Abstract
The fly ash, from the combustion of coal to produce energy and heat, is an industrial waste, in which large accumulations represent a serious environmental threat. To reduce the environmental burden and improve the economic benefits of energy production, the science and industry focus on the transformation of coal combustion byproducts into new functional materials. The fly ash was studied by modern analytical methods. As a result of the hydrothermal reaction, several types of zeolites were synthesised from the fly ash: analcime, faujasite (zeolite X) and gismondine (zeolite P). It was shown that the experimental conditions (temperature, reaction time and alkali concentration) have a significant influence on the type of zeolite and its content in the reaction products. The series of experiments resulted in building approximate crystallisation field of zeolites and other phases as the first stage of the formation of ceramic membrane and other materials.
Introduction
Fly ash is a bulk industrial waste of coal combustion in thermal power plants, steel mills, etc. Therefore, the problem of utilisation of this technogenic waste, occupying large areas and causing damage to the environment, is very important. Many papers were published on the properties of fly ash and possibilities of its use.1 2 3 4 –5 Nevertheless, the development of advanced technologies for the utilisation of fly ash remains an important task. One of their solutions may be represented by the synthesis of zeolites from fly ash. Conversion of the fly ash in zeolites has many applications, including ion exchange, molecular sieves and adsorbents.6 7 –8 Identification of new applications has a real commercial interest: the list of marketable products is expanding and energy costs are reduced, environmental risks are reduced and the efficiency of sustainable development in the region is increased. Coal fly ash contains significant amounts of SiO2, Al2O3 and other oxides, which are regarded as cheap raw for the ceramic industry. Technologies of zeolite synthesis from fly ash are being constantly improved in both the experimental (variations of temperature, pressure, coreagent and other methods of exposure) sphere and the material composition of the initial raw;9 10 –11 the quality, function and cost of final product depend on it. The synthesis of zeolites from fly ash is the first stage in the formation of ceramic materials (ceramic membranes), which defines the significance of this trend of research.
The aim of this work is to develop the scientific basis for the formation of ceramic materials with given properties, in order to achieve that it is important to determine material composition of the fly ash produced from coal combustion at thermal power stations of Pechora coal basin as raw material for ceramic membranes and ceramic materials.
Objects and methods
For the experiments, we used the fly ash from thermal power plants of Pechora coal basin.
The synthesis methods of zeolites were based on Ref. 9. First, using a magnetic separator, we removed ferriferous phases, which do not participate in the synthesis of zeolites. Dry fly ash is mixed with the solution of sodium hydroxide (NaOH) in a certain ratio, mixed thoroughly, and the suspension was placed in an autoclave. The resulting products of hydrothermal reaction was washed with distilled water and dried. This resulted in powders consisting of the mixture of zeolite and unreacted residue in various proportions.
The chemical composition of the fly ash and the products of hydrothermal reaction was determined as follows: Na2O, K2O, FeO, loss on ignition and CO2, obtained by the complete silica analysis, and the other components with the help of X-ray fluorescence analysis (energy dispersive spectrometer MESA500W, Horiba).
The phase composition studies were performed on a powder diffractometer (Shimadzu XRD 6000, radiation Cu Kα, Ni filter) within the range of 2–65° 2θ angle with rate 2° 2θ min− 1. Identification of zeolites was carried out using databases ‘WWW-MINCRYST’ (http://database.iem.ac.ru/mincryst/rus/index.php) and International Zeolite Association (http://www.iza-structure.org/databases).
To study the morphology and chemical composition of the fly ash, we used a scanning electron microscope TESCAN VEGA 3 LMH with energy dispersive Oxford Instruments X-Max.
Results and discussion
Initial fly ash
X-ray diffraction (Fig. 1) showed quartz, mullite, magnetite and hematite in the fly ash. The broad ‘hump’ (area of increased background) on the diffraction pattern in the area of 15–35° 2θ indicates the presence of amorphous phase (probably silicate or aluminosilicate glass).
Diffraction patterns of products synthesised at 80, 95, 140 and 180°C for 12 h (Q: quartz; X: zeolite X; P: zeolite P; A: analcime; K: cancrinite)
The main components of the chemical composition are oxides of silicon (57.78%) and aluminium (18.25%), iron oxide content of ∼9.0%, oxides of other elements of 7.42% and loss on ignition of 7.90% (Table 1).
Chemical composition of fly ash
The fly ash is represented under the electron microscope by globules (Fig. 2), which are divided by the chemical composition to aluminosilicate and iron containing. The aluminosilicate globules composition is predominated by silica (from 41.82 to 61.27%) and alumina (from 17.03 to 22.8%); oxides of iron (up to 8.31%), magnesium (up to 4.83%), potassium (up to 3.05%), titanium (up to 1.04 %) and sodium (up to 0.93%) are also represented. Globule size varies from the first to about hundred micrometres; on the surface, bubbles and elongated structures (Fig. 2b) are observed.
Images (SEM) of surface of a, b aluminium silicate and c, d iron containing globules
On the surface of iron containing globules (Fig. 2c and d), both flat areas and skeletal forms are observed, which are significantly different from each other by their chemical composition. The skeletal forms have a high content of iron oxides (68.14–74.66%) and low silica (1.06–6.22%), alumina (1.33–4.17%) and calcium oxides (0.48–3.59%) contents. On the flat areas, content of iron oxides is greatly reduced (19.29–31.81%), and silica and alumina content is increased (27.12–37.86 and 2.06–6.22% respectively); calcium oxides present in large amounts (10.45–25.3%). Globule size ranges from several to tens of micrometres. Globules, which contain smaller globules within, are often observed (Fig. 1b).
Hydrothermal synthesis
There were two sets of experiments. In the first set, the effect of temperature of hydrothermal reaction on zeolite synthesis was studied (reaction temperature, 80, 95, 140 and 180°C; reaction time, 12 h; the ratio of NaOH/fly ash = 1:1; NaOH concentration, 3.0 mol dm− 3). The second set of experiments studied the influence of reaction time and concentration of alkali on synthesis process (reaction temperature, 140°C; duration, 2, 4, 6 and 8 h; ratio of NaOH/fly ash = 1:1; NaOH concentration, 1.5, 3.0 and 4.5 mol dm− 3).
The process of transformation of the fly ash to zeolites can be represented in the following way:
According to Refs. 10 and 11, this process consists of three stages: dissolution, condensation and crystallisation. When fly ash interacts with sodium hydroxide, it is dissolved, and Si and Al are released to the solution. Then, the condensation of silicon and aluminium ions occurs, followed by gelling and nucleation (forming nuclei or crystallisation centres) and crystallisation of zeolites.
The synthesis results in powders consisting of the mixture of zeolite and unreacted residue in different proportions, in which the output was 70–80% of the weight of the initial fly ash. The bulk density of powder is 0.78–0.83 g cm− 3.
Effect of reaction temperature on synthesis of zeolites
In the result of the reaction at 80°C, the intense reflections of quartz were diagnosed; no newly formed phases were detected (Fig. 1). Electron microscopic studies revealed numerous globules destroyed by alkaline solution.
By increasing the reaction temperature to 95°C silica, the intensity of quartz reflections decreased, i.e. it was dissolved in alkaline solution. Alongside with quartz reflections, the intense reflections were determined, which are characteristic for faujasite zeolite (zeolite A), and weak reflections characteristic for gismondine zeolites (zeolite P). By Si/Al ratio, the zeolites are low silica: silica–aluminium module of zeolite X varies from 1.51 to 1.57, and zeolite P varied from 1.65 to 1.69. Images (SEM) present numerous crystals of zeolite X with octahedral shape with the size of 1–3 mm (Fig. 3a and b). Zeolite P crystals have a rounded shape; their size is ∼5 μm (Fig. 3b).
a accumulation of zeolite X crystals; b crystals of zeolites X and P; c single crystal of zeolite P; d analcime and zeolite P crystals; e single crystal of analcime; f accumulation of crystals of cancrinite
The diffraction patterns of the reaction products obtained at 140°C showed zeolite P and analcime; weak quartz reflections were also present. Zeolite P is more high silica compared to the phase obtained at 95°C: Si/Al ratio varies slightly from 1.93 to 1.94. Silica–aluminium module of analcime varies from 2.12 to 2.21. Images (SEM) showed that zeolite P formed skeletal crystals with size of 10–15 μm (Fig. 3c). Analcime crystals with size of 15–20 μm were observed (Fig. 3d).
The reaction at 180°C resulted in the formation of analcime and cancrinite; no quartz reflections were diagnosed. Si/Al ratio of analcime varies from 2.00 to 2.15. As seen in Fig. 3e, the analcime crystals are formed by tetragonal faces, and their size ranges from 15 to 25 μm. Cancrinite columnar crystals with length of up to 2 μm and about 200–300 nm in diameter (Fig. 3f) are often observed on the surface of analcime, indicating later crystallisation of cancrinite.
These results indicate that the reaction temperature influences the type of synthesised zeolite, which differs by the efficient diameter of pore sizes, and according to the classification, 12 they are divided into narrow, medium and wide porous types. It is determined that increasing reaction temperature results in the formation of narrow porous zeolites: at 95°C, zeolites X formed that are related to wide porous type; at 140°C, zeolite P related to medium porous type; and at 180°C, analcime related to narrow porous zeolite. Pore sizes of zeolites were as follows: analcime, 0.26 and 0.42 × 0.16 nm; zeolite P, 0.31 × 0.45 and 0.28 × 0.48 nm; and zeolite X, 0.74 nm.
Effect of reaction time and alkali concentration on zeolite type
The set of experiments resulted in the approximate crystallisation field of zeolites and other phases (hydrosodalite) at 140°C, reaction time from 2 to 8 h and NaOH concentrations 1.5, 3.0 and 4.5 (Fig. 4).
Diffraction patterns of products synthesised at 140°C for 2, 4, 6 and 8 h at NaOH concentrations of a 1.5, b 3.0 and c 4.5 mol dm− 3 (Q: quartz; P: zeolite P; A: analcime; X: zeolite X; HS: hydrosodalite)
As seen in Fig. 5, the wide porous zeolites X are formed by 4 h of reaction at a high concentration of alkaline solution (4.5 mol dm− 3). Longer reaction leads to the disappearance of the metastable phases of zeolite X and the occurrence of more thermodynamically stable zeolite P and then analcime.
Approximate field of crystallisation of zeolites from fly ash with varying times and NaOH concentrations (X: zeolite X; P: zeolite P; A: analcime; HS: hydroxysodalite); P*: XRD weak peaks, probably of zeolite P, show low crystallisation phases on plot
Zeolite P is crystallised under a wide range of reaction conditions. At the same time, the fields of crystallisation of analcime and zeolite P are significantly overlapped; that is, at the same conditions of the hydrothermal reaction, the mixture of zeolites in various quantitative relations is formed. Higher concentrations of alkali results in the increase in the content of narrow porous phases (analcime) compared to zeolite P and contributes to the formation of non-zeolitic phase, hydrosodalite.
Conclusions
Several types of zeolites, e.g. analcime, faujasite and gismondine zeolites, were synthesised as the result of hydrothermal reaction at 80–180°C from the fly ash of a power plant (Pechora coal basin, Russia) by adding sodium hydroxide at concentration from 1.5 to 4.5 mol dm− 3. The final product was powder: mix of zeolite and unreacted residue in various ratios with yield of 70–80% to the weight of initial fly ash. The bulk density was 0.78–0.83 g cm− 3. We determined that the reaction temperature affects the type of synthesised zeolites: wide porous zeolites were formed at 90–100°C; increasing reaction temperature results in the formation of medium and narrow porous types. It is shown that the type of zeolite and its content in the reaction products are significantly affected by the reaction time and the concentration of alkali. The experiments resulted in the approximate crystallisation field of zeolites and other phases as the first stage of ceramic formation.
Footnotes
Acknowledgement
This work was supported by the Program of UB RAS (project no. 15-18-5-45).