National Technical University of Athens, 9 Iroon Polytechniou Street, Zografou Campus, GR , Athens, Greece

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1 Synthesis of zeolitic materials utilizing CFB-derived coal fly ash as a raw material Angeliki Moutsatsou a*, Olga K. Karakasi a, Nikolaos Koukouzas b, Grigorios S. Itskos b and Charalambos Vasilatos c a Laboratory of Inorganic and Analytical Chemistry, School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechniou Street, Zografou Campus, GR , Athens, Greece b Centre for Research and Technology Hellas, Institute for Solid Fuels Technology and Applications, Mesogeion Avenue, GR , Halandri, Athens, Greece c Department of Geology, National University of Athens, Panepistimioupolis, Ano Ilissia, GR , Athens, Greece Abstract Two different fly ashes (FAs) have been tested for their ability to give synthetic zeolitic products. Polish bituminous (PB) and South African (SA) coal fly ash (FA) samples derived from pilot-scale circulated fluidized bed (CFB) combustion facilities have been utilized as raw material. The two FAs underwent a hydrothermal activation with 1M NaOH solution at 90 C for 24 h. Two different FA/NaOH solution/ratios (50, 100 g/l) were examined for each FA. For lower FA/NaOH solution ratios, the excess of NaOH seems to be prohibitive for the zeolite growth while for a ratio higher than 100 g/l the NaOH quantity seems to be insufficient for the effective activation of the FAs. The experimental products were characterized by means of X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) while X-Ray Fluorescence (XRF) was applied for the determination of their chemical composition and the trace elements concentration. Regarding the PB-CFB FA, the following zeolitic materials were formed: Na-Al-Si Zeolite A (Na), K-Al-Si Zeolite A (K) and Erionite. For the SA-CFB FA, the zeolitic materials formed were: ZSM-18, K-Ca-Al-Si-Hydrate Unnamed Zeolite, Erionite and Linde (L). The zeolitic products were also evaluated in terms of their cation exchange capacity (CEC), specific surface area (SSA), specific gravity (SG), particle size distribution (PSD), ph and the range of their micro- and macroporosity. The products were characterized by finer particle size concerning SA-CFB FA, while PB-CFB FA showed an increase of the percentage of the fine particles. The ph value of FAs before and after the hydrothermal activation for all the proportions is 11.3 to 11.9 and a condensation occurs by nucleophilic attack of OH -. The values of CEC, SSA along with the efficiency of pores were higher for FA/NaOH = 100 g/l. The current work focuses on testing the synthesized zeolitic materials of the aforementioned zeolification process for their potential of retaining heavy metals from a contaminated soil. Keywords: CFB fly ash; synthetic zeolites; alkaline activation; bituminous coal 1. Introduction The combustion of solid fuels using conventional combustion technologies dominates the coal-burning power production. However, more environmental friendly technologies, such as the CFB combustion technology, continuously gain ground. * Author/s to whom correspondence should be addressed: angst@central.ntua.gr (A.Moutsatsou)- Telephone: Fax:

2 Therefore, the amounts of CFB-derived coal fly ash, are steadily increasing, as a result of the continuous development of the CFB technologies (Gutierrez et al., 1993, Koukouzas et al., 2007). It is obvious that alternative applications should be developed in order to recycle the high FA output (Itskos S., 2000). The production of zeolites is one of the potential applications of fly ash production to obtain high value industrial products with environmental technology utilization (Querol et al, 1999). The synthesis of zeolite products from fly ash is analogous to the formation of natural zeolites from volcanic deposits or other high-si-al materials (Steenbrugen and Hollman, 1997, Querol et al, 2002, Itskos G. 2006). Both volcanic ash and FA are fine-grained and contain large amount of active aluminosilicate glass. One of the processes from which zeolites can be naturally formed is through the influence of hot groundwater on the glass fraction of volcanic ash. The particular zeolitic development may take thousands of years in order to form natural zeolites. In the laboratory the process can be speeded up (to days or hours) for both volcanic ash and FA. In that case the activation solution used is an alkaline one, usually NaOH or KOH. The classical alkaline conversion of fly ash is based on the combination of different activation solution/fa ratios with temperature, pressure and reaction time, to obtain different zeolitic types. The methodologies developed on this field aim at the dissolution of Al-Si bearing phases of the FA and the subsequent precipitation of the zeolitic material (Querol et al, 2001; Querol et al, 2006, Moutsatsou et al., 2003; Moutsatsou et al., 2006). NaOH or KOH solutions with different molarities, at atmospheric and water vapor pressure, from 80 to 200 C and 3 to 96 h have been combined to synthesize many different types of zeolites (Moutsatsou and Protonotarios, 2006). The zeolitic content of the resulting products varies depending on the solution/fly ash ratio and reaction time. All the aforementioned procedures utilize coal fly ash derived from conventional combustion and very little research has been conducted on the field of CFB-derived coal FA utilization. The present study deals with the hydrothermal activation of two coal FAs produced in pilotscale CFB combustion facilities. 2. Materials and Methods and derived from CFB facilities underwent an alkaline hydrothermal treatment at 90 C, using NaOH 1M as an activation solution, in a 1L stainless steel reactor. The incubation period was set at 24 h and mixing took place at 150 rpm. After that period, the mixture was filtered and the solid residue was dried at 40 C for 24 h and leached with water until no NaOH was detected. Chemical analyses of PB- and SA- CFB-FAs as raw material as well as zeolitic products were performed by X-Ray Fluorescence (XRF). The mineralogical composition of the zeolitic materials was identified by X-Ray Diffraction (XRD) and Scanning Electron Microscope (SEM). The synthesized zeolitic materials were subjected to N 2 adsorption using BET Method in order to determine their specific surface area (SSA), while their cation exchange capacity (CEC) was evaluated following the US.EPA 9081 Method (sodium acetate). The particle size distribution (PSD) of the initial FAs and the synthesized zeolitic materials was determined by MalVern Mastersizer-S using the wet dispersion method in water. The study of the range of macro- and microporosity in the synthesized zeolitic materials was performed using a porosimeter Autosorb-1 (with crypton analysis, optimum for microporosity) made by Quanta-Chrome. Finally, the ph (ISO 6588) and the specific gravity (SG-ASTM C642) of the initial FAs and the synthesized zeolitic materials were also evaluated. 3. Results and Discussion 3.1 Zeolite synthesis

3 In the present study, NaOH was selected as an activation solution, since its solutions present higher conversion efficiency than the respective KOH under the same temperature (Steenbrugen and Hollman, 1997, Moutsatsou et al, 2006). The experimental conditions (NaOH concentration, temperature) are typical for a pure alkaline activation, taking place at low temperatures and intermediate activation periods (Querol et al, 2001, Moutsatsou 2006). The applied techniques mainly aim at the dissolution of Al-Si bearing phases of the FA and the subsequent precipitation of the zeolitic materials. Table 1 illustrates the impact of the alkaline activation on the chemical composition of the initial fly ashes. Table 1. Effect of hydrothermal activation on the major chemical compounds of CFB fly ashes Before hydrothermal After hydrothermal activation activation Compound (50g/L) (100g/L) (50g/L) (100g/L) SiO Al 2 O Na 2 O SO CaO The final solid products were subjected to XRD analysis and SEM investigation for the identification of known zeolites. The results are presented in Table 2, where mineral phases were also identified and attributed to the initial fly ashes. The formation of the aforementioned zeolitic products was also confirmed by the SEM investigation (Figures 1-6). In Figures 1 and 2 is obvious the zeolitic grain of treated samples (included in Table 2). Figures 3-6 include the SEM photos of the alkaline-treated (50 and 100 g/l FA/NaOH ratios). In these images, along with the identified zeolites for samples, the crystallized remaining NaOH is detected. Table 2. Known zeolites and mineral phases present in the synthesized zeolitic materials Mineral 50 g L g L g L g L - 1 Quartz Calcite Magadiite + + Lime + + Hematite Portlandite Illite + + Zeolite A (Na) + + Zeolite A (K) + + Unnamed Zeolite + + ZSM Linde (L) + Erionite + + +

4 Figure 1: SEM photo of PB, 50 g FA/1L NaOH-analysis 5μm. Figure 2: SEM photo of PB, 100 g FA/1L NaOH-analysis 5μm. Figure 3: SEM photo of SA, 50 g FA/L-analysis 5μm. Figure 4: SEM photo of SA, 100 g FA/L-analysis 10μm. Figure 5: SEM photo of SA, 50 g FA/L-analysis 2μm. Figure 6: SEM photo of SA, 100 g FA/L-analysis 2μm.

5 12, ,5 ph 11,8 11,6 11,4 11,2 11 Specific Gravity (g/ml) 2 1,5 1 0,5 10, Figure 7: PH evolution as a function of FA/NaOH ratio Figure 8: SG evolution as a function of FA/NaOH ratio 3.2 ph measurement The ph values of and along with those of the synthesized zeolitic materials were evaluated and plotted as a function of FA/NaOH ratio (Figure 7). The ph is an important parameter considering possible future utilization of the zeolitic materials. In both cases of CFB-coal fly ashes, ph is higher than this of the initial FAs, due to the excess of the alkali concentration as it is concluded from table 1., 3.3 Specific Gravity Hydrothermal treatment of FA has opposing impacts on SG of the final products. As long as the NaOH solution penetrates the cenospheres of the initial fly ashes, allows the trapped air to escape, thus increasing the SG. Nevertheless, as the zeolitization process progresses, the subsequent crystallization of the experimental products yields to a larger pore volume, thus decreasing the SG. For sample, SG values decrease constantly as FA/NaOH ratio increases. On the other hand, regarding sample, the lowest SG is for 50 g/l FA/NaOH ratio and increases importantly for 100 g/l FA/NaOH ratio, almost reaching the initial FA value. Figure 8 illustrates the evolution of SG as a function of FA/NaOH ratio. 3.4 Specific Surface Area In order the specific surface area (SSA) of the synthesized zeolitic materials to be determined, they were subjected to N 2 adsorption using BET Method. In general, the SSA of the raw FA increases significantly after the alkaline activation, thus enhancing the efficiency of the material, mainly regarding soil remediation procedures. In Figure 9, the comparison of the SSA of the synthesized zeolitic materials is presented. The results for the treated CFB fly ashes indicate their upgraded potential of retaining contaminants from polluted soils. activated under the FA/NaOH ratio of 100 g/l shows the highest value of SSA, increased to the extent of 220% in respect with the same sample under 50g FA/L NaOH and to 190% for treated s (Figure 9).

6 45 0,3000 Specific Surface Area (g/m2) Total Pore Volume (ml/g) 0,2500 0,2000 0,1500 0,1000 0, , Figure 9: SSA (g/m 2 ) of the synthesized zeolitic materials Figure 10: Total Pore Volume (ml/g) of the synthesized zeolitic materials 3.5 Range of porosity, micro-porosity and total volume of the pores The range of the macro- and microporosity in the synthesized materials, along with the total volume of their pores, are crucial factors concerning the industrial applications of the zeolitic materials. The results of the pore characterization are very encouraging. The total pore volume of the material before treatment (normally about 0.01 ml/g) increases significantly after the conversion (up to 0.26 ml/g). Additionally, the synthesized materials include a substantial range of porosity that can reach even the point of 37.5% of the total volume of the material. The range of porous structure in the synthesized zeolitic materials is, to a very important degree, greater in comparison with the normal respective amount in coal fly ashes. treated with 100g/L FA/NaOH ratio presents the most desired properties, concerning the remediation procedures for contaminants with greater ionic radius, while treated with 100g/L FA/NaOH ratio can be easily applicable as smaller ionic retainer. Figures illustrate the differences between the porous structure of the treated materials. 40 3, porosity (%) micro-porosity (%) 2,5 2 1, , Figure 11: Porosity (%) of the synthesized zeolitic materials Figure 12: Micro-porosity (%) of the synthesized zeolitic materials

7 3.6 Particle size distribution The alkaline hydrothermal treatment resulted in a decrease in the mean particle size of the initial material. Half of particles activated under 50 and 100 g/l FA/NaOH ratio lie <22.5 μm, while no particles coarser than 107 and 140 μm, respectively, were detected. Regarding for both FA/NaOH ratios half of the particles were found to have a mean diameter < 27 μm, whereas they do not have particles with diameter >110μm. It must be noted that fine-grained materials are more effective, when utilized for heavy metals retaining, a procedure that is also strongly dependent on the nature of the contaminated soils. 3.7 Utilization of the synthesized zeolitic materials Major prospective applications of the synthesized zeolitic materials are based on their use as high ion exchangers in industrial waste water and soil decontamination. CEC (cation exchange capacity) values are increased in comparison with the normal respective values of the initial fly ashes, reaching a maximum of 1,2 mequiv/g ( activated with 100 g/l FA/NaOH). In general, the results are very encouraging about the future industrial applications of the synthesized zeolitic materials. Additionally, the hydrothermal activation of fly ashes is a sustainable procedure that can be applied in large-scale, thus leading to the significant financial upgrade of the main by-product of power production. In this case, the final products are hybrid materials that combine the preferred properties of fly ash and zeolites in the field of soil reconditioning. The further investigation of the synthesized zeolitic materials, which will be conducted in the near future, includes the test of their ability of precipitating or adsorbing heavy metals by using Cu-Zn-Cd-Pb solution. The results will be mainly linked with the porosity range of the materials, thus confirming the connection of the estimated materials properties with their industrial applications potential. 4. Conclusions Chemical and mineralogical properties of two CFB-derived coal fly ashes make them a suitable raw material for the synthesis of hybrid zeolitic materials with upgraded potential for soil remediation applications. Keeping temperature, NaOH concentration and incubation time constant, the effect of FA/NaOH ratio on zeolite synthesis was investigated. The best FA/NaOH for Polish bituminous CFB-coal fly ash is 100 g/l regarding both quantitative zeolite synthesis and desired properties of the experimental products. As far as South African CFB-coal fly ash is concerned, both ratios are equally effective for mono-mineral zeolite synthesis as well as for the required characteristics of the synthesized zeolitic materials for the contaminated soils reconditioning. Cation exchange capacity, specific surface area and porous structure of the experimental products are improved with respect to the initial CFB-coal fly ashes. All the synthetic materials will be tested in the near future for their capacity in retaining heavy metal from the contaminated soil and wastewater. Acknowledgements The authors gratefully acknowledge financial support from the European Commission Research Fund for Coal and Steel, under contract RFCR-CT Doctor N.K. Kanellopoulos is also acknowledged for the porosity measures performed in the Institute of Materials Science of the NCSR Demokritos.

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