillary (200 °C). The sample was heated at heating rate 10 °C min−1 under flow of argon (100 cm3 min−1) to 1200 °C. The set of 651 infrared spectra was collected in wavenumber region from 400 to 500 cm−1 for either assessment. The 32 scan spectrum with resolution 16 cm−1 was taken every 11.82 s. The applied setting means, that temperature was increasing for 1.97 °C within the range of sampling interval. 3. Results and discussion The typical Effluent Gas Analysis plot of kaolin is shown in Fig. 1. The carbon dioxide and water vapour were detected in the purging gas during thermal treatment of sample. It stands to reason that both gas producing processes have reached the maximum rate at different temperature, but their temperature intervals are overlapping. The combustion of organic compounds starts at 370 °C and the highest concentration of CO2 was detected at 400 °C. The dehydroxylation takes place approximately at the same temperature and the formation of volatile phase continues over next 270 °C. The maximum concentration of water vapour, i.e. maximum rate of process is located at 500 °C. Furthermore, the mutual influence of both processes was observed. Water vapour, which is evolved during dehydroxylation of kaolinite, slows down combustion of the carbon compounds from the sample and the end of process is shifted to higher temperatures. The carbon dioxide
Fig. 2. TG-DTA results of thermal analyzer gathered together infrared spectrometer. The part of TG curve reconstructed from determined kinetic data is plotted by solid line.
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Fig. 3. Baseline corrected infrared spectrum of gas phase generated during EGA of kaolin at 400 °C, 500 °C and 600 °C.
bands can be recognized in the spectra up to 600 °C. The dehydroxylation process is almost complete at this temperature (Fig. 3). The TG-DTA results of EGA apparatus (Fig. 2) show that burning of organic compounds is also detectable on the derivate of mass loss curve (DTG) like the shoulder of dehydroxylation peak. However, heat effect on DTA curve is overlaid by the endothermic peak of dehydroxylation at 511 °C. The mass loss of sample attains 12.69 wt.%. The second sharp exothermic peak at 986 °C belongs to the formation of cubic spinel phase (Eq. (2)). Including an initial drying process, the mass of the sample was reduced at about 13.30 wt.% to the 1200 °C. Although the burning of organic compounds during thermal decomposition of kaolinite can be detected by DTG, the EGA techniques can easily distinguish the individual running processes via using time dependence of absorbance for some of spectral bands of monitored compounds. Fig. 4 shows the Gram–Schmidt reconstruction expressing the relative spectral response changed over the duration of the EGA experiment. This curve is in many respects analogous with DTG, because all gas products can be in this case detected by IR spectroscopy. Further intensities of antisymmetric νas(Σ+ u ) and symmetric ν2(A1) stretching bands of carbon dioxide and water during analysis are plotted, respectively. The great advantage of EGA is the possibility of evolved CO2 monitoring separately from water vapour, even within temperature interval of dehydroxylation. That offers wide area for further research, because some manufacturing defects in ceramics are derived from overlaying of dehydroxylation and burning of organic impurities or plasticizers of working mixtures. The examples are formation of black cores and undesirable coloration of ceramic body [38] due to changes in the course of redox processes.
Fig. 4. Complete Gram–Schmidt reconstruction and intensity of water and carbon dioxide bands in the time dependence of spectra.
Time dependence of ν2(A1) was used to investigation the dehydroxylation of kaolinite. Integration of the peak enables to express the degree of conversion as: y=
It : I∞
ð5Þ
There It and I∞ are integrated intensities for instantaneous and final time of reaction. Calculated time dependence of y is shown on Fig. 5. The most probable mechanism and kinetic parameters of the process was identified by using logarithmic form of modified Coats and Redfern equation: ln
g ðyÞ AR 2RT E 1 AR E 1 1− − ⋅ ≅ ln − ⋅ ; = ln 2 ΘE E R T ΘE R T T
ð6Þ
where g(y) is an integral form of kinetic function. The straight line over wide interval of y was obtained by plotting ln[g(y) / T2] versus reciprocal temperature (T−1) for proper mathematical model g(y) of the reaction mechanism. The overall activation energy and preexponential factor were calculated from the slope and intercept with y-axis of the plot, respectively. Applied theory of non-isothermal kinetic is described in works [39,40]. The most probable mechanism of the dehydroxylation, e.g. mathematical expression of g(y) function, was evaluated using mathematical models published in works [39,40] including processes handled by the rate of chemical reaction, nucleation, reaction on the phase boundary, diffusion and some other kinetic function with unjustified mechanism. Values of the linear regression coefficient (R2) were calculated for y from 0.25 to 0.90 using kinetic equations for more than different 30 mechanisms. Among all applied kinetic
Fig. 5. Time and temperature dependence of degree of conversion during experiment.
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Table 1 Evaluation of the most probably mechanism of dehydroxylation based on calculation of R2 in the conversion range of 0.25 ≤ y ≤ 0.90. Equation name
One-third order Second order Third order Avrami-Erofeev eq., n = 1 Avrami-Erofeev eq., n = 2 Avrami-Erofeev eq., n = 3 Parabola law Valensi equation Jander equation Ginstling-Brounstein eq. Power law-shrinking disk Power law-shrinking cylinder Power law-shrinking sphere
Symbol of g(y) F1/3 F2 F3 A1 , F 1 A2 A3 D1 D2 D3 D4 R1, F0, P1 R2, F1/2 R3, F2/3
Reaction rate determining step
2/3
Chemical reaction
Random nucleation and subsequent growth of nuclei One-dimensional diffusion Two-dimensional diffusion Three-dimensional diffusion Four-dimensional diffusion Reaction on the phase boundary
functions, Table 1 shows results only for the most probable mechanism (marked by bold) and several common or frequently published kinetic functions. The rate of third-order chemical reaction (F3) was evaluated as the most probably mechanism of the dehydroxylation (Fig. 6). Determined values of overall activation energy and frequency factor are 242 kJ mol−1 and 2.21 × 108 s−1, respectively. Reaction mechanism and values of pertinent kinetic parameters (EA and A) found by the EGA were verified using thermogravimetric data (Fig. 2) collected by thermal analyzer, which is connected with an infrared spectrometer. Both applied methods of thermal analysis show the best fit for process handled by the rate of third-order chemical reaction (Table 1). Corresponding values of EA and A are 239 kJ mol−1 and 8.55 × 107 s−1, respectively. Results imply that the good agreement between both experimental techniques, as well as previous published results of isothermal assessment [37], was achieved. Furthermore, collected information about kinetic of thermal decomposition of kaolinite was used for reconstruction of TG curve. Thermogravimetric curve calculated from the kinetic data is plotted in Fig. 2. The good agreement between experimental and reconstructed TG curve indicates that determined kinetic data satisfactorily describe the course of thermogravimetric experiment within the wide interval of the fractional conversion.
Expression of g(y) function 1 − (1 − y) (1 − y)−1 − 1 (1 − y)−2 − 1 −ln(1 − y) [−ln(1 − y)]1 / 2 [−ln(1 − y)]1 / 3 y2 y + (1 − y)ln(1 − y) [1 − (1 − y)1/3]2 1 − (2y / 3) − (1 − y)2/3 y 1 − (1 − y)1/2 1 – (1 − y)1/3
R2 for TA method EGA
TGA
0.893 0.993 1.000 0.956 0.939 0.912 0.877 0.908 0.940 0.921 0.844 0.912 0.929
0.876 0.989 0.999 0.946 0.925 0.891 0.861 0.895 0.929 0.908 0.823 0.897 0.916
isothermal conditions and results were compared with simultaneous thermogravimetric experiment. Effluent Gas Analysis provides possibility to distinguish two concurrent processes—the burning of organic impurities and dehydroxylation. Formation of water vapour slows down the combustion of organic compounds in kaolin and temperature interval of process is broadened. On the contrary, the influence of burning of organic substances on the course of dehydroxylation is undetectable, due to its low content in the applied kaolin. Dehydroxylation of kaolinite is under applied condition handled by the rate of third-order reaction (F3). The overall activation energy and frequency factor correspond to 242 kJ mol−1 and 2.21 × 108 s−1, respectively. The determined value of EA is placed within the most frequently published interval from 140 to 250 kJ mol−1 [30,34,41,42].
Acknowledgments This paper arose out of the research project supported by the Ministry of Education, Youth and Sports No. 1M06005 and project No. CZ.1.05/2.1.00/01.0012 “Centres for Materials Research at FCH BUT” supported by the operational program Research and Development for Innovations.
References 4. Conclusion The thermal decomposition of industrial treated (washed) kaolin process was investigated by Effluent Gas Analysis under non-
Fig. 6. Graphical determination of the reaction mechanism and kinetic parameters.
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Contents lists available at ScienceDirect
Powder Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p ow t e c
The non-isothermal kinetics analysis of the thermal decomposition of kaolinite by Effluent Gas Analysis technique Petr Ptáček ⁎, František Šoukal, Tomáš Opravil, Magdaléna Nosková, Jaromír Havlica, Jiří Brandštetr Institute of Materials Chemistry, Faculty of Chemistry, Brno University of Technology Purkyňova 464/118, Brno, CZ-612 00, Czech Republic
a r t i c l e
i n f o
Article history: Received 11 February 2010 Received in revised form 15 April 2010 Accepted 14 May 2010 Available online 21 May 2010 Keywords: Kaolin Kaolinite Non-isothermal dehydroxylation Activation energy Heterogeneous kinetics Effluent Gas Analysis
a b s t r a c t The thermal decomposition of kaolin with high-content of the medium ordered kaolinite was studied by Effluent Gas Analysis (EGA) under non-isothermal conditions. This technique enables to distinguish two overlaying processes during the thermal decomposition of kaolin: oxidation of organic compounds and dehydroxylation. The kinetic of non-isothermal dehydroxylation of kaolinite is controlled by the rate of the third-order reaction. For the given reaction mechanism, the overall activation energy (EA) and preexponential (frequency) factor (A) values are 242 kJ mol−1 and 2.21 × 108 s−1, respectively. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Kaolin is an important raw material for the industrial production. Washed kaolin is mainly used for manufacturing porcelain and sanitary ceramics, whereas fireclay refractory products are fabricated using the crude kaolin. Beyond the ceramics, kaolin is utilized as filling agent for paper, plastics, rubber, cosmetics, etc. Metakaolin, produced by thermal decomposition of kaolin, has found applications in foodprocessing industry, oil shale processing and ceramics [1–4]. Furthermore, kaolin is very promising raw material for waste management [5,6] and preparation of geopolymers and geopolymer based composites [7–11], zeolites [12–14] and intercalates [15–17]. Kaolinite (Al2Si2O5(OH)4) is an essential component of kaolin. However, there will always be certain amount of other phyllosilicates such as smectite (e.g. montmorillonite), mica group minerals (e.g. illite) and tectosilicates (e.g. feldspars). The kind and amount of impurities depend on conditions of aluminosilicates (mainly feldspars) weathering and applied industrial purification (washing process). Furthermore, quartz (β-SiO2), rutile (TiO2), hematite (αFe2O3), ilmenite (Fe2O3∙TiO2), zircon (ZrO2∙SiO2), carbonates (e.g. FeCO3), sulphides (e.g. FeS2) and various kinds of ferrous and alumina hydroxides or oxo-hydroxides (goethite, gibbsite, boehmite…) are the most common accessory minerals of crude kaolins [1,18–20]. Kaolinite as well as other clay minerals can interact with organic
⁎ Corresponding author. Tel.: + 420 541 149 389; fax: + 420 541 149 361. E-mail address: [email protected] (P. Ptáček). 0032-5910/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2010.05.018
compounds, e.g. humic and fulvic acids, consequently there is always certain content of an organic compounds in the kaolin [21,22]. The thermal treatment of kaolinite leads to formation of metakaolinite (Eq. (1)), Al–Si cubic spinel, amorphous silica phase (Eq. (2)) and thermodynamic stable phases and mullite and cristobalite, are formed above 1050 °C (Eqs. (3) and (4)). The whole process can be described by the following reactions [4,23–25]: 450–700 ˚C Al2 O3 ⋅ 2SiO2 ⋅ 2H2 O → Al2 O3 ⋅2SiO2 + 2H2 OðgÞ
ð1Þ
925–1050 ˚C 2ðAl2 O3 ⋅ 2SiO2 Þ → 2Al2 O3 ⋅3SiO2 + SiO2ðamorphousÞ
ð2Þ
≥1050 ˚C 3ð2Al2 O3 ⋅ 3SiO2 Þ → 2ð3Al2 O3 ⋅ 2SiO2 Þ + 5SiO2
ð3Þ
≥1200 ˚C SiO2ðamorphousÞ → SiO2ðcristobaliteÞ
ð4Þ
Simple Eqs. (1)–(4) are unable to describe the process complexity and non-stoichiometric composition of originating phases, predehydroxylation process [26,35], gradual disappearing of residual hydroxyl groups [35] and mullite formation processes (mullitization) [27]. Process is also influenced by many factors, much-frequently degree of disorder of the kaolinite structure [28], pressure and partial water vapour pressure [29,30], heating rate [23,30], mechanical treatments and ultrasound processing of sample [31,32] are mentioned.
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kaolinite guaranteed by producer is higher than 90 wt.% with equivalent grain diameter median in the range 1.2–1.4 µm. The main impurities are mica group minerals and quartz. The colorant oxide content—hematite (α-Fe2O3) and tetragonal TiO2 (rutile), is lower than 0.85 and 0.2 wt.%, respectively. The initial state, properties and chemical composition of applied kaolin is described elsewhere [37]. 2.2. Effluent Gas Analysis
Fig. 1. Effluent Gas Analysis of kaolinite using heating rate 10 °C min−1.
In published literature [28,29,32–36,41,42] an extensive attention is given to the mechanism and kinetics of thermal decomposition of kaolinite and clay mineral in general. The process was investigated by a broad-spectrum of methods, including molecular spectroscopy [28,29], electron microscopy [33] and thermal analysis techniques [32,34–36]. The aim of the present paper is the investigation of two overlapping processes which take place during thermal decomposition of kaolin, i.e. dehydroxylation of kaolinite and combustion of organic compounds. Furthermore, the mechanism of dehydroxilation and pertinent kinetic parameters (overall activation energy and preexponential factor) will be determined on the basis of EGA experiments and compared with TGA results. 2. Experimental procedure 2.1. Kaolin Washed kaolin Sedlec Ia from the region Carlsbad (Czech Republic) produced by the company Sedlecký kaolin a.s. was used for the study of thermal decomposition of kaolinite. This high quality kaolin, originally mined in open-cast mine near Sedlec, is commercially available since 1892 and it is often considered as be world's standard. The content of
The 40 µm undersize of kaolin with specific surface 20 m2 g−1 (Chembet 3000) was used for the EGA experiments. The 20 mg of sample was introduced into alumina cup and arranged in a uniform layer. The crucible was inserted into TG-DTA analyzer Q600 (TA Instruments) connected with measuring cell (TGA/FT-IR Interface, Thermo Scientific) of FT-IR spectrometer Nicolet iS10 (Thermo Scientific) through heated capillary (200 °C). The sample was heated at heating rate 10 °C min−1 under flow of argon (100 cm3 min−1) to 1200 °C. The set of 651 infrared spectra was collected in wavenumber region from 400 to 500 cm−1 for either assessment. The 32 scan spectrum with resolution 16 cm−1 was taken every 11.82 s. The applied setting means, that temperature was increasing for 1.97 °C within the range of sampling interval. 3. Results and discussion The typical Effluent Gas Analysis plot of kaolin is shown in Fig. 1. The carbon dioxide and water vapour were detected in the purging gas during thermal treatment of sample. It stands to reason that both gas producing processes have reached the maximum rate at different temperature, but their temperature intervals are overlapping. The combustion of organic compounds starts at 370 °C and the highest concentration of CO2 was detected at 400 °C. The dehydroxylation takes place approximately at the same temperature and the formation of volatile phase continues over next 270 °C. The maximum concentration of water vapour, i.e. maximum rate of process is located at 500 °C. Furthermore, the mutual influence of both processes was observed. Water vapour, which is evolved during dehydroxylation of kaolinite, slows down combustion of the carbon compounds from the sample and the end of process is shifted to higher temperatures. The carbon dioxide
Fig. 2. TG-DTA results of thermal analyzer gathered together infrared spectrometer. The part of TG curve reconstructed from determined kinetic data is plotted by solid line.
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Fig. 3. Baseline corrected infrared spectrum of gas phase generated during EGA of kaolin at 400 °C, 500 °C and 600 °C.
bands can be recognized in the spectra up to 600 °C. The dehydroxylation process is almost complete at this temperature (Fig. 3). The TG-DTA results of EGA apparatus (Fig. 2) show that burning of organic compounds is also detectable on the derivate of mass loss curve (DTG) like the shoulder of dehydroxylation peak. However, heat effect on DTA curve is overlaid by the endothermic peak of dehydroxylation at 511 °C. The mass loss of sample attains 12.69 wt.%. The second sharp exothermic peak at 986 °C belongs to the formation of cubic spinel phase (Eq. (2)). Including an initial drying process, the mass of the sample was reduced at about 13.30 wt.% to the 1200 °C. Although the burning of organic compounds during thermal decomposition of kaolinite can be detected by DTG, the EGA techniques can easily distinguish the individual running processes via using time dependence of absorbance for some of spectral bands of monitored compounds. Fig. 4 shows the Gram–Schmidt reconstruction expressing the relative spectral response changed over the duration of the EGA experiment. This curve is in many respects analogous with DTG, because all gas products can be in this case detected by IR spectroscopy. Further intensities of antisymmetric νas(Σ+ u ) and symmetric ν2(A1) stretching bands of carbon dioxide and water during analysis are plotted, respectively. The great advantage of EGA is the possibility of evolved CO2 monitoring separately from water vapour, even within temperature interval of dehydroxylation. That offers wide area for further research, because some manufacturing defects in ceramics are derived from overlaying of dehydroxylation and burning of organic impurities or plasticizers of working mixtures. The examples are formation of black cores and undesirable coloration of ceramic body [38] due to changes in the course of redox processes.
Fig. 4. Complete Gram–Schmidt reconstruction and intensity of water and carbon dioxide bands in the time dependence of spectra.
Time dependence of ν2(A1) was used to investigation the dehydroxylation of kaolinite. Integration of the peak enables to express the degree of conversion as: y=
It : I∞
ð5Þ
There It and I∞ are integrated intensities for instantaneous and final time of reaction. Calculated time dependence of y is shown on Fig. 5. The most probable mechanism and kinetic parameters of the process was identified by using logarithmic form of modified Coats and Redfern equation: ln
g ðyÞ AR 2RT E 1 AR E 1 1− − ⋅ ≅ ln − ⋅ ; = ln 2 ΘE E R T ΘE R T T
ð6Þ
where g(y) is an integral form of kinetic function. The straight line over wide interval of y was obtained by plotting ln[g(y) / T2] versus reciprocal temperature (T−1) for proper mathematical model g(y) of the reaction mechanism. The overall activation energy and preexponential factor were calculated from the slope and intercept with y-axis of the plot, respectively. Applied theory of non-isothermal kinetic is described in works [39,40]. The most probable mechanism of the dehydroxylation, e.g. mathematical expression of g(y) function, was evaluated using mathematical models published in works [39,40] including processes handled by the rate of chemical reaction, nucleation, reaction on the phase boundary, diffusion and some other kinetic function with unjustified mechanism. Values of the linear regression coefficient (R2) were calculated for y from 0.25 to 0.90 using kinetic equations for more than different 30 mechanisms. Among all applied kinetic
Fig. 5. Time and temperature dependence of degree of conversion during experiment.
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Table 1 Evaluation of the most probably mechanism of dehydroxylation based on calculation of R2 in the conversion range of 0.25 ≤ y ≤ 0.90. Equation name
One-third order Second order Third order Avrami-Erofeev eq., n = 1 Avrami-Erofeev eq., n = 2 Avrami-Erofeev eq., n = 3 Parabola law Valensi equation Jander equation Ginstling-Brounstein eq. Power law-shrinking disk Power law-shrinking cylinder Power law-shrinking sphere
Symbol of g(y) F1/3 F2 F3 A1 , F 1 A2 A3 D1 D2 D3 D4 R1, F0, P1 R2, F1/2 R3, F2/3
Reaction rate determining step
2/3
Chemical reaction
Random nucleation and subsequent growth of nuclei One-dimensional diffusion Two-dimensional diffusion Three-dimensional diffusion Four-dimensional diffusion Reaction on the phase boundary
functions, Table 1 shows results only for the most probable mechanism (marked by bold) and several common or frequently published kinetic functions. The rate of third-order chemical reaction (F3) was evaluated as the most probably mechanism of the dehydroxylation (Fig. 6). Determined values of overall activation energy and frequency factor are 242 kJ mol−1 and 2.21 × 108 s−1, respectively. Reaction mechanism and values of pertinent kinetic parameters (EA and A) found by the EGA were verified using thermogravimetric data (Fig. 2) collected by thermal analyzer, which is connected with an infrared spectrometer. Both applied methods of thermal analysis show the best fit for process handled by the rate of third-order chemical reaction (Table 1). Corresponding values of EA and A are 239 kJ mol−1 and 8.55 × 107 s−1, respectively. Results imply that the good agreement between both experimental techniques, as well as previous published results of isothermal assessment [37], was achieved. Furthermore, collected information about kinetic of thermal decomposition of kaolinite was used for reconstruction of TG curve. Thermogravimetric curve calculated from the kinetic data is plotted in Fig. 2. The good agreement between experimental and reconstructed TG curve indicates that determined kinetic data satisfactorily describe the course of thermogravimetric experiment within the wide interval of the fractional conversion.
Expression of g(y) function 1 − (1 − y) (1 − y)−1 − 1 (1 − y)−2 − 1 −ln(1 − y) [−ln(1 − y)]1 / 2 [−ln(1 − y)]1 / 3 y2 y + (1 − y)ln(1 − y) [1 − (1 − y)1/3]2 1 − (2y / 3) − (1 − y)2/3 y 1 − (1 − y)1/2 1 – (1 − y)1/3
R2 for TA method EGA
TGA
0.893 0.993 1.000 0.956 0.939 0.912 0.877 0.908 0.940 0.921 0.844 0.912 0.929
0.876 0.989 0.999 0.946 0.925 0.891 0.861 0.895 0.929 0.908 0.823 0.897 0.916
isothermal conditions and results were compared with simultaneous thermogravimetric experiment. Effluent Gas Analysis provides possibility to distinguish two concurrent processes—the burning of organic impurities and dehydroxylation. Formation of water vapour slows down the combustion of organic compounds in kaolin and temperature interval of process is broadened. On the contrary, the influence of burning of organic substances on the course of dehydroxylation is undetectable, due to its low content in the applied kaolin. Dehydroxylation of kaolinite is under applied condition handled by the rate of third-order reaction (F3). The overall activation energy and frequency factor correspond to 242 kJ mol−1 and 2.21 × 108 s−1, respectively. The determined value of EA is placed within the most frequently published interval from 140 to 250 kJ mol−1 [30,34,41,42].
Acknowledgments This paper arose out of the research project supported by the Ministry of Education, Youth and Sports No. 1M06005 and project No. CZ.1.05/2.1.00/01.0012 “Centres for Materials Research at FCH BUT” supported by the operational program Research and Development for Innovations.
References 4. Conclusion The thermal decomposition of industrial treated (washed) kaolin process was investigated by Effluent Gas Analysis under non-
Fig. 6. Graphical determination of the reaction mechanism and kinetic parameters.
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