Converting Coal Fly Ash (CFA) to Zeolite X by the Fusion Method

ABSTRACT

The possibility in converting coal fly ash (CFA) to zeolite was evaluated. CFA samples from the local power plant in Prachinburi province, Thailand, were collected during a 3-month time span to account for the inconsistency of  the CFA quality, and it was evident that the deviation of the quality of the raw material did not have significant effects on the synthesis. The zeolite product was found to be type X.

The most suitable weight ratio of sodium hydroxide (NaOH) to CFA was approximately 2.25, because this gave reasonably high zeolite yield with good cation exchange capacity (CEC). The silica (Si)-to-aluminum (Al) molar ratio of 4.06 yielded the highest crystallinity level for zeolite X at 79% with a CEC of 240 meq/100 g and a surface area of 325 m2/g. Optimal crystallization temperature and time  were 90 °C and 4 hr, respectively, which gave the highest CEC of approximately 305 meq/100 g. Yields obtained from all experiments were in the range of 50 –72%.

INTRODUCTION

Coal fly ash (CFA) is a byproduct of the combustion of coal in the power generation process. In Thailand, more than 5.1 million t of CFA was generated from power plants, with a tendency to increase every year.1 The management of CFA is therefore an economic and environmental issue. CFA can generally be used as a mixed and blended substance for concrete production, and there is evidence that it can also be applied directly for the removal of dyes4 and metals.5 The selection of application depends significantly on economical and social factors, such as the transportation of CFA between the generator and the target industry (such as a cement kiln) and public perception of the reuse of waste materials.

The main constituents in CFA are silica and alumina, which offer the potential of converting it to  zeolite.6–8 The two most common methods available for this conversion are the hydrothermal and fusion methods,9 in which the fusion method has the advantage of speed of reaction9 and purity of the final product (as high as 62% zeolite).10 However, the hydrothermal method provides a more consistent pattern of the zeolite products. For solid- phase reactions such as the conversion of CFA, the fusion method is considered more suitable; the hydrothermal method is more appropriate for reactions in the liquid phase. Because of differences in the properties of CFA raw materials, the quality of the final zeolite product could vary significantly, depending on the conditions of the selected conversion technique. For the fusion method, temperature was reported to have great influence on the final product quality and yield,9,11,12 with a suitable range of temperature for this synthesis of 500 –550 °C.9,10 The addition of reaction promoters such as sodium hydroxide (NaOH) and/or potassium chloride (KCl) to enhance the reaction activity in forming zeolite and to improve the adsorption properties by increasing the surface area of synthesized zeolite is quite common.9,10,13,14 Often the resulting zeolite has negatively charged functional groups on the surface, offering potential sorption applications particularly for the sequestration of cationic metals.15

In this work, the synthesis of zeolite from the CFA obtained from one of the actual coal-fired combined power plants in Thailand was investigated. NaOH was used to assist the reaction in the fusion process, in which several fabricating factors that could strongly affect the properties of the zeolite product were examined. These factors include the silicon (Si)-to-aluminum (Al) ratio contained in the raw material, the weight ratio between NaOH and fly ash, fusion temperature, crystallization temperature, and time. In addition to the common characteristics of the zeolite, sorption properties of the zeolite product such as the cationic exchange capacity (CEC) were also examined.

EXPERIMENTAL PROCEDURES

Methods for Characterization

Zeolite Type. The X-ray diffraction (XRD) data (from X-ray diffractometer, Bruker AXS D8 Discover) were used to calculate zeolite X percentage (or %zeolite X) where the high intensity of the distinguishable peaks (n = number of peaks, or at least 5) from the samples (Ip) and from the zeolite X standard (Is) were chosen, and

The intensity of zeolite X standard is given in Figure 2b.

Elemental Composition and Surface Morphology. An X-ray fluorescence (XRF) spectrophotometer (Philips, Model PW2400) was used to determine the overall mineral composition of the CFA. The specific surface area based on adsorption characteristics of nitrogen (N2) gas on the sample at 77 K was determined using a Brunauer– Emmett–Teller (BET) surface area analyzer, (Micromeritics, Model FlowSorb II 2300). The surface morphology was examined by scanning electron microscopy (SEM; Jeol, Model Jsm5410lv).

CEC. The CEC of the products was determined using the sodium acetate method under room temperature and atmospheric pressure according to U.S. Environmental Protection Agency Method 9081. In this procedure, 3.2 g of zeolite (quantity m in eq 2) was mixed with 1-N sodium acetate (CH3COONa) in a mechanical shaker for 5 min to saturate the sample. The mixture was then centrifuged until a clear supernatant was visible, which was then decanted from the mixture. This procedure was repeated another two times to ensure that all cations in the zeolite were replaced with sodium (Na). The Na-laden zeolite was washed with 27 mL of 99% isopropyl alcohol and shaken in a mechanical shaker for 5 min. The supernatant was removed, and the procedure was repeated one more time to ensure that the zeolite was clean and laden only with Na. The zeolite was then mixed with an ammonium acetate (CH3COONH4) solution. The same procedure was applied with CH3COONH4 two times to ensure that all Na was replaced by the ammonium ion (NH4+) and after each step, the supernatant of CH3COONa solution was decanted to a 100-mL volumetric flask and filled up with CH3COONH4 to a final volume of 100 mL. Finally, the amounts of Na ions in the exchange solutions were determined by atomic absorption spectrophotometry (AAS; Analytik Jena, Model ZEEnit 700). The CEC values were calculated based on a mass balance concept. The Na bind- ing capacity can then be calculated from

where qe is the exchanged Na ions per weight of zeolite (mg/100 g) or CEC value, co is the Na concentration  (mg/L), and cb is the Na concentration in the blank (mg/ L). co and cb were measured by AAS. V is the volume of the aqueous phase (mL), equal to 100 mL in this experiment; m is the amount of zeolite (g), equal to 3.2 g in this experiment; and MW is the molecular weight of Na or adsorbed cation (g).

Percent Yield of Zeolite. The percent yield is one important parameter that demonstrates the efficiency of the zeolite synthesis procedure. The percent yield can be calculated from

where Wzeolite is the weight of the zeolite product (g), and WCFA is the weight of the raw material (g), which was CFA.

Materials (CFA)

CFA is a byproduct of the combustion process using bituminous coal as fuel in the combustion chamber of the local coal-fired combined power plant in Prachinburi Province, Thailand. During the preliminary test, the sample CFA#0 with a Si/Al ratio of 1.71 was used to investigate the effect of the NaOH/CFA ratio. Further experiments were examined with another three CFA samples (CFA#1–CFA#3) collected throughout a period of 3 months to investigate the effect of property variation in raw materials on the product properties. 

Table 1. Elemental composition of CFA samples analyzed by XRF.

The elemental compositions of the CFA samples and the ratio between Al and Si (calculated as the molar ratio of Si and Al) in the raw materials are reported in Table 1 along with BET surface area. It should be noted that the latter three CFAs were obtained approximately 1 month apart and are re- ferred to as CFA#1, CFA#2, and CFA#3. It can be seen that the CFAs contained a reasonable fraction of Si and Al, which was considered a potential raw material for the synthesis of zeolite.

Zeolite Synthesis Procedure

The method developed by Molina and Poole11 was used in this experiment. The preliminary procedure started by mixing 10 g of CFA#0 (Si/Al = 1.71) with 10 g of ground NaOH anhydrous pellet (to make a NaOH/CFA ratio of 1) in a nickel crucible. The mixture was burnt in air at 550 °C for 1 hr in an arching furnace. The product was crushed and transferred to a 250-mL Erlenmeyer flask with a screw cap containing 85 mL of distilled water, which was then shaken in a water bath at 30 °C for 12 hr. The mixture was subsequently crystallized in an oven (90 °C) for 2 hr. The solid product was separated and washed several times with distilled water until the pH of the solution went down to 10 –11 and then dried overnight at 105 °C. The synthesis was repeated with NaOH/CFA weight ratios of 0.5, 1.25, 1.5, 1.75, 2, 2.25, 2.5, and 3. The NaOH/CFA ratio with the highest zeolite X percentage and the highest CEC value was used further with CFAs#1–3 to investigate the effect of fusion temperature (250 –550 °C), crystallization temperature (60 –120 °C), and then crystallization time (2– 6 hr).

Figure 1. Effect of NaOH/CFA weight ratio on CEC and %zeolite X. Synthetic conditions were CFA with a Si/Al ratio = 1.71, fusion temperature = 550 °C for 1 hr, CFA:water ratio = 0.12 g/mL, mixing at 30 °C for 12 hr, and crystallization at 90 °C for 2 hr.

RESULTS AND DISCUSSION

Effect of NaOH/CFA Ratio

A study of the effect of NaOH/CFA ratio was performed with CFA#0, and the results are demonstrated in Figure 1. The NaOH/CFA ratio of 2.25 seemed to be optimal and gave a product with 79% zeolite X. 

Table 2. Properties of zeolite product obtained from different synthetic conditions.

Figure 2. XRD patterns of zeolite X standard and CFA.

The purity of the zeolite product was shown to be enhanced by increasing the alkaline condition (i.e., high NaOH), which enhanced the zeolization reaction.10 However, a further increase in the NaOH/CFA ratio beyond 2.25 was not recommended because the alkalinity could activate the decomposition  of Si and Al to sodium silicate and aluminate in the framework,16,17 or could form undesired products such as hydroxysodalite instead of Na-X.18 In addition, Breck19 stated that the crystal framework became unstable at extremely high alkalinity. In terms of adsorption capacity, CEC increased gradually within the low NaOH/CFA range and reached the maximum of approximately 153 meq/ 100 g at a NaOH/CFA of 2.25. However, the percent yield decreased with the quantity of NaOH used in the synthesis; for example, at a NaOH/CFA of 1.75, the percent yield was 61%, and this dropped to 43% at a NaOH/CFA of 2.25 and gradually dropped further to 38% at a NaOH/CFA of 3 (see Table 2 and Figure 1). Hence, the ratio of NaOH/ CFA of 2.25 was used in the following experiments.

Effect of Raw Material  Properties (Si/Al Ratio)

The effect of variation in the raw material was examined. The different CFAs (CFAs#1–3) contained different Si/Al ratios and BET surface areas, as shown in Table 1. The XRD pattern in Figure 2a illustrates that the CFA samples were amorphous (only XRD of CFA#2 is illustrated in this figure). However, the XRD patterns of the zeolite from different CFA samples (or hereafter referred to as Z-CFAs#1, 2, and 3 for the zeolite generated from CFAs#1, 2, and 3, respectively) in Figure 3a–3c suggested a possible formation of zeolite X. The highest peak intensity was obtained with CFA#2 (Si/Al = 4.06 in Figure 3b) at the fusion temperature of 550 °C, whereas the lowest was with CFA#1 (Si/Al = 2.22 in Figure 3a). These results are consistent with the finding of Somerset et al.,16 Breck,19 and Rabo and Schoonover,20 who stated that zeolites A, X, Y, and faujasite could form from CFA with Si/Al ratios in the range of 2–5.

As demonstrated in Table 2, at the fusion temperature of 550 °C, %zeolite X increased from 57 to 79% with an increase in the Si/Al ratio from 2.22 (CFA#1) to 4.06 (CFA#2). 

Figure 3. XRD patterns from zeolite products prepared at different fusion temperatures (250 –550 °C) and different Si/Al ratios (Si/Al = 2.22– 4.63) for (a) CFA#1, (b) CFA#2, and (c) CFA#3.

Figure 4. Effect of fusion temperature and Si/Al ratio on CEC and %zeolite X. Synthetic conditions were fusion time = 1 hr, NaOH:CFA ratio = 2.25, CFA:water ratio = 0.12 g/mL, mixing at 30 °C for 12 hr, and crystallization at 90 °C for 2 hr.

A further increase in the Si/Al ratio to 4.63 (CFA#3) caused the %zeolite X to drop to 65%. Similar results were also obtained at 450 °C, where CFA#2 gave the highest %zeolite X. However, at fusion temperatures lower than 450 °C, the effect of the Si/Al ratio on %zeolite X became insignificant (results not shown). CFA#2 seemed to have the highest composition of calcium (at 4.12% by weight), and this seemed to have some relationship with the observed %zeolite X as stated in the report by Somerset et al.16 that higher calcium content in raw materials could lead to a formation of zeolite X with higher purity.

The observed surface area in the zeolite X products seemed to vary according to %zeolite X; that is, Z-CFA#2 exhibited the highest surface area of 325 m2/g, whereas Z-CFAs# 3 and #1 provided the areas of 283 and 236 m2/g, respectively. However, the CEC level of the zeolite product depended more significantly on the effect of the incorporation of an iron framework during formation.19 This meant that Z-CFA with the highest iron content could introduce negatively charged sites on the surface and therefore increase the CEC value9,21,22; that is, the CEC of the Z-CFA#1 was the highest whereas the lowest was for Z-CFA#3.

Note that the percent yield of all types of raw materials was quite similar, all in the range of 55– 68% as shown in Table 2. In subsequent experiments, only CFA#2 would be used as an initial raw material.

Effect of Fusion Temperature

The effect of fusion temperature is illustrated in Figure 4. %zeolite X increased rapidly from 40 to 67% with an increase in the fusion temperature from 250 to 450 °C, after which %zeolite X further increased but in a slower rate to 79% at 550 °C. This result was supported by similar findings of Molina and Poole,11 Ojha et al.,23 and Shigemoto and Hayashi.10 

Figure 5.  Effect of crystallization temperature on %zeolite X and CEC (meq/100 g). Synthetic conditions were fusion temperature = 450 °C for 1 hr, NaOH:CFA ratio = 2.25, CFA:water ratio = 0.12 g/mL, mixing at 30 °C for 12 hr, and crystallization for 2 hr.

Figure 6. Effect of crystallization time (2– 6 hr) on %zeolite X and CEC (meq/100 g). Synthesis conditions were fusion temperature = 450 °C for 1 hr, NaOH:CFA ratio = 2.25, CFA:water ratio = 0.12 g/mL, mixing at 30 °C for 12 hr, and crystallization at 90 °C.

The CEC of the Z-CFA#2 rose from 206 to 241 meq/100 g with an increase in the fusion temperature from 250 to 450 °C, and after that it gradually dropped to 240 meq/100 g at the fusion temperature of 550 °C. This corresponded well with the changes in the surface area; that is, the surface area increased with fusion temperature and reached the highest of 250 m2/g  at 450 °C, whereas at 550 °C, the area slightly dropped to 236 m2/g. Subsequent experiments were conducted at the fusion temperature of 450 °C for energy saving purposes. Note that there was no exact pattern in the variation in percent yield. This could be due to the variation in the properties of the raw material.

Effect of Crystallization Temperature Crystallization is the process that forms the crystal framework of the zeolite and this occurs faster at higher crystallization temperatures.24 This was reflected in the results where %zeolite X of Z-CFA#2 increased linearly from 30 to 67% in the range of crystallization temperature from 60 to 90 °C as demonstrated in Figure 5. However, temperatures above 90 °C saw a drastic drop in %zeolite X, likely caused by water dehydration and the formation of undesired zeolite products with low water content. Moreover, Figure 5 also demonstrates that the CEC value of Z-CFA#2 increased from 174 to 241 meq/100 g in the range of crystallization temperature from 60 to 90 °C, and then declined to 203 meq/100 g at 120 °C. Similarly, the surface area increased from 99 to 289 m2/g with an increasing crystallization temperature from 60 to 90 °C, and then dropped to 215 m2/g at 120 °C.

Effect of Crystallization Time

Figure 6 depicts the effect of duration of crystallization process. The range of crystallization time from 2 to 6 hr of CFA#2 enhanced the %zeolite X from 67 to 79%. However, the CEC value of Z-CFA#2 increased from 241 to 305 meq/100 g when increasing crystallization time from 2 to 4 hr. A further increase in the crystallization temperature to 6 hr caused the CEC to reduce to 216 meq/100 g. The variation of the CEC might be due to the changes in the surface area, which was approximately 275 and 320 m2/g at 2 and 4 hr of crystallization, respectively, and was only 236 m2/g at 6 hr. A reaction time that was too long could cause a collapse in the zeolite framework, which then reduced the surface area. Moreover, a previous report stated that a crystallization time that was too long would potentially change the zeolite structure to zeolite type P.10 –12,23,25,26

Comparison of Zeolites of Different Origins

An SEM photograph of the zeolite product is illustrated in Figure 7, which confirms the formation of zeolite X. In this work, the comparison is based on the CEC, which represents the capability of the zeolite to adsorb metals such as sodium or calcium ions.

Figure 7. SEM image of the zeolite product with Si/Al ratio = 4.06, fusion temperature = 450 °C for 1 hr, NaOH:CFA ratio = 2.25, CFA:water ratio = 1.2 g/mL, mixing at 30 °C for 12 hr, and crystallization at 90 °C for 2 hr.

Table 3. Comparison of CEC values obtained from zeolites with different origins.

Table 3 provides the CEC properties of some zeolite products produced from waste materials, particularly fly ash. It could be seen that CEC could range from as high as approximately 410 meq/100 g to as low as 46 meq/100 g. In this work, the CEC of the zeolite produced ranged among that of the top-quality zeolites. When compared with the conversion conditions from other reports, the conditions proposed in this work were relatively milder than the others; that is, the reaction was achieved at relatively low fusion and crystallization temperatures and a shorter crystallization time.

CONCLUSIONS

This work focused on an alternative synthesis method of zeolite X and illustrates that CFA could be converted to zeolite X by the fusion method. The effects of various synthesis parameters such as the NaOH/CFA weight ratio, type of CFA material, fusion temperature, crystallization temperature, and time were examined where the following conditions seemed to be the best: a NaOH/CFA weight ratio of 2.25, Si/Al ratio of 4.06, fusion temperature of 450 °C, crystallization temperature of 90 °C, and a crystallization duration of 4 hr. Under these conditions, the zeolite product could achieve a relatively high CEC value, the synthesis is given in Tables 4 and 5, which demonstrated that the total expense per each batch, or approximately 68 g of zeolite product, was approximately $6.50 (U.S.) or $96 (U.S.) per kilogram of zeolite. This is considerably low when compared with the current global price of zeolite X (~$243.42 [U.S.] per 10 g; from Sigma- Aldrich). However, commercial-grade zeolites costs range from $25 to $80 (U.S.), which is lower than the actual production cost. Nevertheless, it should be noted that this economic calculation was based solely on the laboratory prices when the chemicals were of high quality. In actual industrial scenarios, it is likely that waste heat and commercial-grade chemicals are used, which should significantly lower the production cost.

Table 4. Estimated electrical production cost of 1 kg of zeolite X

Table 5. Estimated material production cost of 1 kg of zeolite X generated from CFA.29

Source: Duangkamol Ruen-ngam and Doungmanee Rungsuk - Chulalongkorn University, Ronbanchob Apiratikul - Suan Sunandha Rajabhat University, Prasert Pavasant - Chulalongkorn University

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