Physicochemical characterization and thermal behaviour of two Tunisians kaolins Текст научной статьи по специальности «Химические науки»

M. Chelly, E. Srasra

PHYSICOCHEMICAL CHARACTERIZATION AND THERMAL BEHAVIOUR OF TWO TUNISIANS KAOLINS

Unité Matériaux, Technopole Borj Cedria, B.P. 95-2050 Hammam-Lif, Tunisia

mohamed [email protected]

1. Introduction

Ceramic product depends on the nature and the properties of utilised raw material. The increase in temperature for singly fired ceramic clays exceed the energy threshold of the reactivity of mineral material and produces a series of reaction and transformations that lead to the formation of new phases and the disappearance of others [1]. For the some firing conditions the halloysite clay produced a higher bulk density than the kaolinite clay as well as finer mullite crystals. Mullite is formed in kaolinite clay at a lower temperature than in halloysite clay [2]. The kaolinite forms metakaolinite by heating from 550°C to 950°C, and mullite mainly forms above 1200°C [3].

The objective of this paper is to study the thermal transformation of two Tunisians kaolins chooses.

2. Materials and methods

Two samples are selected for this study: Kasserine clay (K) and Tabarka clay (T). We add a suffix "b" or "p" to indicate respectively if the clay is crude or purified.

Purification:

The clay fraction was purified by classical methods [4], repeated cation exchange with 1N NaCl solution, washing and sedimentation and dialysis. The fine clean sediment was freeze dried.

Chemical analysis:

The samples are attacked by a mixture of three acids (HCl, H2SO4 and HNO3); all elements pass in solution only the silica that it measured out by gravimetric [5].

The chemical elements, as Al, Fe, Mg, Ca, Na and K are measured by atomic absorption.

X-ray diffraction:

Mineralogical analysis was performed by XRD, using a Philips diffractometer PW 1050/81, with the Ka12 radiation of the copper. In this method, oriented specimens are prepared on glass microscope slides. The slides have a density of 20 mg of samples and were successively given treatment of:

i) Drying at 20°C

ii) Heating for two hours an 500°C

Infrared spectroscopy:

FT-IR spectra were recorded in the region 4000-350 cm-1 in an EQUINOX model 55 infrared Fourier transform spectrometer, using the KBr pellet technique (about 2 mg of sample and 200 mg of KBr were used in the preparation of the pellets).

Cation exchange capacity:

Clay is saturated first with other solutions. The excess of the reagent added is measured by volumetric method.

N2 adsorption experiments:

The specific surface areas of the samples were determined by the corresponding nitrogen adsorption isotherms at 77 K, obtained from Micrometric Autosorb 2050 Analyser, after out degassing the samples at 120°C, with the residual pressure of 10-5 mmHg. The BET method was used for the corresponding calculations. Total pore volume (Vt) was determined at P/P0=0.98.

3. Results and discussion

3.1 Characterization of the two clays:

The centesimal compositions of raw and purified samples are given in Table 1. The following preliminary points may be noticed:

a) The loss on ignition of the Kasserine clay is very important (33.20 %); it can be due to the presence of a supplementary water layer.

b) The (SiO2/Al2O3) ratio is close to 1 for all samples. Therefore we can deduct that the two minerals are of types 1/1.

© Chelly M., Srasra E., Электронная обработка материалов, 2009, № 1, С. 54-64.

c) The (SiO2/Al2O3) ratio of the Tabarka clay (1.15) is very close to the theoretical kaolinite 1.18 [6].

d) The amounts of SiO2 (38.77 %) and Al2O3 (37.20 %) of purified Tabarka clay are nearly equal to those given by the literature for a kaolinite.

Table ^ 1. Chemical analysis^ of the ^ samples

SiO2 AI2O3 Fe2O3 CaO MgO K2O Na2O ZnO PF

Kb 35,12 28,90 0,19 0,63 0,28 1,29 0,15 0,24 33,20

Kp 45,95 28,52 0,17 0,08 0,12 0,08 0,14 0,25 24,69

Tb 44,36 38,77 0,50 0,33 0,08 0,11 0,02 0,17 15,66

Tp 46,03 37,20 0,46 0,36 0,08 0,05 0,04 0,19 15,59

The XRD pattern of the Tabarka clay (fig.1) shows sharp peaks at d = 7.15, 4.45, 4.35, 4.17, 3.84 and 3.57 A due to well crystallized kaolinite [7].

The XRD pattern of the Kasserine clay (fig.2) shows sharp peaks at d = 10, 4.48, 3.34 and 2.58 A due to halloysite 4H2O [8].

Fig. 1. XRD pattern of crude Tabarka clay

10 20 2 Theta 30

Fig. 2. XRD pattern of crude Kasserine clay

The diffractograms (fig.3) show that these two samples are respectively of kaolinite and halloysite that will be destroyed at 500°C. The confirmation of our results with those gotten by other authors [9] is returned in the following Table.

Table 2. Identification of samples

Minerals Normal Glass Glass heated at 500°C

Kaolinite 7,29 À Détruite

Halloysite 10 À Détruite

& ri

dt ïjkàM

at 20 °C

T at 5G0 °C

11 19 27

< o

< ^ Tt ei

K at 20 °C J I Tt ci

K at 500 °C runty |||)>

3 11 19 27

Fig. 3. XRD of oriented films ofpurified samples and heated at 500°C

The exam of IR spectra shows that (fig. 4, 5):

a) IR spectra of raw and purified samples are identical; so no foulness associated to the two clays. Transmission, % Transmission, %

120

T AM Bute

TAMPure

& S m m

«№HoT fin' C

CC I - 'C 'C = en I - --C

260

220

ISO

140

3500 2500 1500 500

Wave number, cm1

3500 2500 1500 500

Wave number, cm-1

Fig. 4. IR spectra of raw and purified Tabarka clay Fig. 5. IR spectra of raw and purified Kasserine clay

b) In the region between 3500 and 4000 cm-1, the IR spectrum of the Tabarka clay (fig.4) shows thins and intense bands whereas the bands of the kasserine clay (fig.5) are larges and less intense. It proves that Tabarka clay is better crystallized than Kasserine clay.

c) Referring on results of IR spectroscopic analysis for a kaolinite, given by Vander Marel and Ben-tels Pacher in 1976, we deduce that Tabarka clay is a kaolinite well-crystallized (Table 3).

3695, 3660 and 3620 Al-OH of valence

940 and 913 Al-OH of deformation

1032 Si-O-Si of valence

1005 Si-O-Al of valence

695 and 539 Si-O-Al of deformation

d) Fig.5 shows bands at 3698, 3625 1036 and 912 cm-1 characterizes the clayey mineral of kaolin family [10]. The adsorption massif at 3468 cm-1 and the band of deformation at 1640 cm-1 (vibrations OH of the hydration water clay), show that the kasserine clay is a halloysite.

Fig. 6 presents the DTA curves of the studied samples. The main changes pointed out by TG and DTA, during the heating of the samples are the following:

a) Lower to 200°C: a first endothermic pick that can be correlated with a pre-dehydration process, which takes place as a results of the reorganization in the octahedral layer, first occurring at the OH of the surface [11]. This explains the importance of the first loss of the Kasserine sample (Table 4) in relation to the one of the Tabarka sample.

b) 500-600°C: a second endothermic pick due to dehydroxylation of the clay. During this temperature kaolinite forms metakaolinite by dehydroxylation [12].

c) 976-1000°C: an exothermic pick that doesn't correspond to a weight loss but it indicates the transition of phase to high temperature of the amorphous product, due to the decomposition of the kaolinite. In well-crystallized samples the decomposition of kaolinite starts at higher temperature [13]. So the observation of these two picks of our samples permits us to conclude that Tabarka sample is better crystallized than Kasserine sample.

d) The calculation of losses in mass (Table 4) of kasserine clay and Tabarka clay shows that it is respectively a halloysite and a kaolinite [14].

533 °C

Fig. 6. DTA curves of crude and purified samples

Samples 1ist loss (%) 2sd loss (%)

T < 200°C 400 < T< 600°C

Kb 3,89 5,04

Kp 1,44 6,62

Tb 0,50 12,77

T 1,32 13,78

Weight Losses (Ross and Kerr, 1931)

Kaolinite 1 - 2 % 13 %

Halloysite 2 - 4 % 6 - 14 %

Fig.7. presents the isotherms of adsorption-desorption of Kasserine clay and Tabarka clay.

The raw sample of kasserine has an important power of adsorption compared to the purified sample.

The adsorption - desorption isotherms of Tabarka clay are confounded. This clay is not porous characteristic of kaolinite. Whereas the isotherms of desorption of the Kasserine clay present hysteretic buckles what indicates that this clay is porous.

The Kasserine clay presents the best power of adsorption of the two clays.

Specific surface values, determined from the isotherms of adsorption, of the two samples are represented in the following Table:

Table 5. Specific surface of samples

Samples Specific surface ( m2/g)

Kb 233,96

kp 128,27

Tb 24,57

T p 24,60

The specific surface is not sufficient for the characterization of clayey mineral. The specific surface of kaolinite is lower to 25 m2/g [15]. In our case the specific surface value of Tabarka clay (Table 5) its 24.5 m2/g corresponds to kaolinite.

The values of C.E.C of the raw and purified clays are nearly identical (Table 6), what confirm that these two samples don't contain any foulness.

According to the literature values [16] of the C.E.C we noted that:

a) The C.E.C of the Kasserine sample (40.11 meq/100g of clay charred), corresponds to Halloysite

4H2O.

b) The value of the C.E.C of the Tabarka sample (3,15 meq/100gs of heated clay), corresponds to kaolinite.

samples C.E.C ( meq/100g of heated clay)

Kb 40,11

Kp 39,84

Tb 3,15

T 3,15

C.E.C of some clayey minerals ( Kelley, 1948)

Kaolinite 3 - 15

Halloysite 2H2O 5 - 10

Halloysite 4H2O 40 - 50

3.2 Study of the thermal behaviour of the kaolinite of Tabarka and the halloysite of Kasserine: The thermal analysis is realized by heating samples to different temperatures (100, 300, 500, 700, 1000 and 1200°C), during one hour. The heated samples are analyzed by different techniques (XRD, IR and specific surface analysis).

3.2.1 Study of the thermal Behavior by XRD :

Fig. 8, 9 present respectively the XRD patterns to different temperatures of the Tabarka and Kasserine samples.

_A___

............^^^^

1000°c

II I" '^""T"1*^"H-.!^——....... i I » ■ !■ I.

Fig. 8. XRD patterns of Tabarka samples to different temperatures

The XRD patterns at 100°C (fig. 9), shows a sharp peaks at d= 7.41, 4.41 and 3.59 A characteristic of kaolinite, what explains the departure of water molecules of the halloysite. The kaolinite is destroyed to 550°C due to the formation of an amorphous dehydration product identified as metakaolinite after an endo-thermic reaction. At 1000°C, the structure of the two samples is completely changed what interpreted by the formation of crystalline product from the next amorphous intermediate phase results in a prominent exothermic reaction at 980°C [17]. The two difractograms obtained at 1200°C, show a sharp peaks at d=5.41, 3.41, 2.89 and 2,70 A relatives to a new phase that is the mullite.

The diffractogram of Tunisian mullite (fig. 10) gotten by thermal treatment of the Tabarka clay is simulated to XRD pattern of a pure mullite (fig. 11) but picks are less resolute that can be due to a bad crystallization.

Fig. 9. XRD patterns of Kasserine samples to different temperatures

Fig. 10. XRD pattern of Tabarka clay heated at 1200°C and the diffractogram of pure mullite (M. F. de Souza and J. Yamamoto, 1999)[18]

3.2.2 Study of the thermal Behavior by IR:

Fig.11 and 12 present respectively the IR spectera of Kassserine and Tabarka samples, obtained after thermal treatment. After the thermal treatment of samples, the transformation of kaolinite to metakaolinite is confirmed by the absence of detectable Al-O-H bands at 913 cm-1. The reduction of 539 and 913 cm-1 and the appearance of a new band at 786 cm-1 can be connected with the change from octahedral coordination of Al3+ in kaolinite to tetrahedral coordination in metakaolinite [19]. The characteristic bands of kaolinite have been disappeared after the thermal treatment over 500°C. a new band at 2850-3500 cm-1 is observed in the sample treated at 700°C and is reduced as the treatment temperature is increased.

The specters of the two clay samples, heated to 1200 °C, show the apparition of a new phase are similar to the mullite spectera (fig. 13) observed using FTIR. This mullite is prepared from sol-gel process by the synthesis of the pure aluminosiloxane [20].

Fig. 12. IR of Tabarka samples after thermal treatment Transmittance (arbitrary linear scale)

Wavenumbers, cm-1

Fig. 13. Chemical homogeneity in mullite prepared from polymeric precursors can be observed using FTIR (Treadwell et al., 1996)

3.2.3 Study of the thermal Behavior by specific surface:

Fig. 14 presents the adsorption - desorption isotherms of kasserine samples after thermal treatment, show a progressive diminution of the porosity until completely disappears at 1200°C.

□ 0,2 0,4 0,fi 0,8 1,0 1,2

Fig. 14. Adsorption-desorption isotherms of Kasserine samples after thermal treatment 180-

table:

Fig. 15. Adsorption-desorption isotherms of Tabarka samples after thermal treatment The specific surfaces of two clays obtained after thermal treatment are represented in the following

Table 7. Specific surface of samples after thermal treatment^

Temperature (°C) 20 100 300 500 700 1000 1200

Specific surface ( m2/g) K 233,96 178,80 178,12 149,83 130,01 12,29 0,61

T 24,57 22,77 23,92 22,15 21,74 15,19 5,92

The specific surface of Kasserine clay (233.96 m2/g to 20°C) remains very important until the temperature of 700°C, what reveals some catalytic applications to high temperature.

Specific surface, m2/g

30 h

20 10-

0 200 600 1000 1400 T emp erature "C

Fig. 16. Evolution of the specific surface of Tabarka clay according to the temperature

The specific surface of Tabarka clay according to the temperature (fig.16) remains constant until 700°C. One will note the extreme stability of the total surface developed by this clay during the phase of the dehydroxylation. Therefore the dehydroxylation doesn't entail any modifications important of the texture of the kaolinite.

The representation of the evolution of the specific surface of Kasserine clay, according to the temperature (fig.17) show that:

a) A slightly decreases of the surface in the begin of the dehydroxylation.

b) The surface remains constant in the second stage of the deshydroxylation (100°C to 700°C). One will note the extreme stability of the total surface developed by kasserine sample during this phase.

c) An abrupt diminution at 700°C that is interpreted by a process of sintering.

25Q Specific surface, m2/g

150-

50-

0 200 600 1000 1400

T emp erature "C

Fig. 17. Evolution of the specific surface of Kasserine clay according to the temperature 4. Conclusion

The selected clays differ by their mineralogical properties. The Tabarka is a well-crystallized kaolinite. Whereas the Kasserine clay is a hydrated halloysite characterized by a important specific surface (233.96 m2/g). The evolution of specific surface according to the temperature of the Kasserine samples after thermal treatment shows an abrupt diminution at 700°C that are interpreted by a process of sintering. The thermal treatment of the two clays at 1200°C, shows the apparition of a new phase that is the mullite. This mullite is identical to mullite gotten by the process sol-gel from the synthesis of the pure aluminosiloxane.

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Received 25.08.08

Summary

The object of this work is to study the physicochemical proprieties and the thermal behaviour of two Tunisians kaolins. The two samples selected from Tabarka and Kasserine are tested by chemical analysis (CA), X-ray diffraction (XRD), differential thermal analysis (DTA), infrared spectroscopy (IR), cation exchange capacities (CEC) and specific areas (BET) in order to determine their chemical and mineralogical composition and structural differences. This study showed that the clay of Tabarka is a well crystallized kaolinite and the clay of Kasserine is a halloysite 4H20, containing amorphous alumina. The evolution of specific surface according to the temperature of the Kasserine samples after thermal treatment shows an abrupt diminution at 700°C that are interpreted by a process of sintering. The thermal treatment of the two clays at 1200°C, shows the apparition of a new phase that is the mullite. This mullite is identical to mullite gotten by the process sol-gel from the synthesis of the pure aluminosiloxane.