Chemical composition of coal fly ash and synthesized zeolites.
South African coal combustion power utilities generate huge amounts of coal fly ash that can be beneficiated into zeolitic products. This chapter reports on the optimization of the presynthesis and synthesis conditions for a pure‐phase zeolite Na‐P1 from selected South African coal fly ashes. The hydrothermal treatment time, temperature, and molar quantities of water during the hydrothermal treatment step were successfully optimized. The optimum hydrothermal treatment time and temperature were 48 h and 140°C, respectively. Pure‐phase zeolite Na‐P1 was obtained with a molar regime of 1 SiO2:0.36 Al2O3:0.59 NaOH:0.49 H2O at an aging temperature of 47°C for 48 h. The optimized conditions were applied to two fly ashes from two coal‐fired power utilities, and high‐purity zeolite Na‐P1 was obtained. The third coal fly ash with a different chemical composition gave a low‐quality Na‐P1 under the optimized conditions. The cation exchange capacity for the high‐purity zeolite phase was 4.11 mEq/g, indicating that the adjustment of reactant composition and presynthesis or synthesis parameters leads to yields of high‐quality zeolite Na‐P1. The results also show that conversion of the coal fly ash into high‐purity zeolite also depends on the chemical and mineralogical composition of the coal fly ash.
- zeolite Na‐P1
- coal fly ash
- hydrothermal synthesis
- aging step
South Africa relies mainly on coal combustion for power generation . Low‐grade bituminous coal is combusted for power generation; this in turn generates huge volumes of waste materials such as bottom ash, fly ash, boiler slag, flue gas desulfurization sludge, and noncaptured particles [1, 2]. Of most concern is fly ash, which is mainly collected from flue gases by means of mechanical devices [3, 4]. After collection, fly ash is hydraulically transported to holding ponds, lagoons, landfills, and slag heaps, where it can be reacquired for treatment purposes or discarded or conveyed to ash heaps in dry disposal systems. The disposal and management of the huge quantities of coal fly ash has been a concern to industrial environmental managers and the scientific community and are in constant search for bulky beneficial utilization of coal fly ash to offset management costs and protect the environment. Approximately 27 Mt of fly ash is produced annually in South African power utilities . Of this, only 5% is utilized beneficially. The remainder is discarded in surface impoundments, such as ash retention dams, where it continues to evolve chemically and mineralogically with possible environmental impacts . Several researchers have proposed strategies for bulky utilization of fly ash, which include (i) applications as an additive in the manufacturing of cement, concrete, construction materials, and road pavements [6, 7], (ii) utilization of fly ash in zeolite synthesis for wastewater treatment [8–14], and (iii) use of fly ash in the neutralization of acid mine drainage and mine backfill [1, 15]. Most of these approaches are an attempt to beneficially use coal fly ash to supplement the cost of disposal and management and to reduce the negative environmental impact.
The application of coal fly ash in zeolite synthesis is of major interest due to the many industrial applications of zeolites, such as catalysis or catalyst carriers, adsorbents for the removal of inorganic and organic contaminants from wastewaters, management of radioactive wastes, gas separation, slow‐release fertilizer, and in the manufacture of detergents . The conversion of these low‐cost waste products into products of higher value such as zeolites would allow the beneficiation of fly ash in an environment‐friendly condition in addition to economic benefits. South African class F coal fly ash has been confirmed to be a good feedstock of Al and Si for zeolite synthesis because of its compositional dominance of aluminosilicate and silicate phases [14, 16]. Zeolite synthesis from fly ash is one of the potential environmentally useful applications of fly ash to produce high‐value industrial products . The major potential applications of zeolites synthesized from fly ash are based on their use as high capacity ion exchangers in industrial water waste treatment due to their large pore volumes [8, 10, 17]. Zeolites have also proven to be good candidates for use in soil decontamination  and have also shown great potential for use in the removal of postcombustion gases such as SOx and NOx .
Through various synthesis methods, researchers have synthesized various types of zeolites from fly ash, such as zeolite Na‐P1 , zeolite A [21, 22], and zeolite ZSM‐5 . However, few studies have been successful in the conversion of fly ash into pure‐phase zeolites . Querol et al.  evaluated the synthesis of zeolites from fly ash at pilot scale using various Spanish fly ashes. The authors observed that, to obtain pure‐phase zeolites, the optimum synthesis conditions have to be established for each coal fly ash. They attributed this to differences in mineralogical and chemical composition. South African coal fly ashes differ significantly in the chemical and mineralogical composition and this formed the basis of this work, as they have not been evaluated before as feedstocks to synthesize pure‐phase zeolites.
The zeolite Na‐P1 was chosen as the model zeolite to test our hypothesis. Zeolite Na‐P1 belongs to an important group of zeolites that can be synthesized under mild hydrothermal synthesis conditions without using templates, making them potentially economically viable and a green product. The narrow particle size distribution of the zeolite Na‐P1 coupled with the micron‐sized crystallites and the unusual framework flexibility gives this zeolite unique, favorable ion exchange and water sorption properties . Very few studies have reported on the synthesis of pure‐phase zeolites, although numerous studies have been carried out in the synthesis of zeolites from coal fly ash using modified synthesis procedures [13, 21, 26]. Most of the studies have yielded low‐quality zeolites due to the incomplete conversion of the coal fly ash matrix into zeolitic phase. Successful transformation of South African fly ash into impure zeolite P using the mild temperature method (100°C) was achieved by Petrik et al. .
Szostak  classified the factors affecting the zeolite crystallization as composition of the reaction matrix, time of reaction, temperature, and history‐dependent factors such as stirring, aging, nature of mixture, and order of mixing. Casci  observed that the variation of individual reaction components and reaction variables, such as reaction temperature, alkaline concentration, and time of synthesis, influence the type of zeolite synthesized, the quality of the zeolite, and also the efficiency of the synthesis process. To achieve the complete dissolution of coal fly ash matrix and conversion into the zeolitic phase, the modification of presynthesis steps and synthesis and postsynthesis conditions is recommended. A two‐step synthesis procedure developed by Hollman et al. was adapted for our optimization procedures using South African coal fly ashes . The optimized parameters were hydrothermal treatment time, temperature, and water content during hydrothermal treatment. This chapter reports on the successful conversion of South African coal fly ash into pure‐phase Na‐P1 zeolite.
2. Sampling and experimental procedures
2.1. Sample handling
Pulverized coal fly ashes were collected from the ash collection systems at three different coal power utilities in Mpumalanga, South Africa. These coal fly ashes were used as feedstock for the zeolite synthesis. The sampled coal fly ash was stored in tightly locked plastic containers to prevent the ingress of moisture and CO2 and out of direct sunlight. Coal fly ash consists of metastable phases and reactive components, which were formed during high‐temperature combustion, and they are likely to react with CO2 and moisture in the atmosphere forming new products. This might alter the chemical and mineralogical composition of the coal fly ash .
2.2. Synthesis procedures and equipment
A two‐step process for the synthesis of zeolites from fly ash adopted from Hollman et al.  was followed, whereby a mixture of coal fly ash and alkaline solution was subjected to (1) aging and (2) hydrothermal treatment.
The setup for the aging process is shown in Figure 1. In this aging step, coal fly ash was mixed with sodium hydroxide (NaOH) pellets in a ratio of 1:1. The NaOH pellets were first dissolved in 100 mL ultra‐pure water in a separate beaker and then added to the coal fly ash in a 250 mL HDPC temperature‐resistant sealable bottle, and a magnetic bar was added and the mixture in the sealed bottle was then heated on a magnetic stirrer hotplate. The hotplate was adjusted to a predetermined temperature of 47°C, which was controlled by a temperature probe, while the speed of rotation was set at 800 rpm. The aging temperature and time were kept constant at 47°C and 48 h, respectively, as reported from a previous study , to be optimum for the dissolution of aluminosilicate matrix of the coal fly ash in alkaline media.
For hydrothermal treatment, varying amounts of ultra‐pure water were added to the slurry after aging, the mixture was stirred, and the resulting homogenous solution was transferred in aliquots of 10 mL into a 23 mL Parr bomb. The crystallization of the feedstock was achieved by placing the mixture in sealed Parr bombs in a thermostated Memmert hot air oven for a predetermined time and temperature (100–160°C at intervals of 20°C with parallel time variation of between 12 and 48 h at intervals of 12 h).
After hydrothermal treatment at the predetermined time and temperature, the sample bearing Parr bombs were removed from the oven and allowed to cool down to room temperature. The cooled mixture was filtered to obtain the filtrates and the solid residue products. The pH of the filtrate was determined, whereas the solid product was washed thoroughly with ultra‐pure water until the washing attained a pH of 9 to 10. The supernatant solution was acidified and kept refrigerated until analysis for chemical species. When the pH of the rinse solution had reached 9, the phases were separated and the solid product was recovered and dried overnight at 90°C and then transferred into airtight plastic containers before characterization. The pH and EC of solutions and supernatants were measured using HANNA HI 991301 portable pH/EC/TDS/temperature meter, which was calibrated before taking the measurements.
The chemical composition of the coal fly ash and the synthesized zeolitic materials was carried out using X‐ray fluorescence spectroscopy (XRF; Philips PW 1480 X‐ray spectrometer). The samples were prepared by mixing 9 g of coal fly ash or zeolitic material with 2 g of a binder (10% C‐wax binder and 90% EMU powder). The mixture was then thoroughly shaken, poured into the mould, and pelletized at a pressure of 15 tons for ∼1 min using a Dickie and Stockler manual pelletizer. Loss on ignition (LOI) was measured by placing the samples in the furnace at 1000°C for at least 45 min. The instrument operating conditions for major element analysis were on a fused glass bead at 40 kV and 50 mA tube, whereas those for trace species were on a powder briquette at 50 kV and 40 mA tube operating conditions.
The samples for X‐ray diffraction (XRD) were ground to a fine powder. The qualitative and quantitative XRD analyses of the coal fly ash and synthesized zeolite samples were done by placing the powder samples in a sample holder. The sample pattern was generated by a Philips X‐ray diffractometer with Cu Kα radiation. The crystalline phases were identified by matching the obtained XRD profile with the powder diffraction file data (Joint Committee of Powder Diffraction Standards) files for inorganic compounds. The quantification of crystalline and amorphous phases was done as follows: After the addition of 20% Si (Aldrich; 99% purity) for the determination of amorphous content, the samples were milled in a McCrone micronizing mill. XRD profiles were acquired with a PANalytical X'Pert Pro powder diffractometer with X'Celerator detector and variable divergence and receiving slits with Fe filtered Co Kα radiation generated at 20 mA and 40 kV. The phases were identified using X'Pert Highscore Plus software. The relative phase amounts (wt.%) were estimated using the Rietveld method (Autoquan Program).
The morphology of the coal fly ash and the zeolitic products were examined using a scanning electron microscope (Hitachi X‐650, Tokyo, Japan) with a compact detecting unit lead detector at 25 kV. The samples were mounted on aluminum pegs and sput coated with a thin film of gold for conductivity. For analysis by transmission electron microscopy (TEM), the samples were prepared by diluting a suspension of the synthesis products in ethanol, ultrasonicating, and depositing a drop onto S147‐4 Holey carbon film 400 mesh Cu grids. A 200 kV field emission gunlens 1 was used with spotsize 3 at 200 kV using high‐resolution TEM (HRTEM) Tecnai G2 F20 XTwin MAT.
The cation exchange capacity (CEC) was determined following the methods of Amrhein et al. . The untreated coal fly ash was first saturated through three repeated rinsing steps with 1.0 M sodium acetate (pH 8.2) followed by four washings with ultra‐pure water. The extraction of the exchangeable cations was done with three aliquots of 1.0 M ammonium acetate (pH 8.2). A 0.5 g of the zeolitic products was extracted in succession with four 25 mL aliquots of 1.0 M ammonium acetate (pH 8.2). On the addition of the 25 mL extracting solution, the mixture was then continuously agitated for 15 min, centrifuged for 15 min, and decanted. The procedure was repeated four times and the cumulative extract was collected for each sample. The concentrations of exchangeable cations (Na+, Mg2+, Ca2+, and K+) in the final solution were determined by inductively coupled plasma‐atomic emission spectrometry. The CEC was then reported as mEq/g sample.
The surface area and pore size determination of the zeolitic products were carried out using the Brunauer‐Emmett‐Teller (N2‐BET) surface analysis technique. The samples (0.35–0.5 g) were first outgassed at 110°C using helium gas. Micromeritics Tristar instrument (Tristar3000, Micromeritics, Norcross, GA, USA) was used with nitrogen as the analysis gas based on a 5 point with 30 adsorption and 30 desorption points.
3. Results and discussion
3.1. Characterization of coal fly ash and zeolitic products
The chemical composition of the coal fly ash feedstock and the zeolitic products reported as oxides is presented in Table 1.
|Coal fly ash A
|Coal fly ash B
|Coal fly ash C
For all coal fly ash feedstock, the SiO2+Al2O3+Fe2O3 is ≥70%, meaning the fly ashes can be classified as class F fly ash (ATSM method C 618). This is consistent with coal fly ash from the combustion of bituminous coal from South Africa . The LOI ranged from 3.85% to 6.59% (w/w). LOI represents the unburned carbon in coal fly ash. The mean SiO2/Al2O3 ratio ranged from 1.65 for coal fly ash A, 1.57 for B, and 2.01 for C. The SiO2/Al2O3 ratio is important in that it directly governs both the Si/Al ratio of the zeolite product and the incorporation of Al in the framework structure. The presence of CaO and MgO plays a significant role in zeolite synthesis. The Ca2+ and Mg2- ions act as competing cations during synthesis . The lower content of CaO in coal fly ash C could result in the lower alkalinity of the solution during aging and hydrothermal synthesis, which could affect the rate of depolymerization and monomerization process . The SiO2/Al2O3 for zeolite A (from coal fly ash A) was 1.45 less than in the feedstock fly ash, indicating the inefficient conversion of the feedstock. Na2O was observed to be higher in all the zeolites compared to the feedstock; this is because of the incorporation of Na as a charge‐balancing cation, as NaOH was used as the alkaline coal fly ash matrix dissolution agent.
The qualitative XRD results of coal fly ash A are presented in Figure 2 (note that the XRD profile is similar for coal fly ashes B and C and hence not shown), whereas Figure 3 shows a summary of the quantitative analysis of the three coal fly ashes.
The qualitative XRD analysis indicated the presence of the following mineral phases in all coal fly ash, quartz, mullite, magnetite, and hematite (Figure 2). The presence of the amorphous phase can be identified as the broad diffraction “hump” in the region between 18° and 32° 2
3.2. Optimization of zeolite synthesis conditions
The optimization of the zeolite synthesis conditions was carried out using coal fly ash A. These optimized conditions were then applied for zeolite synthesis using coal fly ashes B and C and the properties of the zeolitic material compared to that resulting from coal fly ash A. The optimized conditions were the hydrothermal treatment temperature, time, and amount of water during the hydrothermal treatment process. The results are presented in terms of the chemical and mineralogical analysis of the zeolitic products, morphology, CEC, surface area, and pore volume.
3.2.1. Optimization of the hydrothermal treatment temperature and time
XRD was employed as a tool to monitor the evolution of zeolitic phases during the hydrothermal treatment process. Several authors pointed out that the increase of temperature leads to an increase in mullite dissolution, nucleation, and growth in zeolite crystals and an increased in Si dissolution from coal fly ash leading to an increase in Si/Al ratio, which influences the crystallinity of the final zeolitic product [10, 33, 34].
shows the XRD profile of zeolitic products for a concurrent variation of hydrothermal treatment time and temperature for coal fly ash A feedstock.
shows the XRD profile of the transformation of different phases of the coal fly ash as a function of time at hydrothermal treatment temperature of 100°C.
shows the disappearance of the broad hump between 18° and 32° 2
3.2.2. Effect of variation of water during hydrothermal treatment process
During the aging process, the dissolution of the aluminosilicate matrix of the coal fly ash occurs, releasing Si and Al, which are the main building blocks of the zeolite. Depending on the conditions employed, the saturation of the ionic precursors can occur, hindering more dissolution. The addition of water will tend to dilute the concentration of the ionic precursors leading to the further dissolution of the coal fly ash matrix. Figure 5 shows the XRD profile of zeolitic products obtained on varying the water added during the hydrothermal treatment at 140°C and 48 h treatment time.
The additional water was added after the aging process, as equal amounts of water (100 mL) had been used during the aging process. It is observed that the addition of more water after aging enabled the formation of relatively pure phase of zeolite Na‐P1. A balance has to be struck on the amount of water to be added, as too much water can also change the degree of supersaturation, which can slow the crystallization kinetics in addition to increasing the cost of zeolite synthesis due to the need to manage the disposal of large amounts of effluent generated.
3.2.3. Relative percent crystallinity of the zeolitic products
The relative percent crystallinity of the zeolitic products was done by summing and normalizing the peak heights of five major peaks (2
3.3. Synthesis of zeolite Na‐P1 using other coal fly ashes at optimized conditions
To test the success of the optimization process, the synthesis of zeolite Na‐P1 from different coal fly ash feedstock was attempted at the optimized conditions. The XRD and XRF characterization of these coal fly ashes (coal fly ashes B and C) were presented previously (Figures 2 and 3; Table 1). Figure 7 presents a comparison of the XRD profile of zeolite Na‐P1 synthesized from coal fly ashes A, B, and C at the previously optimized conditions (140°C hydrothermal temperature and 48 h treatment time). Figure 7 indicates that the zeolite Na‐P1 produced with coal fly ash B was similar to that of coal fly ash A. However, coal fly ash C showed an incomplete dissolution of mullite and quartz. This could be attributed to the difference in chemical and mineralogical composition of the coal fly ash C compared to coal fly ashes A and C (Figures 2 and 3; Table 1). This indicates that the conditions employed were not optimum for the dissolution of quartz and mullite in coal fly ash C. Another observable fact about the incomplete zeolitization of coal fly ash C could be due to the low CaO, which is known to contribute to the alkalinity of the solution. Coal fly ash C had the lowest CaO content (Table 1).
3.4. Morphological evolution of the coal fly ashes during the synthesis process
The morphological evolution of the coal fly ash feedstock after aging and hydrothermal treatment is depicted in Figure 8. The coal fly ash has a smooth spherical appearance and typically consists of microspheres. Gitari et al.  have previously reported the smooth spherical appearance of South African coal fly ash. The coal fly ash particles are observed to be smooth spheres, as the glassy phase covers the particles. The coal fly ash after the aging process appears to show a partial disintegration of the spheres, indicating the conditions employed during aging are not sufficient to completely break the matrix. This could also point to the resistance of mullite and quartz to dissolution. However, after hydrothermal treatment, a transformation of the disintegrating microspheres was observed, with granular crystalline particles being observed on the surface of the spheres. These were identified by XRD to be the zeolitic phases. Walek et al.  reported that the hydrothermal crystallization process begins on the surface of the undissolved or partially dissolved coal fly ash particle. These granular particles represented the initial zeolite formation process. A further illustration of the evolution of the crystalline zeolitic phases is depicted during the variation of water during the hydrothermal treatment at 140°C temperature for 48 h (Figure 9). Crystalline granules are clearly evident on the surface of the microspheres, which increases as the amount of water is increased. The crystallinity is also evident from the increasing agglomeration of the crystalline granules. The formation of the well‐ordered crystalline zeolitic phase formed during aging conditions of 47°C for 48 h and hydrothermal treatment for 48 h at 140°C with H2O/SiO2 ratio of 0.49 was confirmed by use of HRTEM (Figure 10).
In conclusion, the observations on the morphological transformation/evolution of the coal fly ash indicate that the glassy phase was dissolved in alkaline media and subsequently transformed into zeolite crystals, which were deposited on the surface of the disintegrating microsphere. The zeolite formation mechanism can therefore be described as (1) the dissolution of the SiO2 and Al2O3 from the glassy phase/mullite/quartz in alkaline media, (2) the formation of aluminosilicate gel as zeolite precursor from SiO4 and AlO4 species, and (3) the crystallization of the zeolite phase at supersaturation of the alkaline media. The addition of water during hydrothermal treatment lowers the supersaturation and allows more dissolution of the glassy phase/mullite/quartz and the release of more SiO4 and AlO4 species leading to increased formation of the zeolite phase. The disintegration of the fly ash microsphere is predicted to continue with increased dissolution of the resistant mullite and quartz phases, as more hydroxyl ions migrate through the microsphere with continued hydrothermal treatment and subsequent formation of crystalline zeolitic phases (Figure 9).
3.5. CEC and surface area
The CEC of the zeolitic products was determined to provide further evidence to the purity of the phases. An increase in CEC was observed in the zeolitic products compared to the feedstock coal fly ash. Coal fly ash had a CEC of 0.48 mEq/g, zeolite product at 100°C/48 h of 2.98 mEq/g, and zeolite product at 140°C/48 h of 3.91 to 4.11 mEq/g. These results correlate well with the increasing crystallinity observed in the optimization stages (Figures 4 and 9). The results also compare closely to the CEC value of commercial zeolite Na‐P1 (5 mEq/g). The addition of water during the hydrothermal treatment enhanced the CEC of the zeolitic products (zeolitic product obtained at 140°C/48 h with a H2O/SiO2 molar ratio of 0.49 was 4.10 mEq/g). This again confirms the high crystallinity and purity of the zeolitic phase obtained at this synthesis conditions. The BET analysis of surface area showed that the surface area of the material obtained after aging was low (30.5693 m2/g), but this was observed to increase significantly on hydrothermal treatment at 24 h/140°C (58.6358 m2/g) and 48 h/140°C (67.6329 m2/g), which again confirms the high‐purity and crystallinity of the zeolitic phase.
South African coal fly ashes have been proven to be suitable feedstock for hydrothermal conversion into high‐purity zeolite Na‐P1. The compositional and mineralogical analysis of the South African coal fly ashes identified mineral phases quartz, mullite, and amorphous glassy phase, which acts as crucial ingredients for the zeolite synthesis process. The optimization of the hydrothermal treatment process identified the conditions for the synthesis of high‐purity zeolite Na‐P1 as molar regime of 1 SiO2:0.36 Al2O3:0.59 NaOH:0.49 H2O and synthesis conditions of aging slurry at 47°C for 48 h. This was followed by hydrothermal treatment at 48 h and 140°C. The addition of water after the aging step expressed as H2O/SiO2 molar ratio was observed to play an important role in the hydrothermal conversion of coal fly ash into high‐purity zeolites. The results show that a simple adjustment of reactant composition and presynthesis or synthesis parameters leads to almost complete dissolution of coal fly ash matrix and the conversion of the Si/Al ionic precursors into high‐purity zeolite Na‐P1. The high purity and CEC (4.11 mEq/g) imply that the zeolites from the class F South African coal fly ash can meet high‐end applications such as wastewater treatment or application in detergents.
The authors would like to acknowledge the Water Research Commission (WRC), National Research Foundation (NRF), and Coaltech 2020 for funding this research work.
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