The reuse of industrial wastes from a coal-fired power plant and a plasma electrolytic oxidation process was attempted to realize a zero discharge. The batch composition was adjusted by adding sodium hydroxide and sodium aluminate. A single-mode microwave oven equipped with reflux condenser was used for crystallization under atmospheric pressure. The synthesized samples were characterized by X-ray diffraction, scanning electron microscopy, BET, thermogravimetric analysis, and cation-exchange capacity (CEC) measurement. Analytical results indicated that Na-A zeolite with a defined maximum crystallinity could be successfully synthesized by hydrothermal treatment of fly ash with wastewater. Due to the high CEC, the product can be applied for gas purification and soil remediation processes.
1. Introduction
The abundance of amorphous aluminosilicate in fly ash as a by-product of coal-fired power stations makes it an important source material for synthetic zeolites. Zeolitic aluminosilicates are valuable inorganic porous materials that can be synthesized by various techniques and serve for a wide range of diverse applications as adsorbent, catalyst, adsorbent in the form of powder, formed particulates, and membranes [1–6]. Over the last few decades, the synthesis of different zeolites from fly ash has been of great interest to many researchers. Two well-known activation processes are generally employed for the conversion of coal fly ash (CFA) to zeolite. The first one is a two-step process, in which pretreatment fusion at high temperature is fol- lowed by a hydrothermal process. Different zeolites including Na-A [7–9], Na-X [10–14], and Na-P [15] were synthesized by means of this indirect two-stage method. The second method is a direct hydrothermal process, in which the fusion step is eliminated [16] and different zeolites such as Na-A [17, 18], Na-Y [19], Na-P [20–23], and hydroxy sodalite [24] are synthesized. The direct and indirect zeolitization processes of CFA are being studied in terms of type and concentration of the alkaline solutions, solution/solid ratio, aging time, mixing technique, and time, temperature, and pressure of the crystallization reaction using different thermal sources such as conventional and microwave techniques. Numerous publications including review articles [25–27] indicate the increasing interest in using fly ash as a source of Si and Al for manufacturing different porous zeolitic aluminosilicates.
All methodologies for zeolitization of CFA involve three major stages of dissolution, nucleation, and crystal growth. The conventional techniques for synthesis of different zeolites from CFA are very time-consuming taking 12–48 h on average. Currently, microwave-assisted hydrothermal processes are being considered as one of the most effective techniques for chemical synthesis including zeolite crystallization [28–30].
Most studies utilized deionized, distilled, or industrial water during the synthesis, but few reports analyzed different sources of water. Belviso et al. [10] synthesized zeolite X from CFA and seawater by pretreatment fusion with NaOH followed by hydrothermal crystallization. Results obtained with seawater and distilled water were compared in [10]. The synthesis yield at different crystallization temperatures was higher using seawater. Hussar et al. [31] synthesized zeolite A by hydrothermal process using sodium silicate, sodium aluminate, and the by-product of an aluminum etching process. The chemical composition of the aluminum etching by-product consisted of the main oxides of Al2O3 (92 %), Na2O (6 %), and SiO2 (0.5 %). Their results indicated that a higher synthesis reaction temperature and a longer reaction time led to improved synthesis of zeolite A.
Figure 1. Schematic illustration of different steps involved in microwave-assisted zeolitization of CFA using PEO wastewater.
The effect of taking industrial waste brine solution instead of ultrapure water was investigated during the synthesis of zeolite by Musyoka et al. [24]. They used CFA as silicon feedstock and high-halide brine obtained from the retentate effluent of a reverse-osmosis mine water treatment plant. The brine contained high sodium and potassium levels and low concentrations of toxic elements. In addition, there was a trace of aluminum equal to 48.38 mg L–1 and no silicon. The use of brine as a solvent resulted in the formation of hydroxy sodalite zeolite. Unconverted mineral phase of mullite and hematite from the fly ash feedstock were found in the synthesis product. Musyoka et al. [12] used two types of mine waters, i.e., acid and circum- neutral, obtained from coal mining operation instead of pure water to manufacture Na-X and Na-P zeolites by means of a two-step indirect method, i.e., fusion followed by hydrothermal crystallization, and a direct method, respectively. The important cationic species of circumneutral water were 952 mg L–1 Na, 38 mg L–1 Mg, 19 mg L–1 Ca, 1.2 mg L–1 Si without any Al. The acid drainage water contained 4694 mg L–1 Fe, 68 mg L–1 Na, 386 mg L–1 Mg, 458 mg L–1 Ca, 613 mg L–1 Al, and 31 mg L–1 Si. The use of circumneutral mine water resulted in Na-P and X zeolites of similar quality whereas the application of acidic mine drainage led to the formation of hydroxy sodalite zeolite.
Plasma electrolytic oxidation (PEO) is a relatively novel technique to produce functional oxide coatings on the surface of metals such as aluminum, magnesium, and titanium and their alloys. The application of light metals, e.g., aluminum and magnesium, is increasing in different industries because of their strength to weight ratio, low density, and ease of fabrication. Recently, the PEO process has attracted a lot of research interest due to its capability to create coatings on these sub- strates with considerably improved wear and corrosion resistances compared to traditional anodizing techniques. The configuration used in PEO is similar to conventional anodizing; however, the coating process performs at much higher potentials. The electrolytes used in PEO typically contain low con- centrations of alkaline solutions [32].
CFA is not only a waste and environmental hazard, but also its disposal is a financial burden. Wastewater of plasma electrolytic oxidation process is also considered an environmental problem and requires expensive technologies to treat and dispose. The volume of generated industrial waste will continue to grow with the increased industrial and domestic energy demand. In order to reduce the environmental impacts of industrial wastes, potential utilization of the waste toward value- added products should be explored. The use of industrial wastewater is considered as a solution for high water consumption demand of zeolite synthesis technologies. Furthermore, it is a potential capital-saving option for handling these two industrial solid and liquid wastes. Single-mode microwave irra- diation shortens the zeolitization time leading to a facile and efficient process at larger scale.
2. Experimental
2.1 Materials
CFA was obtained from Nanticoke coal-fired power plant, Ontario, Canada, owned by Ontario Power Generation OPG. The CFA sample was stored in a sealed container before use. All chemicals including sodium hydroxide (Alphachem, Canada) and anhydrous sodium aluminate (Sigma-Aldrich, USA) were of analytical grade and used as received without further purification. Wastewater of the PEO process [32] was employed instead of distilled water for the preparation of the solutions.
2.2 Hydrothermal Synthesis
The reaction mechanism of the hydrothermal microwave-assisted zeolitization of CFA involves three stages of dissolution, nucleation, and crystallization [22], as illustrated in Fig. 1. A typical synthesis experiment for production of Na-A zeolite from coal fly ash (CFAZA) involved the addition of 1.82 g of sodium hydroxide (granules) and 1.82 g of fly ash to 17 mL wastewater, i.e., the NaOH/CFA ratio was 1. The mixture was then agitated by a mechanical stirrer for 12 h at 60 °C (Sol A).
Afterwards, in order to balance the Si/Al molar ratio to 1, 3 mL of sodium aluminate solution (Sol B) (0.155 g L–1 in the PEO wastewater) was added to Sol A and mixed thoroughly for 2 h by means of a mechanical stirrer. The molar composition of the final aluminosilicate gel was 4.45 Na2O:1 Al2O3:1.79 SiO2:193.78 H2O. The mixture was subjected to microwave radiation at different power levels and time for crystallization of zeolite LTA. The crystallization experiments were carried out with a single-mode microwave device equipped with a condenser system for reaction heating under atmospheric pressure. A cylindrical PTFE reactor (28 mm ID, 108 mm length) was used to conduct the reaction inside the microwave chamber at various microwave power, reaction time, and reaction temperature.
The experiments were performed in a self-adjusting microwave oven in single-mode at 2.45 GHz (CEM Corp., Discover, USA), where the reaction temperature and power were recorded by a real-time monitoring system. The temperature was measured with an infrared pyrometer installed in the microwave chamber. Fig. 2 presents an overview of the crystallization system. After a given period of microwave irradiation, the solid products were filtered, thoroughly washed with deionized water to remove unreacted and water-soluble components, dried overnight at 100 °C in an electric oven, then characterized.
Figure 2. Single-mode microwave reactor setup.
2.3 Characterization
X-ray diffraction (XRD) data of the synthesized samples were collected by a Rigaku-MiniFlex powder diffractometer (Rigaku, Japan) using CuKa (l for Ka = 1.54059 Å) over the range of 5° < 2q < 40° with a step width of 0.02°. The obtained zeolite crystalline phase was compared with the standard peaks of a reference zeolite [33]. The crystallinity of the products was determined by the peak-fitting algorithm in the MDI-Jade v 7.5 software. Crystal size distribution and morphology of the zeolite were studied by scanning electron microscopy (SEM) JSM 600F (Joel, Japan) operating at 10 keV of acceleration voltage. Specific surface area and pore size of the prepared samples were determined by means of a BET analyzer (Micrometrics ASAP 2010). Known amounts of samples, e.g., 50–80 mg, were loaded into the BET sample tube and degassed under a vacuum of 10–5 Torr at 125 °C for 6 h. Thermogravimetric analyses (TGAs) were performed on a Mettler Toledo TGA/SDTA 851e model (Switzerland) with version 6.1 Stare software. The samples were heated from 30 °C to 600 °C at a heating rate of 10 °C min–1 under nitrogen purge.
where WCFAZA is the weight of the zeolite product (g) and WCFA is the weight of the CFA (g).
2.4 Experimental Design
Cation exchange capacity (CEC) is the maximum quantity of total cations that a zeolite is capable of holding and is available for exchange with the cations in the surrounded medium. It is expressed as milliequivalent of ion per gram or meq per 100 g of dry zeolite. It is a specific characteristic of each zeolite, which depends on the structural chemistry of a particular zeolite, e.g., the Si/Al ratio. Theoretically, the Si/Al molar ratio determines the number of exchangeable cations and the CEC. The CEC values of the zeolitized CFA samples were measured using the ammonium acetate saturation standard procedure over five days [34].
The whole process includes three main steps: (i) replacing exchangeable ions of zeolite with ammonium (NH4+) using 1 N solution of ammonium acetate; (ii) extract of NH + ions through washing dry sample acidified solution (0.005 M HCl) of 10 % NaCl; and (iii) measurement of released NH + by means of the Kjeldahl technique.
In order to measure the concentration of leached ions from the tested samples, the supernatant solution of the CEC tests was analyzed by an inductively coupled plasma atomic emission spectrometer (ICP-AES; Varian, Australia) with ICP expert software v 4.0. The product yield of the zeolitization process demonstrating the efficiency of the zeolite synthesis procedure was calculated by [13]:
The CFAZA synthesis was optimized by means of a statistical approach using a two-level central composite, double-factorial design. The influence of each reaction parameter on the crystallinity of Na-A as a response was examined with Design-Expert 7.1.5 software (StatEase, Minneapolis, USA). The effects of the two most important variables, namely microwave power at three levels of 100, 200, and 300 W and irradiation time at three levels of 10, 20, and 30 min, for continuous microwave irradiation under atmospheric pressure were investigated.
3. Results and Discussion
3.1 Chemical Composition
The chemical and mineralogical compositions of the CFA used as starting materials and main source of Si and Al are given in Tab. 1. According to the XRD data, the main components of the CFA are amorphous aluminosilicate as well as quartz and mullite that exist as crystalline phases. The SiO2/Al2O3 ratio was found to be 2.06, thus being appropriate for the synthesis of low-silica zeolite crystallites such as LTA-type zeolite.
Table 1. Chemical composition of CFA under study obtained by XRF analysis; particle size £ 600 mm, SiO2/Al2O3: 2.13.
The chemical composition of the wastewater used in this study, measured by ICP-AES, is reported in Tab. 2. The alkaline pH of the wastewater, pH 12, due to the presence of NaOH and KOH as well as trace amounts of Si and Al makes this waste a suitable medium for zeolite synthesis.
Table 2. Major elemental composition of the used PEO waste- water (pH 12).
3.2 XRD Analysis
The NaOH/CFA ratio was set at a minimum value of 1 based on the amount of alkaline source present in the wastewater. Fig. 3 illustrates the XRD pattern of raw and alkaline-treated CFA before subjecting to microwave irradiation. Quartz, mulite, calcite, and hematite are the main crystalline phases existing in CFA. During the process of alkaline treatment of CFA with sodium hydroxide, the calcite phase was removed or dissolved and no relevant peak was observed. No noticeable disso- lution of quartz happened under the mild alkaline conditions indicating the resistance of quartz during direct hydrothermal treatment.
The low-intensity peaks indicate the residual amounts of mullite and hematite phases after alkaline treatment. Low levels of the mullite phase are attributed to the fact that mullite is resistant to dissolution during direct hydrothermal treatment. The kinetics of dissolution of silicon and aluminum strongly depends on the concentration of the hydroxyl ion OH–, which is responsible for hydrolysis and dissolution of the reagents. There is no significant difference between XRD results of CFA before and after gelation with sodium aluminate solution.
The XRD patterns of the zeolite synthesized by microwave crystallization are presented in Fig. 4. Most of the peaks appear at 2q ranging from 5° to 40° and match well with those for Na- A zeolite, confirming the successful formation of LTA zeolite as the major crystalline phase of the zeolitized CFA [35]. It can be seen that the majority of quartz phase in the CFA remained intact when the digestion or dissolution process is carried out at 60 °C for 12 h.
As reviewed by Querol et al. [27], the main limitation of the processes for synthesizing zeolites from fly ash is the slow reaction rate and the relatively high temperatures (125–200 °C) needed to dissolve Si and Al from fly ash particles. Under these conditions, many of the high-CEC and large-pore zeolites such as Na-A zeolites cannot be synthesized. When the temperature is reduced, then the synthesis yield decreases considerably and a very long activation time of 24–48 h is required. The advantage of microwave-assisted crystallization process is to reduce the activation time from hours to a few minutes when complete dissolution of Si and Al happens. Increasing both power and time, the crystallinity of the product will be enhanced.
At a constant power of 100 W, the average temperature of activation solution was 116 °C according to Fig. 4 a. The quantities of mullite and hematite were low because not all of the CFA was converted to zeolite. The presence of unconverted mullite and hematite in the products was also confirmed by XRD. Increasing the power to 300 W resulted in a solution temperature of 127 °C; see Fig. 4 c. Therefore, zeolite synthesis from coal ash is an equilibrium reaction between the alkaline solution and solid phase. By raising the temperature, the solubility of silica and alumina ions increases and the formation of crystal nuclei is initiated. The crystal growth leads to a complete dissolution of the original material for the formation of zeolite crystalline phases.The characteristics of CFAZA synthesized using CFA and wastewater as starting materials by means of microwave-assisted hydrothermal technique were compared with the synthesized samples with distilled water. The XRD patterns of the samples synthesized under experimental conditions of 300 W and 30 min are illustrated in Fig. 5. Most of the peaks match well with those obtained for distilled water. The larger height of the peaks corresponding to distilled water indicates more crystallinity.
A statistical method based on a two-level full- factorial design with one center point per block, i.e., nine experimental runs in total, was applied to optimize the synthesis conditions. Design-Expert 7.1.5 software was employed to study the influence of microwave irradiation power and time on crystallinity and yield of the final products; the results are summarized in Tab. 3. According to the XRD data, Na-A zeolite with a maximum crystallinity of 67.24 % was successfully synthesized by a hydro- thermal treatment of fly ash with wastewater at 300 W for 30 min, which is very close to the crystallinity of the zeolitic product obtained by distilled water under similar reaction conditions, i.e., 73.47 %. It is noteworthy that the yields of all syn-water under similar reaction conditions, i.e., 73.47 %. It is noteworthy that the yields of all synthesized CFAZA were quite similar to the product yield of the zeolite produced by CFA and distilled water, being in the range of 82.0–84.1 %.
Figure 3. XRD patterns of the raw and treated CFA samples prior to the micro- wave irradiation. M, mullite; Q, quartz; H, hematite; C, calcite.
The influence of different variables on the crystallinity was evaluated by analysis of variance (ANOVA). Tab. 4 presents the results of preliminary statistical analysis and the effects of the variables. The F-test was used to identify the most significant variables in the hydrothermal microwave-assisted process. The significance level (p-value) adopted was 0.05. A mathematical polynomial model (quadratic design) was constructed in terms of the alphabetically coded input significant variables, i.e., P for microwave power, q for microwave time. According to the data reported in Tab. 4, the crystallinity is affected by both micro- wave power and time. The higher F-value of 150.3 of the microwave power indicates that crystallinity is strongly influenced by microwave power rather than by microwave irradiation time with an F-value of 51.38.
3.3 SEM Analysis
SEM images of the raw CFA and the synthesized zeolitic products are illustrated in Fig. 6. The pres- ence of characteristic cubic crystallites of Na-A zeolite validates the XRD data. The amount of amor- phous phase decreases and more cubic crystals appear at higher microwave power. It seems that the zeolite crystallization takes place at the interface between the undissolved CFA particle and the alka- li solution leading to the deposition of zeolite crystallites. This means that the nucleation mechanism of the CFA zeolitization is heterogeneous. After 30 min of microwave irradiation at 100 W, a few cubic structures appear; however, the cubic structure is not abundant and is embedded in the flower-like or ball-of-yarn-like crystalline structure. Furthermore, the size of the zeolite crystallites is in- creased at higher microwave power. The Na-A zeolite morphology is characterized by regulated cu- bic-shaped crystals with approximate dimensions of 2–5 mm. The morphology of the CFAZA was a chamfered-edged cube due to the initial SiO2/ Al2O3 concentration used in the production process. With higher microwave irradiation power, the abundance and size of cubic zeolite crystals increase, indicating that crystal growth is strongly related to microwave irradiation power.
3.4 BET Surface Area Analysis
Figure 4. XRD patterns of synthesis CFAZA at three levels of microwave power versus time. Power: (a) 100 W, (b) 200 W, (c) 300 W.
The BET surface area of the CFAZA sample synthesized under optimized conditions of 300 W for 30 min was 47.14 m2g–1. This surface area is 10 % larger than that of the sample synthesized at 100 W, which was 43.8 m2g–1. On the other hand, the micropore area and external surface area of the sample produced at 300 W were 5.69 and 41.45 m2g–1, respectively. The pore volume and the mean pore diameter for the best sample were 0.132 cm3g–1 and 8.274 nm, respectively; see Tab. 5. The specific surface area of CFAZA obtained by wastewater is three times larger than that of CFA.
Figure 5. XRD patterns of CFAZA obtained with distilled water and wastewater under microwave power of 300 W for 30 min.
Table 3. Crystallinity and yield for the synthesis of Na-A zeolite
The shape of the adsorption/desorption isotherm and its hysteresis pattern provide valuable information about the physisorption mechanism, which can be used to qualitatively predict the type of pores present in the adsorbent. The hysteresis between the adsorption isotherm and desorption isotherm is thought to be due to the different pore sizes being combined. The obtained adsorption-desorption curve of CFAZA sample is a special type of isotherm shape, classified as type IIB [36]. Based on International Union of Pure and Applied Chemistry (IUPAC) isotherm classification [37], the adsorption branch of the isotherm has a general shape like type II isotherms typical of nonporous aluminosilicate. This adsorption isotherm profile represents a monolayer-multilayer adsorption mechanism of the gas on the open and stable solid surface. A type II isotherm is normally associated with monolayer-multilayer adsorption on an open and stable external surface of a powder, which may be nonporous, macroporous, or even to a limited extent microporous.
The desorption branch of the isotherm is a distinct H3-type hysteresis loop indicating mesoporous aluminosilicate [36]. The H3 hysteresis loop type is mostly found in materials with platy particles having slit-shaped mesopores. These materials are not purely mesoporous as there is no indication of the completion of mesopore filling that would result in a plateau at higher relative pressures as in a typical type IV isotherm. Characteristic features of the type IV isotherm are its hysteresis loop, which is associated to capillary condensation taking place in mesopores, and the limiting uptake over a range of high relative pressures. A significant volume of macropores in these materials results in the absence of the isotherm plateau at high relative pressures as seen in type IV isotherms.
Another important feature observed in BET isotherms is the forced closure of desorption branch to adsorption branch where relative pressures are low. Disappearance of the hysteresis, i.e., the tensile strength effect, is due to the instability of the hemispherical meniscus during capillary evaporation in pores with diameters smaller than approximately 4 nm. In these pores, the surface tension forces are stronger than the tensile strength of the liquid causing the meniscus collapse which leads to a spontaneous evaporation of the bulk liquid phase. The presence of forced closure in the isotherm shape may indicate a significant volume of pores with diameters smaller than 4 nm.
3.5 Thermogravimetric analysis (TGA)
The TGA curves of CFA and its zeolitized counterpart are displayed in Fig. 7. CFA exhibited a weight loss of 6.1 %, which mostly occurs at 105 °C. The gentle slope associated with the weight variations and the trend of heat flow changes are a particular behavior of CFA and attributed to the reversible adsorption of atmospheric moisture on the external surface and macro-pores of CFA. The TGA curves of both CFAZA samples show a 10 % weight loss with a point of inflection at approximately 160 °C. This weight loss indicates that the water content in these samples is higher than in the CFA sample confirming the obtained BET micropore surface area. This could be attributed to evaporation of adsorbed water molecules on the porous structure of the synthesized zeolite.
Table 5. Pore structure specifications of CFA and CFAZA samples.
3.6 Cationic Exchange Capacity (CEC) and Heavy Metals in the Leachate
Figure 6. SEM images of the raw CFA and microwave-assisted synthesized CFA- ZA samples after 30 min microwave irradiation at different power values. (a) CFA, (b) CFAZA (100 W), (c) CFAZA (200 W), (d) CFAZA (300 W).
The CECs of CFA and synthesized CFAZA samples were found to be 0.30 and 1.82 meq g–1, respectively. This very high CEC, which is comparable to the zeolitized CFA reported by other researchers [8, 30] indicates that the produced zeolite has a great potential as adsorbent in different environmental remediation processes.
Results of typical leaching tests obtained by elemental analysis data of the leached ions after soak- ing the CFA and zeolite samples in ammonium accelerate solution for seven days at 25 °C are summarized in Tab. 6. The concentration of different elements such as Fe, V, B, Be, Cr, Se, Co, and Pb, which were analyzed by the ICP technique, were remarkably lower in the leachate solution of the synthesized zeolitic product compared to the original CFA. Accordingly, the leach resistance of the zeolitized sample toward the studied ions is as follows: Fe > V > B > Be > Cr > Se > Co > Pb. Due to the initial composition of the used wastewater, the concentration of some elements such as K, Na, Ba, Cu, and Ni in the supernatant liquid is higher for CFAZA than CFA [38]. The aluminum concentration of the leachate solution of the zeolitized sample by means of wastewater was higher than of the distilled water.
Figure 7. TGA pattern of CFA and CFAZA.
Hydrothermal microwave-assisted zeolitization is a fast and energy-effective process compared to the conventional hydrothermal techniques. The microwave irradiation energy, which is a function of both microwave power and irradiation time, influences the crystallinity of the final product. The required energy for CFAZA crystallization was between 9:1 Wh g—1 , corresponding to 10 W for 10 min, and 82:4 Wh g—1 , corresponding to 300 W for 30 min. When compared to the high conduction heating energy required for 2–90 h of ordinary crystallization processes, the low micro- wave energy consumption makes this technology economically viable.
Concerning the process scale-up, although microwave energy has found commercial applications in many areas including food processing, heating and vulcanization of rubber, drying processes, etc., however, there are some technical drawbacks related to large-scale applications of microwave energy. Consequently, designing a new chemical process that is economically viable and environmentally sustainable is of paramount importance to the inorganic chemical industry. Essential improvements in this area can be achieved by reorganization of chemical manufacturing processes, including new reactor configuration and novel process conditions. The chemical industry has started to develop different approaches of process intensification technologies, which often need significant changes in traditional processing technologies.
Table 6. Composition of supernatant liquids obtained by soaking CFA and synthesized CFAZA in distilled water [mg L–1].
The idea of exploring very unusual process conditions to intensify chemical reactions is a recent concept called novel process windows, which is operating under process conditions that remarkably increase reaction rates without sacrificing the selectivity. To overcome the technical limitations of the large-scale batch microwave-assisted synthesis, recent efforts have been focused on developing a microwave processing system under continuous-flow conditions. The typical short reaction times of few minutes or seconds that can be achieved by means of microwave energy sources provide an ideal platform for continuous-flow manufacturing, in which short residence times are essential to gain a high yield. Using various single-mode or multimode microwave systems, numerous examples of successful microwave-assisted continuous-flow synthesis on a gram- or kilogram-scale have been reported. Nevertheless, much more research and development works are needed to overcome major technical barriers.
From an environmental and economic point of view, the process developed in this study can only reduce the environmental burden associated with these wastes but could also offer extra income generated from the reuse of wastewater instead of water. Further studies are under way to develop this process as an eco-friendly manufacturing technique toward a zero-discharge process in order to reduce the generated aqueous residuals and its environmental impact.
4. Conclusions
A novel approach to address environmental concerns related to CFA and PEO wastewater as a solid and a liquid industrial waste stream is proposed. In this regard, the production of a value-added zeolitic product, i.e., Na-A zeolite, is studied using CFA and PEO wastewater. This process can be considered for large-scale conversion of CFA. The microwave-assisted production of the LTA zeolite with CFA and PEO wastewater as starting materials was systematically analyzed and optimized by means of a two-level full-factorial design with one center point per block technique. Applying microwave irradiation at relatively low power of 100–300 W for a short period of time, i.e., 10–30 min, proved to be a feasible technique for scale-up. The process developed in this study will help to reduce the environmental burden associated with these wastes and also offer a new source of revenue for the coal-fired power plant. The zeolitized CFA produced in this research provides a relatively high CEC and good water adsorption capacity, and its high leaching resistance to toxic elements makes this value-added product a promising candidate for environmental remediation applications.
Source: Jamshid Behin, Syed Salman Bukhari, Vahid Dehnavi, Hossein Kazemian, Sohrab Rohani - Western University (UWO), Department of Chemical and Biochemical Engineering, London, ON, Canada
Advertisement
The 10 largest coal producers and exporters in Indonesia:
- Bumi Resouces
- Adaro Energy
- Indo Tambangraya Megah
- Bukit Asam
- Baramulti Sukses Sarana
- Harum Energy
- Mitrabara Adiperdana
- Samindo Resources
- United Tractors
- Berau Coal
