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Effect of Crystallization Time on the Hydrothermal

Synthesis of Zeolites from Kaolin and Bauxite

B. Kwakye-Awuah, E. Von-Kiti, R. Buamah, I. Nkrumah, C. Williams

Abstract— Kaolin and bauxite were used as alumina and silica sources to synthesize zeolites hydrothermally. The source mate- rials as well as the synthesized zeolites were characterized by X-ray diffraction (XRD) scanning electron microscopy (SEM), en- ergy dispersive x-ray analysis (EDX) and Fourier transformed infrared spectroscopy (FTIR). XRD spectra of the bauxite showed Gibbsite phase whereas that of kaolin gave 32.4 % quartz and 67.6 %. The main phases of zeolites obtained after hydrothermal crystallizations were zeolite types LTA, analcime and zeolite X. Longer crystallization time resulted in phase change of the zeo- lites into sodalite. Hence, natural raw materials such as bauxite and kaolin have the attractive features of providing the staring reagents for the synthesis of ultrapure synthetic zeolites.

Index Terms— Synthesis, hydrothermal, zeolites, characterization, kaolin, bauxite, time.

1 INTRODUCTION

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Zeolites are microporous aluminosilicate with crystalline structure and unique ion exchange and sorption properties. They consist of either a silicon or aluminum atom as the center linked with four oxygen atoms at the corners in a tetrahedral manner. The tetrahedra are connected in such a way that cavi- ties form [4 – 10]. The negative charge of the tetrahedra is bal- anced by cations such as ions of hydrogen, sodium, potassi- um, magnesium, calcium and ammonium. They are known as molecular sieves since the dimension of the pore tend to ad- sorb molecules of certain dimensions while rejecting those of larger dimensions [14]. Zeolites are of considerable commer- cial importance in their applications ranging from water sof- tening to catalysis in the petrochemical industry [18] Zeolites occur naturally and synthetically (laboratory tailored ones). Extensive use of zeolites in terms of their applications has re- sulted in rapid increase in the large scale consumption which requires developments to synthesize zeolites from inexpensive sources. Synthetic zeolites are used more often than natural zeolites due to the purity of crystalline products and the uni- formity of particle sizes [11]. Since the principal raw materials used to manufacture zeolites are silica and alumina, which are amongst the most abundant mineral components on earth, the potential to supply raw materials for zeolites synthesis is vir-

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• B. Kwakye-Awuah is currently a lecturer in the Department of Physics, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana. E-mail: bkwakye- awuah.cos@knust.edu.gh

• E. Von-Kiti is currently pursuing PhD program Materials Science in Kwame

Nkrumah University of Science and Technology, Kumasi, Ghana. E-mail:

vonliz_05@yahoo.com

• R. Buamah is a Senior Lecturer in the Department of Civil Engineering, Kwame

Nkrumah University of Science and Technology, Kumasi, Ghana. E-mail:

rabuamah.coe@knust.edu.gh

• I. Nkrumah is a Lecturer in the Department of Physics, Kwame Nkrumah Universi-

ty of Science and Technology, Kumasi, Ghana. E-mail: inkrumah.sci@knust.edu.gh

• C. Williams is a Professor of Mesoporous and Microporous materials in the

University of Wolverhampton, UK. Email: c.williams@wlv.ac.uk

tually unlimited [7].
Different clay minerals have been used as a starting material for zeolites synthesis: kaolinite [17, 20]., montmorillonite [20], bentonite [15], halloysite [2], interstratified illite–smectite [1], among others. Natural zeolites have been also used to synthe- size zeolites possessing properties better than those of the starting material [19]. Kaolin is one of the most versatile indus- trial minerals and used extensively for many applications [21]. The synthesis of NaA zeolite from kaolin source has been worked on since 1970 [11] by the hydrothermal reaction of dehydroxylated kaolin with NaOH solution. Kovo and Holmes [2] and Reyes et al [12] have also synthesized zeolite from kaolin. Kang et al [19] synthesized 4A zeolite by mixing
5–15 wt.% bauxite with kaolin and calcination at high temper- ature, followed by reaction with sodium hydroxide, filtration, and drying. Successful synthesis of zeolite 4A from bauxite and kaolin has been reported by Xu et al [16] using different molar ratios of starting materials. The fusion temperature and time of 600 °C and 2 hours were maintained for optimum re- sults. Furthermore, Kang et al [19] synthesized zeolite X using bauxite with chemical composition (wt. %) SiO2 : 58.1 and Al2 O3 : 33.7 as starting material and sodium silicate solution. In this work, synthetic zeolites were synthesized from kaolin and bauxite both abundant in Ghana.

2.1 Materials and preparation

Kaolin was purchased from Anfoega in the Volta Region of Ghana. Bauxite was obtained from the Awaso Mines in the Western Region of Ghana. Sodium hydroxide was purchased from Sigma Aldrich, UK. Distilled water was obtained in the Water Research laboratory, Department of Civil Engineering, KNUST. The kaolin was ground and sieved with a 0.25 μm

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mesh. Bauxite from Awaso was obtained from the field and sieved to remove impurities.

2.2 Zeolite synthesis

50 g of kaolin was calcined at 600 °C for 2 hours to obtain me-
culate. Sodium Amalgam was then added to disperse adher- ing particles. Measurements were then recorded on a comput- er connected to the instrument. The vibrational properties were investigated by Fourier transformed infrared
spectrometry (FTIR). Measurements were done using 100
takaolin after which it was added to 2 M NaOH solution and

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scans at 4 cm
resolution, units of log (1/R) (absorbance), over
heated at 110 °C for 2 hours. Meanwhile, 25 g of bauxite was
heated in caustic soda (1 M NaOH) at 150 °C for 5 hours after which the slurry was filtered. The filtrates from the metakaolin and bauxite were added in varying ratios to obtain a molar ratio of SiO2 /AlO3 = 1-1.5. For the reaction mixture, the sodi- um aluminate solution (from bauxite) was added to the sodi- um silicate solution (from kaolin) to obtain a gelatinous solu- tion. The gel was stirred for 20 minutes until it was homoge- nized. The reaction was then aged at 24 hours at room temper- ature in Teflon bottles and later crystallized at 95 oC in an elec- tric oven for 3 hours. The whole procedure was repeated for crystallization times of 5, 24 and 94 hours. Following filtration and washing with distilled water, the samples were dried overnight at 100 °C, crushed into powder and stored in plastic containers.

2.3 Characterization technique

To asertain the crystal structure and the composition of the synthesized zeolite, it was essential to characterize the zeolite. The X-ray diifraction XRD) pattern of the un-heated and heat- ed bauxite which gives a measure of phase purity were recorded were recorded on Empyrean X-ray powder diffractometer over range of 3o to 70o (PANalytical, UK Ltd, Cambridge). The diffractometer was equipped with a graphite monochromated radiation source (8987 eV;
). Data processing was carried out using High Score Plus software with a search/match facility and an ICDD database on a DEC Microvax Minicomputer interfaced to the
diffractometer. The surface morphology of the un-heated and


heated bauxite as well as the elemental composition was examined by scanning electron microscopy (SEM) using a Zeiss EVO 50 equipped with energy dispersive X-ray spectrometer (EDX) (Zeiss, UK). Aluminium stubs were pre- pared prior to the analysis with an adhesive coating. The sam- ples were sprinkled on the stubs. Where necessary, the sam- ples were gold-coated using an Emscope SC500 Sputter coater to reduce static charging. Electron micrographs were obtained at various magnifications. Particle size distribution per unit volume was analyzed using a Mastersizer long bed analyzer (Malvern Instruments, UK). Samples of the un-heated and heated bauxite were taken for particle size analysis. Before measurements were made, the laser lenses were aligned in a straight line. Settings were set to obscuration value of 0.2. Af- ter steady conditions, 0.1 mg of each sample was loaded into an MSX 15 sample handling unit that uses the mechanical ac- tion of stirring to ensure that the zeolite particles did not floc-

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the mid-infrared mid-IR region of 1200-400 cm . An air back-

ground spectrum was collected at the start of the sample anal- ysis. A small sample of each of the un-heated or heated bauxite was centred on the ZnSe plate to ensure that it covered the entire crystal surface, and a pressure clamp was used to apply pressure on the sample. The zeolite samples were analyzed three times for three different samples. A background spec- trum was measured before samples were scanned to compen- sate for atmospheric conditions around the FT-IR instrument. Thermogravimetric analysis (TG –DTA) was performed using a Perkin Elmer TGA 7 (Perkin Elmer, UK) with differential thermal analysis. The study was carried out by heating at 10
°C /min heating rate starting from room temperature, with
sensitivity of 0.2 mV in atmospheric conditions, and the changes in the sample due to heating were observed. The temperature range for the analysis was 50 oC to 800 oC.
images as separate files.

3 RESULTS AND DISCUSSIONS

Kaolin and bauxite samples were both analyzed for their chemical and mineralogical properties. The main crystalline phases in bauxite were gibbsite and rutile. Kaolin from Anfoega had its main crystalline phase being kaolinite (67.6%) and quartz (32.4%). XRF analysis was used to ascertain the chemical compositions of both bauxite and kaolin (Table 1). XRD pattern as shown by Figure 1 for bauxite exhibited gibbs- ite as its major mineral composition of 99.6 %. Gibbsite is known to have high solubility in H2 SO4 , HCl and NaOH (ref ). Bauxite reacts with NaOH (caustic soda) to form sodium alu- minate (Na 2 Al 2 O4 ) while the silica reacts to form a complex compound of Na2 O∙Al 2 O3 ∙SiO2 . Since the silica content of the bauxite used in the study was relatively low, the formation of this compound is negligible. The impurities like Fe2 O3 and TiO2 remain insoluble in the mud.

Kaolinite activity can be increased by altering its properties with thermal, mechanical or chemical treatment. Calcining above 550 °C, converts kaolin into metakaolin by removal of structural OH ion making it reactive.

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TABLE 1

International Journal of Scientific & Engineering Research, Volume 5, Issue 2,CFehbermuaircya-l20c1o4mposition of bauxite and kaol7i3n6 as determined by EDX

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Kaolinite reacted with NaOH to form sodium aluminosilicate

solution with a high saturation towards its silicate content. SiO 2 Al2 O3 K2 O TiO 2 Fe 2 O3 Na2 O MgO

Bauxite wt.% Kaolin wt.%

1.12 98.6 trace 1.2 0.4 - -

69.58 20.54 5.59 0.95 1.08 0.28 1.98

Fig 1: XRD spectrum bauxite. The predominant phase was Gibbsite (99.6 %) with a minor rutile phase of 0.4 %

Fig 2: XRD spectrum kaolin. The predominant phase was kaolinite (67.6 %) with a minor low quartz phase of
0.4 %

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Fig 3: XRD spectrum of synthesized zeolite from bauxite and kaolin for a crystallization time of 3 hours. Zeolite A (=LTA) was obtained as the major phase.

Fig 4: XRD spectrum of synthesized zeolite from bauxite and kaolin for a crystallization time of 5 hours. The main phases were zeolite LTA, analcime and zeolite X.

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Int

ISS

Fig 5: XRD spectrum of synthesized zeolite from bauxite and kaolin for a crystallization time of 24 hours. The main phase was zeolite zeolite X with the appearance of ice crystals.

Fig 6: XRD spectrum of synthesized zeolite from bauxite and kaolin for a crystallization time of 94 hours. The main phases were sodalite, with some formation of zeolite ZK and SAPO-43.
It was found that the 94 hour crystallization yielded 72.3% of basic sodalite, 16% of zeolite ZK-14 and 11.7% of SAPO- 43. Prolonged crystallization time in zeolite A (LTA) and zeolite X
has been observed to dissolve to form zeolites sodalite (SOD)
and P (GHS) respectively when synthesized in alkaline alumi-

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nosilicate gel (ref ). As reported by Chandrasekhar and Prama- da (1999), formation of multiphase (zeolites) crystalline prod- ucts is achieved from the use of heterogeneous aluminosili- cates as against the use of soluble silicates and aluminates that often result in the formation of pure zeolite types. The FTIR spectra of the kaolin and metakaolin showed different peaks in their structure. The intense bands occurred at 900–1200 cm-1 and 430–520 cm-1 for kaolin and 900 – 1050 cm-1 and 430 – 500 cm-1 for metakaolin. The first is assigned to an asymmetric stretching mode and the second one to a bending mode of the T–O bond. All these bands are more or less dependent on the crystal structure. The transformation of kaolinite to me- takaolinite can be observed by the loss of peaks at 3618 -3370 cm -1 being the stretching vibrations of hydroxyl groups pre- sent in kaolin but absent in metakaolinite. the loss of Al-OH bands (925 and 910 cm-1), changes in the Si-O stretching bands (1002 cm-1, 1019 cm-1) and the reduction in intensity of Al-O-Si bands at 796 and 778 cm-1 confirm the transformation.
TABLE 2
at 913 cm-1 is due to Al – O - H group deformation vibration. Bands appearing at around 448, 540, 662, 1100 cm-1 are as- signed to the Si–O vibration. Assignments of vibrational framework of the synthesized zeolites at different synthesis products are given in Table 2. The SEM photographs of the synthesized zeolites are showed in fig. 8. The shape of the crystals for 3 and 5 hour crystallization were cubic. The cubic crystalline shape is associated with zeolite A. The average crystal sizes were 4.4 μm and 5 μm respectively. However, increased synthesis time resorted to the zeolite A modifying to X with hexagonal crystal shapes after 24 hours typical of zeo- lite X from reagents. The crystal size was approximately 3 μm. The 94 hour synthesis time had orthorhombic shaped crystals with a size of 1.65 μm.
Bands obtained from the FTIR spectra (not shown) for bauxite and kaolin and for hydrothermal synthsis
at 3, 5, 24 and 94 hours respectively

These changes are similar to those reported in other studies [4
– 9, 12, 23 – 25]. The characteristic bands observed in the baux- ite were 1017, 790, 738, 662, 555, 448 and 407 cm-1. Bauxite showed in the region of Al - O - H vibration stretching bands around 3618 – 3370 cm-1. The bands around 3524, 3452 and
3392 cm-1 are assigned to O-H stretching of gibbsite. The band

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a b

c d

Fig 7: Scanning electron micrographs of the zeolite crystals obtained for crystallization time of (a): 3 hours, (b): 5 hours, (c): 24 hours and (d): 94 hours.

4 CONCLUSION

Zeolite synthesis was successful from bauxite and kaolin. The main crystalline phases in kaolin were quartz and kaolinite whilst that of bauxite was gibbsite. These phases were con- verted to zeolite types at varying crystallization times. The effect of crystallization time on the type of zeolite was investi- gated. 87.2% zeolite A was obtained at 5 hours crystallization time and zeolite Na LSX with a yield of 93% was obtained af- ter 24 hours. Comparing the results showed that zeolite syn- thesis is greatly affected by the crystallization time. Zeolite synthesis has been confirmed to be successful at low tempera- tures (ref ). The cost of synthesized zeolite was lower than that of commercial zeolite as well as that prepared from reagents.

ACKNOWLEDGMENT

The authors wish to thank the University of Wolverhampton for assisting in the characterization of samples. This work was supported in part by a grant from Royal Society, UK and Grand Challenges Canada.

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