An optimization of synthesis technique for Na-X zeolite from coal-fly ash using Taguchi experimental design

Na-X Zeolite was made from Class-F coal fly ash from a thermal power plant in the Indian state of Gujarat. The objective of this study is to maximize Na-X zeolite crystallization by using alkaline fusion hydrothermal treatment. The authors have also optimized the process variables using the L9 orthogonal Taguchi method. The operating parameters investigated are fusion temperature, fusion time, liquid/solid (L/S) ratio, and alkaline solution concentration. The XRF analysis of coal fly ash reveals that quartz (SiO2), mullite (Al2O3.SiO4), and a small fraction of hematite and magnetite are the major minerals. The highly crystalline zeolite (ZT-5) was synthesized in a 0.3 M NaOH solution at a fusion temperature of 550 °C for 12 h. with a 1.3 L/S ratio. Contrary to the CFA graph, the zeolite (ZT-5) XRD pattern displays prominent peaks between 20 and 40°. The SEM images illustrate the transition from a smooth spherical texture to a highly crystalline lattice structure in the morphology of zeolite. A sharp stretch around 440 and 460 cm−1 and weak bend around 660 and 694 cm−1 show the relative O-T-O deformation and the change in shape of the Al-O-H bonds, as measured by FT-IR absorption.


Introduction
The exponential rise of the economy and reliance on basic energy sources has adversely impacted all aspects of the environment. The disposal of coal fly ash (CFA) from coalbased power plants is a common problem of concern on a global scale (Gollakota et al. 2020;Asl et al. 2018;Seidler et al. 2020). In order to preserve valuable land, the Third Amendment Notification issued by the Ministry of Environment, Forest and Climate Change of the Government of India on January 25, 2016 prohibits the dumping and disposal of fly ash. To achieve zero waste, it is necessary to utilize 100% of the fly ash generated. The cement industry consumes only the first and second grades of fly ash, which accounts for 63% of the fly ash produced (Rastogi et al. 2020). The rest is disposed of in ash ponds. Based on the analysis, by 2032 we would consume more than 1.8 billion tonnes of coal annually. (Surabhi 2017). Due to its geographical location, Indian Coal has an ash proportion between 35 and 45%; on rare, it even surpasses 50% (Ahmed et al. 2016). As per the amount of coal ash produced annually by coalbased thermal power plants is approximately 197 million tonnes, and by 2032, approximately 600 million tonnes of coal ash will be generated annually (Jindal 2019). To minimize the pozzolanic characteristics of fly ash, a traditional disposal strategy including the disposal of fly ash slurries, is chosen. This disposal method requires not only thousands of hectares of land as a resource, but also thousands of millions cubic metres of water every year. Since it has the potential to harm the environment, it requires proper attention.

Scope of research work
Various researchers have indeed been investigating the multiple potential applications of CFA. The CFA can be utilized to synthesize a range of products, including zeolites, catalysts, geopolymers, bricks, cement, and photocatalysts (Gollakota et al. 2020;Asl et al. 2018;Al-dahri et al. 2020). The CFA is a prudent option as a raw material because of its unique mineralogical composition, such as aluminosilicate, and the high cost of the above-mentioned commercial varieties (Ren et al. 2020;Sivalingam et al. 2019). The transformation of CFA-based adsorbent involves both physical and chemical processes. The fly ash can be transformed into zeolites, so resolving the problem of waste disposal and transforming industrial waste into a marketable commodity (Deng et al. 2016).

Utilization of CFA as zeolite
The majority of adsorbents derived from CFA are produced via alkaline treatment followed by acidic treatment using traditional techniques such as hydrothermal treatment, alkaline fusion hydrothermal treatment, and microwave-assisted approach. Holler and Wirsching (1985) sought to turn coal fly ash into zeolite via caustic processing. Many researchers have since created fly ash-based adsorbents by altering the conditions of synthesis. Lin and Hsi (1995) conducted extensive research on the impact of process factors, including fusion temperature, caustic reagent, molarity, and reaction duration, on the properties of synthesized zeolite. Ojha et al. (2004) reported the effect of varying parameters; they tuned the settings in order to produce zeolite-X with similar adsorption qualities to commercially available zeolite 13X. Iqbal et al. (2019) successfully synthesized zeolite 4A by hydrothermal temperature changes. The fusion with sodium hydroxide is the most cost-effective and efficient method for transforming fly ash into zeolite. After production, adsorbents were frequently coated with polymeric films, metallic compounds, and non-metallic compounds to increase their adsorption efficiency and prolong their lifetime. In order to simplify the systematic approach to the synthesis, Table 1 provides a brief summary of numerous CFA-based techniques along with parameters for zeolite production.
The current study focuses on the alkaline fusion hydrothermal synthesis of Na-X zeolite from an Indian thermal power plant. The prospective study involved the use of a variety of analytical methods to compare the synthesized zeolite with the reference zeolite 13-X. (commercial grade). The achievement of high crystallinity was prioritized. The fly ash-based zeolite Na-X (as an inexpensive supporting material/adsorbent) was created with the purpose of absorbing dye waste effluent from the region's textile sector as a future work.

Materials
The CFA was collected from two different thermal power plants of Gujarat, India, i.e. Kutch Thermal Power Plant (KTPP) and Mundra Thermal Power Plant (MTPP). Table 2 displays the chemical composition of raw fly ash received from both the thermal power plants. The XRF study reveals that both the CFA's are having approximately same composition therefore for the experimental work we have utilized the CFA from Mundra Thermal Power Plant (MTPP). Sodium aluminate (99%) was purchased from Shreekala Pvt. Ltd. Vadodara, Gujarat. NaOH (Merk) and HCl (Merk) were used for alkaline treatment and acid treatment respectively.

Synthesis
The best parameters for the synthesis of zeolite from CFA were determined using the Taguchi orthogonal design technique. The statistical methods are efficient and able to interpret the interaction between the desired parameters Shilapuram 2015, 2017;Gunasekaran et al. 2020;Ismailzadeh et al. 2020;Manjunatha et al. 2020;Movahhedi et al. 2020;Demiral and Samdan 2022;Gebreegziabher et al. 2023). In this strategy, orthogonal arrays are used to design tests, resulting in fewer experiments being conducted (Ding et al. 2007;Srivastava et al. 2007;Bayat et al. 2018). In addition to being efficient and less time-consuming, orthogonal arrays provide one valuable approach for assessing the impacts of various variables on response (Srivastava et al. 2007). Taguchi approach classifies the variance owing to controllable (design elements) and uncontrollable (noise factors) components. Taguchi has developed a strategy for decreasing variation by matching the appropriate controllable element options to the noise component (Furness 1996). The desired experimental data were used to calculate the mean values and signal-to-noise ratios (S/N) of the quality/ response characteristics of each parameter. The following equation shall be used in this calculation for something similar to the "larger is better" S/N will be preferred.
In this model, n is the number of observations and y i is the characteristic index value for each observation I (Park 1996). The levels and components of the experiment are listed in Table 3. To validate the results, three levels were selected for each element and four factors were selected for each approach. The orthogonal arrays for L9 are displayed in Table 3. This study utilized four variables: the fusion temperature (°C), the fusion time (h), NaOH concentration (M), the ratio of NaOH to fly ash (ml g −1 ).  1 3 Based on previous work (Cundy and Cox 2005) to develop Na-X zeolite, three values for each component were selected. The temperature-parameter's range of 500-600 °C was selected. The synthesis of zeolitic material is influenced by the fusion temperature. Since the ratio of SiO 2 /Al 2 O 3 began to decrease at temperatures beyond this range, it was observed that by using temperatures above this range had a negative impact on the crystallinity of Na-X zeolite. The level of NaOH was reported to be between 1 and 1.5 M. The hydrothermal synthesis time was chosen to be between 12 and 48 h.
A parametric effect plot is made using Minitab software based on the results of an analysis of means (ANOM) test. On the basis of the S/N ratio, analysis of variance (ANOVA) is then used to quantify performance characteristics and the contribution of each parameter. The details are presented in Table 4. Figure 1 depicts the plots illustrating the effect of factor interaction on the Na-X zeolite concentration based on the S/N ratio. As demonstrated in Fig. 1a, the amount of Na-X zeolite is influenced by the relationship between hydrothermal temperature and synthesis time. At 12 h. and 15 h., the Na-X zeolite concentration was increased due to the low and high temperatures. The composition of zeolite increased at low temperatures and decreased at intermediate temperatures during the 12 h. synthesis period. When  1 Interaction plots of synthesis parameters for zeolite-X crystallization: a temperature-time; b temperature-NaOH; c temperature-L/S ratio, d time-NaOH; e time-L/S ratio; f NaOH-L/S ratio the synthesis temperature was raised from 500 to 600 °C, the proportion of Na-X type zeolite was found to rise. These findings are consistent with those of other studies (Labik et al. 2020;Derkowski et al. 2006;Charnell 1971). The relationship between temperature and NaOH concentration is depicted in Fig. 1b. It has been demonstrated that altering the NaOH pattern at elevated temperatures does not appreciably increase Na-X content. To maximize the fraction of Na-X at low or moderate temperatures, a medium concentration of NaOH was recommended.

The interaction amongst experimental circumstances and the proportion of Na-X zeolite
In contrast, when the concentration of NaOH was moderate, the Na-X concentration increased at each temperature. At each temperature level, a high L/S ratio was required for the improvement of the Na-X concentration. In addition, with the low temperature range and moderate L/S ratio, the Na-X concentration grew (Fig. 1c). The relationship between the synthesis period and the NaOH concentration is depicted in Fig. 1d. At low NaOH concentration, it takes a considerable amount of time to increase the Na-X zeolite content. Increasing the concentration of NaOH to 1.3 M resulted in a low Na-X content at longer times and a high Na-X content at shorter or intermediate times. The Na-X zeolite content skyrockets as the NaOH concentration is steadily increased. At a NaOH concentration of 1.2 M, all degrees of synthesis time were shown to have increased Na-X concentrations. Under these conditions, the high-content Na-X zeolite crystallization increases the CEC of the produced samples. Figure 1e depicts the relationship between the L/S ratio and synthesis duration. At the low and high levels of synthesis time, a moderate degree of L/S decreased the concentration of Na-X. At modest levels of fusion duration and L/S ratio, a rise in Na-X content has been reported. The relationship between the L/S ratio and the NaOH concentration demonstrates that at high concentrations of NaOH and high Na-X content, adjustments in the L/S ratio had no appreciable effect on the Na-X content (Fig. 1f). For the low level of NaOH template, however, the Na-X zeolite concentration decreased from low to medium levels of the L/S ratio, whereas the Na-X content increased from low to higher levels of NaOH concentration.

Optimization using ANOM approach
Through analysis of means (ANOM), the optimal model parameters and the significant effect of each parameter can be evaluated. The divergence a parameter level causes from the average response is what defines its influence. Equation (2) may be used to calculate the value is the average (m) of the S/N ratio (n) for each parameter (j) at each level where i is the number of levels: The process variable level that provides the highest S/N ratio is the optimal level. Figure 2 depicts the ANOM output for the S/N ratio. Therefore, in the present study, the best combinations for obtaining the maximum Na-X zeolite crystallinity are as follows: (1) high temperature (550 °C), (2) high L/S ratio (1.33), medium NaOH concentration (1.2 M), and medium fusing time (12 h).

Variance analysis of empirical observations
The analysis of variance (ANOVA) method was utilized to determine the contributory elements of the indicated independent parameters. To identify the effects of each parameter, ANOVA was done. The total sum of squares given a set of information (output), i.e. Y 1 , Y 2 ,…, Y N , can be calculated using the following equations, where TT represents the total number of outcomes and N represents the range of outcomes.
Factor sums of squares can be computed by using Eq. 4 where K A is the multitude of aspects A levels A i is the total number of experiments in which factor A takes level I and n Ai is the total number of results (Y i ) associated with factor A i . The primary parameters required for ANOVA calculations are the total and factor sums of squares. Other ANOVA-derived quantities are all obtained from the basic sums of squares.

Synthesis of zeolite-X from CFA
Prior to modification, samples of raw fly ash were screened through a BSS Tyler screen with 80-mesh openings to remove bigger particles. The process of calcination at 600 °C for three h. has eliminated unburned carbon (4-6%) and other volatile compounds present in fly ash. To stimulate the formation of zeolite, samples of fly ash were treated with 1 M HCl. The weight ratio between sodium hydroxide and fly ash varied between 1 and 1.5. A predetermined amount of sodium hydroxide and fly ash (calcined and HCl treated) were crushed and fused in a stainless-steel tray for one hour at temperatures ranging from 500 to 600 °C. After the mixture cooled to room temperature, it was further crushed and mixed with water (10 g fly ash/100 ml water). The slurry was agitated in a stirrer at room temperature for two hours. For hydrothermal treatment, mixtures were baked for six hours at 90 °C. The precipitate was then washed twice with distilled water to remove excess sodium hydroxide, filtered (pH 10) and dried at 800 °C in the muffle furnace. Figure 3 represents the flow of synthesis.

Characterization Techniques
• X-ray diffraction: Philips X'pert MPD instrument at CSMCRI Bhavnagar facility was utilized to obtain XRD patterns of CFA and synthesized zeolite. The samples were imaged between 10° and 50° (2θ, θ where is the diffraction angle). The crystallinity was quantified by summing the heights of the major peaks in XRD patterns. • Morphological analysis by Scanning Electron microscope: The JEOL JSM 7100F equipment was utilized for the structural investigation of CFA and produced zeolite. The bulk and elemental content of CFA and produced zeolite were analysed by SEM/EDAX, which revealed the fraction of oxides present in both materials. • Fourier transform infrared (FTIR): To investigate the modifications in bond structure and acidity Perkin Elmer's G-FTIR spectrometry was used to perform infrared spectroscopy between wave numbers 400 cm −1 and 4000 cm −1 . Figure 4 exhibits the XRD patterns of both CFA and synthesized zeolite obtained by alkaline fusion hydrothermal treatment. SiO 2 , Al 2 O 3 , Fe 2 O 3 and oxides of Mg, Ca, P, and Ti comprise the majority of the composition of coal fly ash. In addition to quartz (SiO 2 at 26.85°) and mullite (Al 2 O 3 . SiO 2 ), minor amounts of hematite and magnetite were also detected in the fly ash; nevertheless, the majority of the fly ash was mainly composed of amorphous material. The study confirms the existence of a glass phase in CFA, which gives rise to a large peak between 18° and 32°. On the other hand, sharp peaks were recorded for the 26.95°,29.76°, and 34°) and 15.93°,21.49°,23.49°,26.95°,29.76°, 34°, 52.43°). The hydrothermally produced products have sharper intensities in the XRD diagram, which may be due to a higher zeolite's concentration. The presence of crystalline phase is signified by the disappearance of the broad bump. The fact that the mullite phase shifted minimally during the hydrothermal reaction indicates that it was a relatively stable phase. For the identification of mineral phases, Brags' formula provides a unique fingerprint based on the lattice Fig. 2 Main effects of each parameter on crystallization of Na-X zeolite parameter of the structure, as each mineral has a unique set of d-spacing values compared to published data (Deng et al. 2016;Querol et al. 2001). The XRD patterns of the treated fly ash (KTPP and MTPP fly ash is taken as the typical, Fig. 4a and b) corresponded identically with those of the X-type zeolite (Boycheva et al. 2020;Pathak et al. 2014).

Scanning electron microscope
Tables 2 and 5 illustrate the chemical components of CFA and the produced zeolite, respectively. Since the overall  proportion of SiO 2 , Al 2 O 3 , and Fe 2 O 3 is greater than 70% and CaO is less than 20%, the form of CFA employed in this study can be categorized as the class F type (Ojha et al. 2004;Boycheva et al. 2020). SiO 2 and Al 2 O 3 constitute the majority of the CFA, with a ratio of 1.92 SiO 2 to Al 2 O 3 . Thus, CFA is ideally suited for use as a low-cost raw material in material synthesis. This is comparable to other CFA samples investigated and utilized in the literature for zeolite synthesis (Chigondo and Nyamunda 2013;Jha et al. 2009). The majorities of the particles were round or elliptical and measured between 0.1 µm and 1.2 µm in diameter. The capture of sodium ions during hydrothermal processing of zeolite results in a considerable increase in the fraction of Na 2 O in the zeolite material relative to CFA (from 1.87 to 12.73%). This aligns quite well with the findings of Jha et al. (2009). The hydrothermal treatment of fly ash led to a high conversion of fly ash to crystalline zeolite, as evidenced by the absence of spherical particles in the treated fly ash (as seen through the SEM images). Both ZT-4 and ZT-5 zeolites had excellent crystallinity. (Chigondo et al. 2013;Jha et al. 2009; Nawagamuwa and Wijesooriya 2018). (Fig. 5).

Fourier transform infrared spectroscopy
The structural characteristics and acidity are examined using FTIR analysis. Figure 6 demonstrates the IR spectrum of the synthetic zeolite. It is scanned between 400 and 4000 cm −1 . The IR spectrum is divided into two distinct categories: (a) internal vibration owing to TO 4 (Si and Al) framework (b) external connectivity of TO 4 unit in  (Cundy and Cox 2005). The 550 cm −1 and 800 cm −1 stretches indicate quartz and mullite, respectively. Figure 6 depicts a more specific illustration of the two most significant bands for zeolites, which typically occur between 800 and 1250 cm −1 and 420 and 500 cm −1 . The first band is attributed to an asymmetrical stretching mode of the T-O bond, while the second is a bending mode. In this analysis, it is observed that each of these bands is dependent on the crystal structure to some degree. A Sharp stretch about 440 cm −1 , 460 cm −1 , and weak bend around 660 and 694 cm −1 suggest the internal deformation of O-T-O, but the upper range 3447 indicates a rather strong OH bond. T-O band typically occurs at 1053 cm −1 in CFA, but the presence of zeolite is verified by 440-460 cm −1 sharper and shifted to a lower frequency. In the mid-infrared region of the electromagnetic spectrum, the fundamental framework vibration of Si(Al) O 4 groups can be observed. Figure 6 provides a summary of the IR spectrum information for synthesized and published Na-X type zeolites (Ojha et al. 2004;Shukla et al. 2019). In the infrared spectrum, the produced zeolite exhibits an absorption band between wave numbers 990 and 1440 cm −1 , indicating the presence of substituted Al atoms in the tetrahedral forms of silica frameworks. All of these data indicate that fly ash treated with alkali and hydrothermally yields zeolites of type X. (Ding et al. 2007;Srivastava et al. 2007).
The summary of some observations based on the XRD, IR, and SEM data of the Na-X type zeolite generated by activating fly ash from Indian thermal power plant is represented in the following section.
1) The treatment conditions and raw material concentrations had a significant impact on the characteristics of the zeolitic material that was produced. The developed zeolite's crystallinity appeared to alter with fusion temperature, reaching its highest value at 823 K. The similar trends are already studied and evidenced by Bai et al. (2018), Shigemoto et al. (1993) & Guozhi et al. (2019. 2) NaOH/ Fly ash ratio plays a crucial role in the zeolitazation process. Higher sodium content in the reactant mixture will result in a greater production of water-soluble sodium silicates. In subsequent stages, increased formation of sodium silicates enhances the yield of zeolitic materials. Furthermore, is Na + cation crucial for zeolitization. The sodium ions are known to stabilize the sub-building units of zeolite frameworks, specifically the six-membered ring, which is critical to the synthesis of zeolite in hydrothermal environments (Ojha et al. 2004). ANOM analysis shows that zeolite crystallinity increases with the concentration of NaOH up to 1.2 M, but gradually decreases thereafter.
3) Besides SiO 2 and Al 2 O 3 , fly ash also contains several additional elements that are undesirable in synthetic zeolites. The acid treatment process during pre-treatment can eliminate a number of unwanted contaminants from produced zeolite, including iron (III) oxide, calcium oxide and magnesium oxide, which act as hindrance during crystallization phase. In the current investigation, HCl was used to diminish the concentration of iron and alkali oxides, which were found in fly ash but were primarily concentrated in the outer part of the ash particles (Natusch and Taylor 1980). 4) An outstanding relative crystallinity was established by the XRD investigation. The surface morphology's cubic nature was established by SEM. It was proposed that fly ash would be a realistic and cost-effective raw material for the actual industrial manufacture of zeolite 13-X. In a subsequent study, we will assess the effectiveness of zeolite materials used as an adsorbent for treating dye waste effluent.

Conclusion
The starting material for this investigation is one of the byproducts of thermal power plants. The sustainability strategy is the primary emphasis of this study. The hydrothermal alkaline treatment was utilized to create CFA-based adsorbent (Na-X Zeolite). Employing the Taguchi L9 design, the effects of fusion temperatures, NaOH concentrations, and L/S ratios on the crystallization of Na-X zeolite were analysed. The expected amount of Na-X zeolite was utilized to determine a response parameter. As part of our analysis of zeolite crystallization, we employed XRD, FTIR spectroscopy, SEM-EDX, and particle size analysis. As a result, the chemical and morphological characteristics of fly ash have altered, demonstrating its transition into zeolite materials. The formation of zeolitic material was strongly reliant on the proportion of the raw material and treatment conditions. By altering the reaction parameters, zeolites with varied surface area, silica/alumina ratio, and crystallinity were synthesized. The best surface area and crystallinity were obtained under the following conditions: NaOH/fly ash ratio of 1:3, fusion temperature of 550 °C, duration of 12 h, and hydrothermal treatment time of 6 h. By seeding the raw material with sodium aluminate, a structure comparable to commercial zeolite 13X was produced. The work conducted represents a possible profile to use the discarded product i.e. coal fly ash, to produce an inexpensive supporting adsorbent for the possible removal of dye waste effluent from different sources.