- Email: [email protected]
Influence of an aging step on the synthesis of zeolite NaA from Brazilian Amazon kaolin waste
j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):2924–2929
Available online at www.sciencedirect.com
www.jmrt.com.br
Original Article
Influence of an aging step on the synthesis of zeolite NaA from Brazilian Amazon kaolin waste Ana Áurea B. Maia a,∗ , Rafael N. Dias a , Rômulo S. Angélica b , Roberto F. Neves b a
Faculdade de Engenharia Industrial, Universidade Federal do Pará, Campus Abaetetuba, Rua Manoel de Abreu s/n, CEP: 68440-000, Abaetetuba, Pará, Brazil b Programa de Pós-Graduac¸ão em Geologia e Geoquímica, Universidade Federal do Pará, Campus Guamá, Rua Augusto Corrêa, 01, 66075-110 Belém-Pará, Brazil
a r t i c l e
i n f o
a b s t r a c t
Article history:
The synthesis of zeolite NaA from kaolin waste was conducted subjecting the reaction mix-
Received 24 May 2018
ture to an aging step before autoclaving at high temperature. This waste, from Brasilian
Accepted 8 February 2019
Amazon, is deposited in large basins and display excellent quality. In this aging step, the
Available online 25 May 2019
reaction mixture was agitated for 24 h at room temperature, and after this time, the synthesis
Keywords:
ture of 110 ◦ C. The kaolin waste and synthesis products were characterized through X-ray
was performed for varying times of 1, 2, 3, 4, 6, 12, 18, 20 and 24 h at a constant temperaKaolin waste
diffractometry, scanning electron microscopy and chemical analysis. The results showed
Aging
that the kaolin waste consists primarily of kaolinite. Zeolite NaA was obtained after 4 h.
Zeolite NaA
However, zeolite NaA was obtained with a high degree of purity and a high degree of structural order only for times of 6, 12, 18, 20 and 24 h. This study showed that zeolite NaA can be produced in shorter times through the synthesis route used. The production of zeolite NaA from kaolin waste could be an excellent way to minimize environmental impacts and decrease processing costs. © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1.
Introduction
There are several ways to synthesize zeolites, and the goal of all of them is to obtain zeolitic products with high purity under viable, economical conditions. In this context, the use of industrial waste to produce new materials, in addition to be
∗
a challenge worldwide, can generate zeolitic products through economically and environmentally favorable processes. The kaolin waste from the Amazon, which originates from the kaolin beneficiation process for paper covering, is essentially made of kaolinite [1–3]. This material is considered a waste due to its granulometry which is inadequate for paper covering. Various basins are necessary to deposit a large quantity of waste de kaolin produced for this mining industry. Therefore, this waste is an excellent raw material for the synthesis of zeolites, such as phase NaA [1–5], sodalite [6,7], faujasite [8,9], phase NaP [10] and phases KA, CaA and MgA [4].
Corresponding author. E-mail: [email protected] (A.Á. Maia). https://doi.org/10.1016/j.jmrt.2019.02.021 2238-7854/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):2924–2929
Kaolinite is one of the clay minerals most commonly used in the synthesis of zeolites due to the presence of 4coordinated Al and Si after the thermal treatment of kaolin to obtain metakaolinite [11]. In the zeolite structures, Al and Si are also tetrahedrally coordinated, which explains the high reactivity of metakaolinite when it presents these atoms in tetrahedral coordination [3,11]. ˚ howZeolite NaA display pore size of approximately 4 A, ever, this pore dimension can be variable depending on the ˚ and Ca (5 A). ˚ These cationic type of exchangeable ion: K (3 A) forms (KA and CaA) can be obtained through ion exchange process from NaA phase. This remarkable characteristic turn zeolite NaA to be the zeolitic material most synthesized worldwide [11]. In the synthesis of zeolite NaA, which presents a structural Si/Al ratio equal to 1, as in kaolinite, a sodium source is necessary in addition to the clay mineral. Thus, the synthesis of zeolite usually occurs in two steps. In the first step, the thermal treatment of kaolin to obtain metakaolin occurs. In the second step, the reaction of metakaolinite with the sodium source, typically sodium hydroxide, takes place. In this step, the chemical composition of the reactional mixture can vary in mass, and the variation of the temperature and crystallization time can also be studied [11]. All these parameters can be varied to obtain zeolite NaA with a high degree of purity under economic conditions. In the case of the aging method, a pre-crystallization of the reaction mixture needs to occur in the synthesis of zeolites before crystallization. Aging can be accomplished by stirring the reactional mixture for a certain time at a temperature close to room temperature [12], at lower temperatures, such as 4 ◦ C [13], or even at higher temperatures, such as 50 and 70 ◦ C [14]. The subsequent crystallization of zeolites occurs at a temperature higher than that for the aging and in a shorter time than when using the conventional method [12,14,15]. The aging step allows greater control of the size of the crystals and ensures the purity of the desired phase [16]. According to these authors, the crystals formed through this synthesis method are smaller than those formed through the conventional method because structural elements related to nucleation are already being formed during the aging process. According to Jihong [17], the aging of the reaction mixture influences the nucleation and crystallization of zeolites by increasing the nucleation rate, reducing the induction period and the duration of the crystallization, reducing the crystal size and increasing the amount of crystals. Slangen et al. [12] verified that when the reaction mixture is agitated at room temperature for enough time (24 h) at the aging step, the synthesis of zeolite NaA can be accomplished in just 1 h via microwave heating at 120 ◦ C, resulting in crystals with a relatively uniform size distribution. For shorter stirring times, such as 5 min (pre-crystallization), the obtained product was 100% amorphous after heating the reaction mixture using microwave for 5 minutes at 100 ◦ C (crystallization). For an aging time of 2 h (pre-crystallization), the product obtained under the same conditions cited above (crystallization) was approximately 10% hydroxysodalite and 90% amorphous. Thus, the use of an aging step in the synthesis of zeolites will increase the degree of structural order
2925
of these materials if favorable conditions are selected for the experiments [13]. The zeolites, due to the characteristics of their structure, are used in various applications, such as: cracking catalysis, adsorption and ion exchange [11,18,19]. Thus, this study aims to propose a new method to synthesize zeolite NaA, using kaolin waste from the Brazilian Amazon region, by adding an aging step to minimize the crystallization time, which will allow reduced process costs. In the future, the zeolite NaA can be used as adsorbent for the removal of the heavy metals, for example.
2.
Materials and methods
2.1.
Materials
Kaolin waste from the Capim region was used as a starting material for the synthesis of zeolite, as a source of Al and Si. A solution of 5 M NaOH was used as a sodium source.
2.2.
Methods
2.2.1.
Treatment of kaolin
The kaolin waste was thermally treated to obtain a reactive metakaolinite in the synthesis of zeolite NaA. Thus, a temperature of 700 ◦ C and a time of 2 h were used because, according to Maia et al. [3], these conditions are optimal for obtaining highly reactive metakaolinite from this waste. Kaolin waste and the product of its treatment were characterized by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The total chemical composition of the waste was analyzed by combined methods, such as emission spectroscopy and induced coupled plasma mass spectrometry.
2.2.2.
Synthesis process
The composition of the reaction mixture for the synthesis of zeolite NaA used in this study was the same as that used by Maia et al. [4], who also used kaolin waste from the Capim region to synthesize this microporous material. In that research, zeolite A was obtained with high purity and a high degree of structural order for a Na/Al molar ratio of 1.64 in the reaction mixture and a synthesis temperature of 110 ◦ C. In this study, an aging step was used in the process of zeolite NaA synthesis, and the reaction mixture was then subjected to 110 ◦ C. For the aging of the reaction mixture, metakaolin and NaOH solution (5 N) were transferred to a flask, corresponding Na/Al and Si/Al molar ratios of 1.64 and 1, respectively and 15 mL of distilled water were added to it. This mixture was stirred at room temperature with a magnetic stirrer for 24 h. After the aging step, the reaction mixture was transferred to the reactor cup, and 10 mL of distilled water was added. The reactor used in the synthesis consists of an external part made of stainless steel and an internal part (cup with an approximately 50 mL capacity) made of Teflon. The reaction mixture was then placed in an oven at 110 ◦ C for varying times of 1, 2, 3, 4, 6, 12, 18, 20, and 24 h. Thus, in this study, the synthesis temperature and the composition of
2926
j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):2924–2929
Fig. 1 – XRD results of kaolin waste and metakaolin. K: kaolinite; An: anatase; Q: quartz.
Fig. 2 – SEM images of kaolin waste and metakaolin.
the reaction mixture were held constant while the synthesis time was varied. After the established synthesis time, the reactors were removed from the oven and subjected to rapid cooling, and the products were then washed and filtered to neutral pH. Later, the washed products were dried in the oven at 110 ◦ C for 6 h. The products were characterized by XRD and SEM. The entire synthesis process was conducted in duplicate.
3.
Results and discussion
3.1.
Starting materials
According to the X-ray diffractometry (XRD) results presented in Fig. 1, the kaolin waste used in the synthesis of zeolite NaA consists mainly of kaolinite. This figure also shows that the kaolinite present in this waste has a high degree of structural order, because the elevate intensity of this peaks. The metakaolin diffractogram, also seen in Fig. 1, showed that kaolinite was entirely transformed into metakaolinite because the peaks of kaolinite are no longer present. In addition, a rise in the background is observed, which shows the presence of a non-crystalline (amorphous) phase. The metakaolin diffractogram also presents anatase (An) and quartz (Q) peaks, mineralogical impurities normally found in the kaolin from the Capim region and consequently in the waste from this kaolin processing [1,2]. The peaks of these impurities were not observed in the mineralogical diffractogram of kaolin waste because they occur in small concentrations in relation to kaolinite. Scanning electron
Table 1 – Chemical analysis of kaolin waste. Component
wt.%
SiO2 Al2 O3 Fe2 O3 TiO2 CaO MgO Na2 O K2 O LOI
46.11 38.27 0.57 0.35 <0.01 0.02 0.05 0.14 14.46
microscopic images of the starting material (kaolin waste) and the intermediate material (metakaolinite) are presented in Fig. 2 and show that the starting material, without thermal treatment, is primarily composed of many clustered particles of pseudo-hexagonal morphology, which can be called poorly selected “coarse particles” because they are industrial waste. The metakaolinite resulting from kaolinite calcination at 700 ◦ C for 2 h presents large agglomerations of particles, with the pseudo-hexagonal morphology being maintained. The results of the chemical analysis of the kaolin waste and weight loss due to heating the material are presented in Table 1. These results show that the waste used in this study consists primarily of kaolinite, as the levels of alumina and silica and the weight loss due to heating are close to the values specified by Murray [20], namely, 39.50% for Al2 O3 , 46.54% for SiO2 , and 13.96% for H2 O. The other constituents found in
j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):2924–2929
2927
Fig. 3 – XRD patterns of the products obtained at 0 h (aging), 1, 2 and 3 h. An: anatase; Q: quartz.
Fig. 4 – XRD patterns of products obtained for crystallization times of 4–24 h. ZA: zeolite A.
Fig. 5 – SEM images of products synthesized at 6 h and 20 h.
the material are considered impurities, such as TiO2 (0.35%), which is related to anatase, previously identified by XRD.
3.2.
Synthesis of zeolite NaA
The diffractograms of material obtained at the aging step (0 h) and of the products synthesized at 110 ◦ C for times of 1, 2 and 3 h that underwent the aging step before crystallization are presented in Fig. 3. The characteristic peaks of zeolitic material were not found in the diffractogram of the material obtained in the aging step (0 h). The quartz and anatase peaks from the starting material are still detected. The crystallization times of 1, 2 and 3 h were not enough to produce zeolite NaA. However, the presence of non-crystalline material is observed based on the rising background in all diffractograms. Fig. 4 presents the diffractograms of products crystallized for 4, 6, 12, 18, 20 and 24 h. Zeolite NaA was formed from
4 h onward. From 4 h to 24 h, only zeolite A was synthesized, and the characteristic peaks of this zeolitic material are welldelineated, indicating that all the metakaolinite had reacted, which shows that it is an excellent starting material. The graph in Fig. 4 shows that it was possible to synthesize zeolite NaA at 4 h, but the intensity of its peak was lower than that at 6 h. In this case, at 6 h of synthesis, zeolite NaA, probably, display higher structural order degree. Fig. 4 shows that from 6 h, all peaks have the same intensity, i.e., there is no variation in these peaks with time. This result reveals that zeolite NaA can be produced in only 4 h of synthesis, using the conditions and steps described in this study (thermal treatment of the kaolin waste, aging of the reaction mixture, and crystallization). Therefore, the aging of the reaction mixture (pre-crystallization step) contributed for the decrease of the zeolite NaA crystallization time. Jihong (2007) has supported that aging of zeolite synthesis contributed to reduce a duration of crystallization [17].
2928
j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):2924–2929
In addition, zeolite NaA probably presents a highly stable structure, as there is no phase change for longer times, such as 24 h. According to Breck [11], zeolite A is a metastable phase that turns into sodalite over time. According to Cundy and Cox, the aging step probably contributes to the formation of more stable or pure phases [16]. The scanning electron microscopy images of the products synthesized at 6 and 20 h presented in Fig. 5, respectively, show well-formed cubic crystals. According to Breck [11], this morphology is typical of zeolite NaA. This morphology was also observed in the products synthesized by Maia et al. [1,2]. In those studies, zeolite NaA was also synthesized from kaolin waste from the Capim region using multiple synthetic conditions. Thus, the SEM images show that at 110 ◦ C and at 6 and 20 h, zeolite NaA was the only crystalline material, confirming the X-ray diffractometry results (Fig. 5). In the sample produced at 6 h is yet observed an undefined morphology, that probably can be non-reactive metakaolinite (Fig. 5a). In this study, comparing the SEM images of products synthesized under the same synthesis conditions (T = 110 ◦ C, molar ratio Na/Al = 1.64) using the traditional method [4] with the ones produced using the aging step, it appears that the size of the crystals of zeolite NaA was smaller for the latter method. According to the SEM images of the product synthesized by the conventional method, the crystals of zeolite NaA have sizes of approximately 10 m, whereas when the aging step is included before crystallization, the crystal size is approximately 4 m. A decrease in the size of zeolite NaA crystals was also observed by Slangen et al. [12].
4.
Conclusions
An alternative route for the synthesis of zeolite NaA was developed using waste kaolin. This waste is an excellent raw material for producing pure zeolite NaA and provides an alternative approach to minimizing environmental impact. The aging step was effective, producing zeolite NaA after 4 h, which had not been previously achieved for this time and this route (1-thermal treatment, 2-aging, and 3crystallization). The zeolite NaA reached maximum crystallization at 6 h. However, zeolite NaA can be obtained at 4 h. The zeolite NaA, synthesized in this route, display, probably, a stable structure, as there was no formation of sodalite which is theoretically the most stable phase, with increasing synthesis time.
Conflicts of interest The authors declare no conflicts of interest.
Acknowledgements We would like to express gratitude to the Pro-Rector of Research and Graduate Studies Office (Pró-Reitoria de Pesquisa e Pós-Graduac¸ão - PROPESP) of the Univeristy of Pará (UFPA) for the support and the Pará Research
Foundation (Fundac¸ão de Amparo e Desenvolvimento da Pesquisa do Estado do Pará - FADESPA) for the translation of this article.
references
[1] Maia AAB, Saldanha E, Angélica RS, Souza CA, Neves RF. The use of kaolin wastes from the Amazon region on the synthesis of zeolite A. Cerâmica 2007;53:319–24. [2] Maia AAB, Angélica RS, Neves RF. Use of industrial kaolin waste from the Brazilian Amazon region for synthesis of zeolite A. Clay Miner 2011;46:127–36. [3] Maia AAB, Straub C, Saalwaechter K, Neves RF, Angélica RS, Pöllmann H. Use of 29 Si and 27 Al MAS NMR to study thermal activation of kaolinites from Brazilian Amazon kaolin wastes. Appl Clay Sci 2014;87:189–96. [4] Maia AAB, Neves RF, Angélica RS, Pöllmann H. Synthesis, optimisation and characterisation of the zeolite NaA using kaolin waste from the Amazon Region. Production of zeolites KA, MgA and CaA. Appl Clay Sci 2015;108:55–60. [5] Silva Filho SH, Bieseki LAR, Maia AAB, San Gil RAS, Pergher SBC. Synthesis of Zeolite A employing Amazon kaolin waste. Cerâmica 2015;61:409–13. [6] Paz SPA, Angélica RS, Neves RF. Síntese hidrotermal de sodalita básica a partir de um rejeito de caulim termicamente ativado. Química Nova 2010;33: 579–83. [7] Maia AAB, Neves RF, Angélica RS, Pöllmann H. Synthesis of sodalite from Brazilian kaolin wastes. Clay Miner 2015;50:663–75. [8] Hildebrando EA, Angélica AS, Neves RF, Valenzuela-Diaz FR. Synthesis of faujasite-type zeolite from kaolin waste. Cerâmica 2012;58:453–8. [9] Maia AAB, Alves MS, Angélica RS, de R, Neves F. Aging effect of reaction mixture constituted of Amazon kaolin waste on faujasite zeolite synthesis. Cerâmica 2017;63:485–548. [10] Hildebrando EA, Andrade CGB, Rocha Junior CAF, Angélica RS, Neves RF. Synthesis and characterization of zeolite NaP using kaolin waste as a source of silicon and aluminum. Mater Res 2014;17:174–9. [11] Breck DW. Zeolitic molecular sieves: structure, chemistry and use. New York: Wiley; 1974. [12] Slangen Slangen PM, Jansen JC, van Bekkum H. The effect of ageing on the microwave synthesis of zeolite NaA. Micropor Mater 1997;9:259–65. ˘ H, Sarıog˘ lan A, Tatlıer M, Erdem-S¸enatalar A, [13] Jülide Köroglu Tunc¸ Savas¸c¸ı Ö. Effects of low-temperature gel aging on the synthesis of zeolite Y at different alkalinities. J Cryst Growth 2002;241:481–8. [14] Zhou Z, Ge Jin, Liu H, Wu J, Mei J. Crystallization mechanism of zeolite A from coal kaolin using a two-step method. Appl Clay Sci 2014;97–98:110–4. [15] Byrappa K, Yoshimura M. Handbook of hydrothermal technology. A technology for crystal growth and materials processing. Norwich, NY: William Andrew Publishing; 2001. [16] Cundy CS, Cox PA. The hydrothermal synthesis of zeolites: precursors, intermediates and reaction mechanism. Micropor Mesopor Mater 2005;82:1–78. ˇ [17] Jihong Y. Synthesis of zeolites. In: Cejka Jiˇrí vBHCA, Ferdi S, editors. Studies in surface science and catalysis, vol. 168. Amsterdam: Elsevier; 2007. p. 39–103. [18] Payra P, Dutta PK. Zeolites: a primer. In: Auerbach SM, Carrado KA, Dutta PK, editors. Handbook of zeolite science and technology. United States of America: Marcel Dekker; 2003.
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ˇ [19] Maese T. The zeolite scene – an overview. In: Cejka Jiˇrí vBHCA, Ferdi S, editors. Studies in surface science and catalysis, vol. 168. Amsterdam: Elsevier; 2017. p. 1–12.
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[20] Murray HH, editor. Applied clay mineralogy. Occurrences, processing and application of kaolins, bentonites, palygorskite-sepiolite, and common clays. Amsterdam: Elsevier; 2007.
Available online at www.sciencedirect.com
www.jmrt.com.br
Original Article
Influence of an aging step on the synthesis of zeolite NaA from Brazilian Amazon kaolin waste Ana Áurea B. Maia a,∗ , Rafael N. Dias a , Rômulo S. Angélica b , Roberto F. Neves b a
Faculdade de Engenharia Industrial, Universidade Federal do Pará, Campus Abaetetuba, Rua Manoel de Abreu s/n, CEP: 68440-000, Abaetetuba, Pará, Brazil b Programa de Pós-Graduac¸ão em Geologia e Geoquímica, Universidade Federal do Pará, Campus Guamá, Rua Augusto Corrêa, 01, 66075-110 Belém-Pará, Brazil
a r t i c l e
i n f o
a b s t r a c t
Article history:
The synthesis of zeolite NaA from kaolin waste was conducted subjecting the reaction mix-
Received 24 May 2018
ture to an aging step before autoclaving at high temperature. This waste, from Brasilian
Accepted 8 February 2019
Amazon, is deposited in large basins and display excellent quality. In this aging step, the
Available online 25 May 2019
reaction mixture was agitated for 24 h at room temperature, and after this time, the synthesis
Keywords:
ture of 110 ◦ C. The kaolin waste and synthesis products were characterized through X-ray
was performed for varying times of 1, 2, 3, 4, 6, 12, 18, 20 and 24 h at a constant temperaKaolin waste
diffractometry, scanning electron microscopy and chemical analysis. The results showed
Aging
that the kaolin waste consists primarily of kaolinite. Zeolite NaA was obtained after 4 h.
Zeolite NaA
However, zeolite NaA was obtained with a high degree of purity and a high degree of structural order only for times of 6, 12, 18, 20 and 24 h. This study showed that zeolite NaA can be produced in shorter times through the synthesis route used. The production of zeolite NaA from kaolin waste could be an excellent way to minimize environmental impacts and decrease processing costs. © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1.
Introduction
There are several ways to synthesize zeolites, and the goal of all of them is to obtain zeolitic products with high purity under viable, economical conditions. In this context, the use of industrial waste to produce new materials, in addition to be
∗
a challenge worldwide, can generate zeolitic products through economically and environmentally favorable processes. The kaolin waste from the Amazon, which originates from the kaolin beneficiation process for paper covering, is essentially made of kaolinite [1–3]. This material is considered a waste due to its granulometry which is inadequate for paper covering. Various basins are necessary to deposit a large quantity of waste de kaolin produced for this mining industry. Therefore, this waste is an excellent raw material for the synthesis of zeolites, such as phase NaA [1–5], sodalite [6,7], faujasite [8,9], phase NaP [10] and phases KA, CaA and MgA [4].
Corresponding author. E-mail: [email protected] (A.Á. Maia). https://doi.org/10.1016/j.jmrt.2019.02.021 2238-7854/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):2924–2929
Kaolinite is one of the clay minerals most commonly used in the synthesis of zeolites due to the presence of 4coordinated Al and Si after the thermal treatment of kaolin to obtain metakaolinite [11]. In the zeolite structures, Al and Si are also tetrahedrally coordinated, which explains the high reactivity of metakaolinite when it presents these atoms in tetrahedral coordination [3,11]. ˚ howZeolite NaA display pore size of approximately 4 A, ever, this pore dimension can be variable depending on the ˚ and Ca (5 A). ˚ These cationic type of exchangeable ion: K (3 A) forms (KA and CaA) can be obtained through ion exchange process from NaA phase. This remarkable characteristic turn zeolite NaA to be the zeolitic material most synthesized worldwide [11]. In the synthesis of zeolite NaA, which presents a structural Si/Al ratio equal to 1, as in kaolinite, a sodium source is necessary in addition to the clay mineral. Thus, the synthesis of zeolite usually occurs in two steps. In the first step, the thermal treatment of kaolin to obtain metakaolin occurs. In the second step, the reaction of metakaolinite with the sodium source, typically sodium hydroxide, takes place. In this step, the chemical composition of the reactional mixture can vary in mass, and the variation of the temperature and crystallization time can also be studied [11]. All these parameters can be varied to obtain zeolite NaA with a high degree of purity under economic conditions. In the case of the aging method, a pre-crystallization of the reaction mixture needs to occur in the synthesis of zeolites before crystallization. Aging can be accomplished by stirring the reactional mixture for a certain time at a temperature close to room temperature [12], at lower temperatures, such as 4 ◦ C [13], or even at higher temperatures, such as 50 and 70 ◦ C [14]. The subsequent crystallization of zeolites occurs at a temperature higher than that for the aging and in a shorter time than when using the conventional method [12,14,15]. The aging step allows greater control of the size of the crystals and ensures the purity of the desired phase [16]. According to these authors, the crystals formed through this synthesis method are smaller than those formed through the conventional method because structural elements related to nucleation are already being formed during the aging process. According to Jihong [17], the aging of the reaction mixture influences the nucleation and crystallization of zeolites by increasing the nucleation rate, reducing the induction period and the duration of the crystallization, reducing the crystal size and increasing the amount of crystals. Slangen et al. [12] verified that when the reaction mixture is agitated at room temperature for enough time (24 h) at the aging step, the synthesis of zeolite NaA can be accomplished in just 1 h via microwave heating at 120 ◦ C, resulting in crystals with a relatively uniform size distribution. For shorter stirring times, such as 5 min (pre-crystallization), the obtained product was 100% amorphous after heating the reaction mixture using microwave for 5 minutes at 100 ◦ C (crystallization). For an aging time of 2 h (pre-crystallization), the product obtained under the same conditions cited above (crystallization) was approximately 10% hydroxysodalite and 90% amorphous. Thus, the use of an aging step in the synthesis of zeolites will increase the degree of structural order
2925
of these materials if favorable conditions are selected for the experiments [13]. The zeolites, due to the characteristics of their structure, are used in various applications, such as: cracking catalysis, adsorption and ion exchange [11,18,19]. Thus, this study aims to propose a new method to synthesize zeolite NaA, using kaolin waste from the Brazilian Amazon region, by adding an aging step to minimize the crystallization time, which will allow reduced process costs. In the future, the zeolite NaA can be used as adsorbent for the removal of the heavy metals, for example.
2.
Materials and methods
2.1.
Materials
Kaolin waste from the Capim region was used as a starting material for the synthesis of zeolite, as a source of Al and Si. A solution of 5 M NaOH was used as a sodium source.
2.2.
Methods
2.2.1.
Treatment of kaolin
The kaolin waste was thermally treated to obtain a reactive metakaolinite in the synthesis of zeolite NaA. Thus, a temperature of 700 ◦ C and a time of 2 h were used because, according to Maia et al. [3], these conditions are optimal for obtaining highly reactive metakaolinite from this waste. Kaolin waste and the product of its treatment were characterized by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The total chemical composition of the waste was analyzed by combined methods, such as emission spectroscopy and induced coupled plasma mass spectrometry.
2.2.2.
Synthesis process
The composition of the reaction mixture for the synthesis of zeolite NaA used in this study was the same as that used by Maia et al. [4], who also used kaolin waste from the Capim region to synthesize this microporous material. In that research, zeolite A was obtained with high purity and a high degree of structural order for a Na/Al molar ratio of 1.64 in the reaction mixture and a synthesis temperature of 110 ◦ C. In this study, an aging step was used in the process of zeolite NaA synthesis, and the reaction mixture was then subjected to 110 ◦ C. For the aging of the reaction mixture, metakaolin and NaOH solution (5 N) were transferred to a flask, corresponding Na/Al and Si/Al molar ratios of 1.64 and 1, respectively and 15 mL of distilled water were added to it. This mixture was stirred at room temperature with a magnetic stirrer for 24 h. After the aging step, the reaction mixture was transferred to the reactor cup, and 10 mL of distilled water was added. The reactor used in the synthesis consists of an external part made of stainless steel and an internal part (cup with an approximately 50 mL capacity) made of Teflon. The reaction mixture was then placed in an oven at 110 ◦ C for varying times of 1, 2, 3, 4, 6, 12, 18, 20, and 24 h. Thus, in this study, the synthesis temperature and the composition of
2926
j m a t e r r e s t e c h n o l . 2 0 1 9;8(3):2924–2929
Fig. 1 – XRD results of kaolin waste and metakaolin. K: kaolinite; An: anatase; Q: quartz.
Fig. 2 – SEM images of kaolin waste and metakaolin.
the reaction mixture were held constant while the synthesis time was varied. After the established synthesis time, the reactors were removed from the oven and subjected to rapid cooling, and the products were then washed and filtered to neutral pH. Later, the washed products were dried in the oven at 110 ◦ C for 6 h. The products were characterized by XRD and SEM. The entire synthesis process was conducted in duplicate.
3.
Results and discussion
3.1.
Starting materials
According to the X-ray diffractometry (XRD) results presented in Fig. 1, the kaolin waste used in the synthesis of zeolite NaA consists mainly of kaolinite. This figure also shows that the kaolinite present in this waste has a high degree of structural order, because the elevate intensity of this peaks. The metakaolin diffractogram, also seen in Fig. 1, showed that kaolinite was entirely transformed into metakaolinite because the peaks of kaolinite are no longer present. In addition, a rise in the background is observed, which shows the presence of a non-crystalline (amorphous) phase. The metakaolin diffractogram also presents anatase (An) and quartz (Q) peaks, mineralogical impurities normally found in the kaolin from the Capim region and consequently in the waste from this kaolin processing [1,2]. The peaks of these impurities were not observed in the mineralogical diffractogram of kaolin waste because they occur in small concentrations in relation to kaolinite. Scanning electron
Table 1 – Chemical analysis of kaolin waste. Component
wt.%
SiO2 Al2 O3 Fe2 O3 TiO2 CaO MgO Na2 O K2 O LOI
46.11 38.27 0.57 0.35 <0.01 0.02 0.05 0.14 14.46
microscopic images of the starting material (kaolin waste) and the intermediate material (metakaolinite) are presented in Fig. 2 and show that the starting material, without thermal treatment, is primarily composed of many clustered particles of pseudo-hexagonal morphology, which can be called poorly selected “coarse particles” because they are industrial waste. The metakaolinite resulting from kaolinite calcination at 700 ◦ C for 2 h presents large agglomerations of particles, with the pseudo-hexagonal morphology being maintained. The results of the chemical analysis of the kaolin waste and weight loss due to heating the material are presented in Table 1. These results show that the waste used in this study consists primarily of kaolinite, as the levels of alumina and silica and the weight loss due to heating are close to the values specified by Murray [20], namely, 39.50% for Al2 O3 , 46.54% for SiO2 , and 13.96% for H2 O. The other constituents found in
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Fig. 3 – XRD patterns of the products obtained at 0 h (aging), 1, 2 and 3 h. An: anatase; Q: quartz.
Fig. 4 – XRD patterns of products obtained for crystallization times of 4–24 h. ZA: zeolite A.
Fig. 5 – SEM images of products synthesized at 6 h and 20 h.
the material are considered impurities, such as TiO2 (0.35%), which is related to anatase, previously identified by XRD.
3.2.
Synthesis of zeolite NaA
The diffractograms of material obtained at the aging step (0 h) and of the products synthesized at 110 ◦ C for times of 1, 2 and 3 h that underwent the aging step before crystallization are presented in Fig. 3. The characteristic peaks of zeolitic material were not found in the diffractogram of the material obtained in the aging step (0 h). The quartz and anatase peaks from the starting material are still detected. The crystallization times of 1, 2 and 3 h were not enough to produce zeolite NaA. However, the presence of non-crystalline material is observed based on the rising background in all diffractograms. Fig. 4 presents the diffractograms of products crystallized for 4, 6, 12, 18, 20 and 24 h. Zeolite NaA was formed from
4 h onward. From 4 h to 24 h, only zeolite A was synthesized, and the characteristic peaks of this zeolitic material are welldelineated, indicating that all the metakaolinite had reacted, which shows that it is an excellent starting material. The graph in Fig. 4 shows that it was possible to synthesize zeolite NaA at 4 h, but the intensity of its peak was lower than that at 6 h. In this case, at 6 h of synthesis, zeolite NaA, probably, display higher structural order degree. Fig. 4 shows that from 6 h, all peaks have the same intensity, i.e., there is no variation in these peaks with time. This result reveals that zeolite NaA can be produced in only 4 h of synthesis, using the conditions and steps described in this study (thermal treatment of the kaolin waste, aging of the reaction mixture, and crystallization). Therefore, the aging of the reaction mixture (pre-crystallization step) contributed for the decrease of the zeolite NaA crystallization time. Jihong (2007) has supported that aging of zeolite synthesis contributed to reduce a duration of crystallization [17].
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In addition, zeolite NaA probably presents a highly stable structure, as there is no phase change for longer times, such as 24 h. According to Breck [11], zeolite A is a metastable phase that turns into sodalite over time. According to Cundy and Cox, the aging step probably contributes to the formation of more stable or pure phases [16]. The scanning electron microscopy images of the products synthesized at 6 and 20 h presented in Fig. 5, respectively, show well-formed cubic crystals. According to Breck [11], this morphology is typical of zeolite NaA. This morphology was also observed in the products synthesized by Maia et al. [1,2]. In those studies, zeolite NaA was also synthesized from kaolin waste from the Capim region using multiple synthetic conditions. Thus, the SEM images show that at 110 ◦ C and at 6 and 20 h, zeolite NaA was the only crystalline material, confirming the X-ray diffractometry results (Fig. 5). In the sample produced at 6 h is yet observed an undefined morphology, that probably can be non-reactive metakaolinite (Fig. 5a). In this study, comparing the SEM images of products synthesized under the same synthesis conditions (T = 110 ◦ C, molar ratio Na/Al = 1.64) using the traditional method [4] with the ones produced using the aging step, it appears that the size of the crystals of zeolite NaA was smaller for the latter method. According to the SEM images of the product synthesized by the conventional method, the crystals of zeolite NaA have sizes of approximately 10 m, whereas when the aging step is included before crystallization, the crystal size is approximately 4 m. A decrease in the size of zeolite NaA crystals was also observed by Slangen et al. [12].
4.
Conclusions
An alternative route for the synthesis of zeolite NaA was developed using waste kaolin. This waste is an excellent raw material for producing pure zeolite NaA and provides an alternative approach to minimizing environmental impact. The aging step was effective, producing zeolite NaA after 4 h, which had not been previously achieved for this time and this route (1-thermal treatment, 2-aging, and 3crystallization). The zeolite NaA reached maximum crystallization at 6 h. However, zeolite NaA can be obtained at 4 h. The zeolite NaA, synthesized in this route, display, probably, a stable structure, as there was no formation of sodalite which is theoretically the most stable phase, with increasing synthesis time.
Conflicts of interest The authors declare no conflicts of interest.
Acknowledgements We would like to express gratitude to the Pro-Rector of Research and Graduate Studies Office (Pró-Reitoria de Pesquisa e Pós-Graduac¸ão - PROPESP) of the Univeristy of Pará (UFPA) for the support and the Pará Research
Foundation (Fundac¸ão de Amparo e Desenvolvimento da Pesquisa do Estado do Pará - FADESPA) for the translation of this article.
references
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