Mössbauer and X-ray Studies of Phase Composition of Fly Ashes Formed after Combustion of Ekibastuz Coal (Kazakhstan)

Abstract

Mössbauer spectroscopy and X-ray diffraction have been used to study samples of fly ashes formed after combustion of coal from the Ekibastuz basin at the thermal power plants TPP-2 and TPP-3 in Almaty (Kazakhstan). It has been established that the fractions of fly ashes contain iron in the form of magnetite Fe₃O₄ and hematite α-Fe₂O₃. The mixed valence of iron Fe³⁺ ↔ Fe²⁺ in the octahedral sublattice of magnetite is destroyed by isostructural substitution impurities. Maghemite γ-Fe₂O₃ is additionally present in the fly ash of TPP-3 as a product of magnetite slow oxidation. It was shown that at T ≥ 1400 °C the proportion of magnetite in fly ashes increases due to decomposition of hematite, maghemite, hercynite and the drop of iron content in mullite. It was concluded that the amount of iron in magnetite is a temperature indicator of fly ashes formation. The parameters of hyperfine interactions have been determined in the iron-containing minerals of fly ashes. It was identified that formation of the fly ashes structure occurs in oxidizing atmosphere, since no traces were revealed of reducing environment effect on the phase composition.

1. Introduction

Kazakhstan is the country with the huge hydrocarbon reserves, which have a significant impact on the formation and state of the global power market. About 80% of Kazakhstan’s energy supply comes from the use of solid fuel–coal. The main source of fuel for thermal power plants (TPP) is the Ekibastuz coal basin. Coal from this deposit is relatively cheap, but it is characterized by a high ash content of 50% [1,2].

Ash is formed from the mineral part of the fuel after its complete combustion and precipitates from the combustion gases by the ash collecting devices; it is finely dispersed material with ≤ 0.16 mm grains. Currently, a huge amount of ash and slag mixture has been accumulated in the ash dumps of Kazakhstan, which creates a great threat to the environment and human health. Fly ashes of TPP plants are widely used as a raw material for industry [3,4,5,6]. Today in Kazakhstan less than 1.9 million tons of ash-slag waste is processed, which is 8% of its annual output. Low level of fly ashes application is explained, in particular, by the fact that they are heterogeneous and have a complex mineral and chemical composition. Chemical composition of fly ash of Ekibastuz coal is indicated in many papers, in particular, in [4], the content of the main components is as follows: SiO₂—61.5%, Al₂O₃—27.4%, Fe₂O₃—5.65%, TiO₂—1.49%, CaO—1.17%, MgO—0.49%, K₂O—0.42%, Na₂O—0.32%, P₂O₅—0.52, MnO₂—0.17 and SO₃—0.57.

Many papers have been devoted to the study of fly ashes. It has been shown in [7,8,9,10,11,12] that the main magnetic components of fly ashes are magnetite Fe₃O₄ and hematite α-Fe₂O₃. The presence of maghemite γ-Fe₂O₃ in fly ash was detected in [11,13]. It is known that fly ashes contain minerals such as hercynite [13] and mullite [14], as well as silicate glasses in the form of quartz and various calcium silicates with the impurities of iron. However, in contrast to [13], the presence of hercynite in fly ash was not found in [5,14]. Despite the fairly extensive research, the detailed interpretation of the physical–chemical properties of fly ashes is still missing. In many of these studies, the Mössbauer spectroscopy, as a unique method for studying the phase composition of iron-containing phases, has not been used quite effectively. The mineral composition and properties of the ash-and-slag waste are directly related to the geological origin of the original coal, as well as to other several factors [11,15,16] determined by the technical features of certain power plants. Therefore, we should always consider local technological processing of the ash-and-slag waste in a particular region and even in a particular fuel plant.

The purpose of this paper is to study the fly ashes generated upon the combustion of the Ekibastuz coal at TPP-2 and TPP-3 in Almaty, identification of common patterns and individual characteristics of mineral components formation of fly ashes, their phase composition and dependence on the technological parameters of the furnace units.

2. Materials and Methods

Coal samples (Ekibastuz, Kazakhstan) from the Ekibastuz coal basin and fly ashes FA1 and FA2, produced by its combustion at the Almaty thermal power plants TPP-2 and TPP-3, respectively, were studied. TPP-2 is equipped with 8 power boilers BKZ-420-140 (Barnaul Boiler Plant, Barnaul, Russia) with a temperature in the torch core of 1500–1920 °C. TPP-3 is equipped with 6 boilers BKZ-160-100 (Barnaul Boiler Plant, Barnaul, Russia) with a temperature in the torch core of 1400–1500 °C. The powdered highly reactive substance is obtained as a result of the preliminary preparation of the fuel, which is actively ignited when it is mixed with air in the boiler furnace. Primary air is supplied through the burners at a rate of 25–30 m/s; secondary air is supplied at a rate of 40–60 m/s. Fine particles of coal combustion products were removed from the filters of the flue gas cleaning system and the samples were prepared for investigations by the methods of Mössbauer spectroscopy (MS) and X-ray diffraction (XRD). In some cases, fly ash was divided into magnetic and non-magnetic fractions. The method of magnetic separation was used for this purpose.

For the MS investigations the samples of fly ashes were mixed with paraffin at a rate of 100 mg/cm² and pressed into tablets. The Mössbauer spectra of the samples were recorded in the transmission geometry. The source of γ-radiation was ⁵⁷Co in the chromium matrix. The spectra were recorded at 300 K on a nuclear gamma-resonance spectrometer MS1104Em (Research Institute of Southern Federal University, Rostov-on-Don, Russia). The reference sample α-Fe was used for spectra calibration. The Mössbauer spectra were processed using the software SpectrRelax (Version 2.4, Lomonosov Moscow State University, Moscow, Russia) by the methods of model fitting and restoration of distributions of hyperfine parameters [17]. This program is characterized by unique capabilities in the study of locally heterogeneous systems, such as fly ash.

The X-ray diffraction studies were performed on a diffractometer D8 Advance (Bruker AXS GmbH, Karlsruhe, Germany) with copper radiation λ_κα = 0.154051 nm and the graphite reflector. The powder X-ray files ASTM and 7CPDS were used to identify the phases.

3. Results

3.1. Mössbauer Spectroscopy

Mössbauer spectroscopy at ⁵⁷Fe nuclei is the best method for detecting the iron-containing compounds and determination of the chemical state, electronic and magnetic structure of iron in them [18,19,20,21]. The MS spectrum of the Ekibastuz coal sample consists of two doublets (Figure 1). In terms of hyperfine interactions parameters (δ-isomer shift; ε-quadrupole shift) the first of them (the predominant one) belongs to the bivalent iron Fe²⁺ in the crystal structure of iron carbonate–siderite FeCO₃ (δ = 1.230 ± 0.005 mm/s, ε = 0.888 ± 0.005 mm/s). The remaining part of the experimental curve (13% ± 1%) is well interpreted via Mössbauer parameters of the trivalent iron Fe³⁺ in the crystal structure of iron sulfide–pyrite FeS₂ (δ = 0.286 ± 0.008 mm/s, ε = 0.307 ± 0.008 mm/s).

Figure 2a,b shows the MS spectra and distributions of the hyperfine fields for the samples of fly ashes FA1 and FA2 respectively.

Table 1. Hyperfine parameters of the magnetic fraction obtained from the MS spectrum of the fly ash FA1.

Subspectrum	I, (%)	δ, mm/s	ε, mm/s	Hn, kOe	Assignment
H1	3.4 ± 0.8	0.39 ± 0.01	−0.09 ± 0.01	512 ± 1	Hematite
H2	19.9 ± 0.9	0.32 ± 0.01	0.01 ± 0.01	482 ± 1	Magnetite [Fe3+]tetra
H3	15.1 ± 0.9	0.55 ± 0.01	0.03 ± 0.01	468 ± 1	Magnetite [Fe3+]octa
H4	18.0 ± 0.9	0.57 ± 0.01	0.02 ± 0.01	443 ± 2	Magnetite [Fe2+]octa
H5	7.4 ± 0.4	0.60 ± 0.01	0.00 ± 0.01	414 ± 3	Magnetite [Fe2+]octa
H6	3.0 ± 0.4	0.63 ± 0.01	0.02 ± 0.01	385 ± 4	Magnetite [Fe2+]octa
H7	1.5 ± 0.3	0.66 ± 0.01	0.03 ± 0.01	358 ± 2	Magnetite [Fe2+]octa

I: relative spectral area; δ: isomer shift relative to metallic iron at 300 K; ε: quadrupole shift; H_n: hyperfine field at ⁵⁷Fe nuclei at maximum of the distribution.

shows the Mössbauer parameters of the magnetic part of spectrum of the fly ash FA1 and their interpretation. As can be seen, the spectra have a rather complicated structure. The iron atoms occupy several positions in magnetically ordered and paramagnetic states. Therefore, for processing of the experimental data, the methods of model interpretation and restoration of distributions of hyperfine parameters of the Mössbauer spectra were used. The MS spectrum of the magnetic part of the ash FA1 is represented by a sextet with a hyperfine magnetic field of 512 kOe (H1) and distributions of the hyperfine magnetic fields of ⁵⁷Fe in the intervals of 470–510 (H2) and 350–480 kOe (H3–H7). Both distributions of P(H) were carried out by representing one Mössbauer phase. Subsequent calculation of the hyperfine parameters with approximation of the spectra by the Lorentzian shape of the spectral lines was not carried out. Data presented in Table 1 and

Table 2. Hyperfine parameters of the magnetic fraction obtained from the MS spectrum of the fly ash FA2.

Subspectrum	I, (%)	δ, mm/s	ε, mm/s	Hn, kOe	Assignment
H1	13.7 ± 0.9	0.34 ± 0.01	−0.09 ± 0.01	512 ± 1	Hematite
M1	4.2 ± 0.9	0.39 ± 0.01	0.00 ± 0.01	495 ± 1	Maghemite [Fe3+]tetra
M2	4.4 ± 0.2	0.48 ± 0.02	0.00 ± 0.01	477 ± 2	Maghemite [Fe3+]octa
H2	8.2 ± 0.9	0.28 ± 0.02	0.00 ± 0.01	482 ± 1	Magnetite [Fe3+]tetra
H3	10.0 ± 0.9	0.52 ± 0.01	0.03 ± 0.01	458 ± 4	Magnetite [Fe3+]octa
H4	6.8 ± 0.9	0.53 ± 0.01	0.02 ± 0.01	434 ± 5	Magnetite [Fe2+]octa
H5	4.6 ± 0.5	0.54 ± 0.01	0.02 ± 0.01	402 ± 5	Magnetite [Fe2+]octa
H6	2.2 ± 0.4	0.55 ± 0.01	0.00 ± 0.01	359 ± 2	Magnetite [Fe2+]octa

were obtained from the analysis of the areas under the maxima of the distribution function of hyperfine magnetic fields. Due to heterogeneity of the local environment of the Mössbauer absorber nuclei, the shape of the resonance line was described by the PseudoVoigt function:

Z_PV(v) = (1 − α) Z_L(v) + αZ_G(v),

where Z_L(v) is the Lorentz function and Z_G(v) is the Gaussian function.

Table 1 shows that the main component of fly ash FA1 is magnetite Fe₃O₄. This mineral has a spinel structure. In the stoichiometric magnetite, iron is in two crystallographically non-equivalent positions: tetrahedral (A) and octahedral (B) [22]. The spectrum of pure magnetite [Fe³⁺]^tetra[Fe²⁺, Fe³⁺]^octaO₄ consists of only two sextets with the hyperfine magnetic fields 490 and 460 kOe, representing 1/3 and 2/3 of the entire area of magnetite spectrum, respectively; i.e., S(A)/S(B) = 0.5. The first sextet corresponds to Fe³⁺ cations with oxygen atoms in the tetrahedral environment, and the second sextet corresponds to Fe²⁺ and Fe³⁺ cations in the octahedral environment. In this case, the octahedral node can be assumed to be occupied by Fe^2.5+ cations. The analysis of the spectrum of ash FA1 shows that the sextet H1 has the parameters inherent to hematite α-Fe₂O₃. The sextet H2 is specified by the Fe³⁺ ions in the tetrahedral position of Fe₃O₄. The sextet H3 corresponds to the Fe³⁺ ions and the sextets H4–H7 correspond to the Fe²⁺ ions in the octahedral position of Fe₃O₄. The presence of two types of sextets H3 and H4–H7 is associated with the significant non-stoichiometry of magnetite. The occupancy ratio for the positions S(A)/S(B) = 0.44 ± 0.05, which is somewhat less than in the case of stoichiometric magnetite. This may be caused by the presence of some magnetically ordered phase in the fly ash, for example, magnesioferrite MgFe₂O₄.

The composition and properties of magnetite depend on conditions of its formation. This feature is associated with the fact that the isomorphic capacity of Fe₃O₄ is largely a function of temperature. At high temperatures, magnetite is able to capture impurities of several elements [23]. As a result of processing of the MS spectrum of the fly ash FA1, it has been established that the distribution of magnetic fields at ⁵⁷Fe nuclei in the octahedral sublattice has several peaks. In addition, there is a correlation observed between the values of magnetic fields and corresponding isomer shifts. A smaller field corresponds to a larger isomer shift (see Table 1). The growth of impurity ion number in each of the nearest coordination shells around Fe atoms affects the magnetization and magnetic susceptibility. This leads to a change in the effective magnetic field at ⁵⁷Fe nuclei–ΔH_n [23,24]. The quite detailed explanation of the effect of substitutional impurities for certain elements on the values of the isomeric shift and hyperfine magnetic field is given in [25,26,27]. It is known [18,25] that the value of the isomeric shift depends on the degree of filling of s- and d-electron shells. Since the radius difference between the ground and excited states at ⁵⁷Fe nuclei is negative, the isomeric shift increases with decreasing density and decreases with increasing density of external 4s-electrons. The dependence of the isomeric shift on the number of 3d-electrons, which screen 4s-electrons, is opposite. The value of the ΔH_n contributions for the elements to the left of Fe in the periodic table of elements is negative and does not significantly depend on the nature of impurities. Thus, the isomer shift for ⁵⁷Fe nuclei increases with the growth in the number of aluminum atoms in the nearest neighborhood of the iron atom [26]. This fact is explained by the transition of part of aluminum external electrons into the 3d-zone of iron.

Based on [23], magnetite formed at high temperature is capable of containing a number of isomorphic impurities (Al, Ti, Mg, Mn, Cr, Co, etc.). These impurities most often occupy the places in octahedral coordination, replacing Fe²⁺ or Fe³⁺. Aluminum mainly enters the octahedral coordination, replacing Fe³⁺ ions. [23]. Titanium replaces Fe²⁺ ions in the same coordination of magnetite [23]. Therefore, sextets H3-H7 with hyperfine fields of 358–468 kOe can be interpreted as a superposition of sextets, corresponding to ⁵⁷Fe nuclei in the octa position of Fe₃O₄ with isomorphic impurities (Al and Ti).

The paramagnetic part of the spectrum of fly ash FA1 was fitted by two quadrupole shift distributions, separately for Fe²⁺ and Fe³⁺ ions with the relative spectral areas of 18.6% ± 0.7% and 13.1% ± 0.8%, respectively.

The spectrum of the ash FA2 was processed in the same model as the spectrum of FA1. Content of hematite α-Fe₂O₃ (sextet H1) in the fly ash FA2 was significantly higher than in the fly ash FA1. As in the case of the fly ash FA1, the sextet H2 was assigned to the ions Fe³⁺ in the tetrahedral position of magnetite Fe₃O₄. The sextets H3 and H4–H7 were specified by unequal positions of Fe³⁺ and Fe²⁺ ions respectively in the octahedral position. As it can be noted from Figure 2, distribution of hyperfine magnetic fields in the tetrahedral sublattice of the magnetite of ash FA2 was broader compared to the similar distribution of ash FA1. Since S(A)/S(B) = 0.62 ± 0.09, it should be assumed that the sextet H2 was complemented by the contribution due to foreign iron-containing impurities. It should be considered that such an impurity is a maghemite γ-Fe₂O₃. Maghemite, like magnetite, has a reversed spinel structure; therefore, its MS spectrum contains two sextets due to Fe³⁺ ions in the tetrahedral position in the A-sublattice and Fe³⁺ in the octahedral position in the B-sublattice [28,29]. Owing to the fact that the values of hyperfine parameters from Fe³⁺ ions in the A- and B-sublattice are close in value, the spectrum of γ-Fe₂O₃ is often interpreted as one sextet [11,13]. Considering above, the spectrum of the ash FA2 was processed according to the model that contained the added two sextets with the Mössbauer parameters of maghemite [28]. Fitting of the spectrum of the fly ash FA2 (Figure 3) showed that the magnetic part of the spectrum was best described by four sextets and one distribution of ⁵⁷Fe hyperfine magnetic fields in the interval of 350–480 kOe. The parameters and interpretation of the magnetic part of the spectrum for the ash FA2 are provided in Table 2. S(A)/S(B) = 0.36 ± 0.09, which corresponded to the population of Fe³⁺ ions in the A- and B-positions of magnetite. In this case, the described above hypothesis on the presence of certain magnetically ordered phase in the fly ash was considered. It should be noted that the magnetic fields at ⁵⁷Fe nuclei in α-Fe₂O₃, γ-Fe₂O₃ and Fe₃O₄ had lower values compared with those for the well-crystallized minerals. This is typical of the finely dispersed magnetic materials.

The paramagnetic part of the spectrum of the fly ash FA2, as well as FA1, is represented by two quadrupole shifts distributions, separately for Fe²⁺ and Fe³⁺ ions with relative spectral areas of 25.7% ± 0.9% and 20.2% ± 0.9%, respectively.

The samples of the ashes FA1 and FA2, subjected to magnetic separation, were studied for interpretation of the paramagnetic part. As it was mentioned in the Introduction the non-magnetic part of fly ashes contains minerals such as hercynite and mullite. Hercynite is a ferrous mineral with the structure of normal spinel, a paramagnet with cubic syngony, general formula is FeAl₂O₄. The theoretical composition of hercynite is FeO—41.34% and Al₂O₃—58.66% [30]. Often Fe is isomorphically replaced by Mg, sometimes by Mn; Al is replaced by Fe and Cr. Based on the structural type of normal spinel, the bivalent cations (Fe²⁺) are surrounded by four oxygen ions in tetrahedral coordination; while the trivalent cations (Al³⁺, Fe³⁺) are surrounded by six oxygen ions at the vertices of the octahedra. It should be noted that each ion of oxygen is associated with one bivalent and three trivalent cations. The structure of hercynite is characterized by a combination of isometric tetrahedra and octahedra in such a way that each vertex is common for one tetrahedron and three octahedra [31]. The structural formula of this mineral can be represented as follows: [Fe²⁺ IV]^tetra[Fe²⁺ VI (Al³⁺ VI)]^octa. The symbols IV and VI correspond to tetra and octa coordination, respectively.

Mullite is one of the main components of fly ash and the only mineral from the aluminosilicate family that is stable at high temperature (T_melt = 1810–1830 °C) [32]. It belongs to the most important technical materials and therefore is the subject of numerous studies, which are summarized in the review [33]. Mullite has a variable composition, ranging from 3Al₂O₃·2SiO₂ to 2Al₂O₃·SiO₂. Its structure is rhombic. According to the authors of [34], mullite is a chain silicate rich in aluminum. Aluminum ions can be contained in the lattice with a coordination number 6 and 4, appearing both in the octahedral and tetrahedral positions. The structural formula of mullite can be written as follows [4Al³⁺ VI]^octa [Al³⁺ IV (Si₂⁴⁺ IV)]^tetra O₁₃. Mullite is able to form the solid solutions with various oxides [35]. The substitutional solid solution is formed when Fe₂O₃ is added. In this case, the Fe³⁺ ions replace the Si⁴⁺ ions in octahedral and tetrahedral positions. When FeO is added, the Fe²⁺ ions can replace the Si⁴⁺ ions in the tetrahedral position, but the probability of such chemical transformation is low [36,37]. In mullite, isovalent and heterovalent isomorphism is observed. According to Goldschmidt’s law [38], if the crystal lattice has vacant sites, then charge for heterovalent isomorphism is compensated by additional ions. As a result, one ion of higher valence is replaced by two ions of lower valence. It should be expected that iron in mullite due to isomorphic substitution can appear in at least three non-equivalent positions: (Fe³⁺ → Al³⁺)^octa, (Fe³⁺ → Al³⁺)^tetra, (Fe²⁺ → Si⁴⁺)^tetra. Partial isomorphic substitution of Al³⁺ by Fe³⁺ in the lattices of hercynite and mullite occurs due to proximity of the ionic radii of Al³⁺ and Fe³⁺ (0.57 and 0.67 Å respectively).

Considering above, the MS spectra of fly ashes FA1 and FA2 after magnetic separation (Figure 4) were processed with sufficient quality using two groups of doublets. The first group consists of three doublets, characterizing the Fe²⁺ ions in the tetrahedral and the Fe²⁺, Fe³⁺ ions in the octahedral environment in the crystal lattice of hercynite. The parameters of these doublets are similar to the parameters of hercynite produced by the arc-melting method under the protective argon atmosphere [39]. The second group also consists of three doublets, defined by Fe³⁺ ions in the octahedral Fe³⁺, Fe²⁺ in the tetrahedral environment and in the crystal lattice of mullite. In addition, the spectrum contains a doublet related to Fe²⁺ in the octahedral environment of silicate glass [11,14]. The parameters of doublets describing the non-magnetic fraction of the fly ashes FA1 and FA2 are provided in

Table 3. Hyperfine parameters of the non-magnetic fraction of the fly ashes FA1 and FA2.

Sample	Subspectrum	I, (%)	δ, mm/s	ε, mm/s	Assignment
FA1	D1	27.7 ± 0.9	1.16 ± 0.01	0.82 ± 0.01	Hercynite [Fe2+]tetra
	D2	5.0 ± 0.1	1.33 ± 0.04	1.26 ± 0.03	Hercynite [Fe2+]octa
	D3	3.3 ± 0.2	0.30 ± 0.09	0.35 ± 0.09	Hercynite [Fe3+]octa
	D4	27.0 ± 0.7	0.41 ± 0.05	0.67 ± 0.09	Mullite [Fe3+]octa
	D5	9.4 ± 0.1	0.44 ± 0.09	0.35 ± 0.08	Mullite [Fe3+]tetra
	D6	4.1 ± 0.3	0.78 ± 0.05	0.66 ± 0.05	Mullite [Fe2+]tetra
	D7	23.5 ± 0.9	1.10 ± 0.01	1.23 ± 0.01	Silicate [Fe2+]octa
FA2	D1	29.2 ± 0.9	1.17 ± 0.01	0.83 ± 0.01	Hercynite [Fe2+]tetra
	D2	5.2 ± 0.1	1.35 ± 0.04	1.26 ± 0.03	Hercynite [Fe2+]octa
	D3	3.5 ± 0.2	0.30 ± 0.09	0.35 ± 0.09	Hercynite [Fe3+]octa
	D4	27.4 ± 0.9	0.42 ± 0.05	0.60 ± 0.09	Mullite [Fe3+]octa
	D5	9.6 ± 0.1	0.40 ± 0.09	0.32 ± 0.08	Mullite [Fe3+]tetra
	D6	4.1 ± 0.1	0.78 ± 0.05	0.66 ± 0.05	Mullite [Fe2+]tetra
	D7	21.0 ± 0.9	1.14 ± 0.01	1.26 ± 0.01	Silicate [Fe2+]octa

.

Thus, the spectra of the fly ashes FA1 and FA2 are generally similar but differ in two important aspects. The amount of hematite α-Fe₂O₃ was almost 4 times more in the ash FA2. It is known [13,16] that magnetite is a more stable oxide at high temperatures, while hematite is more stable at T < 1388 °C. To confirm this fact, coal was burned at the temperature of 800–900 °C [40]. It turned out that magnetically ordered iron was predominantly in the form of α-Fe₂O₃ in the composition of the fly ash, iron in the form of Fe₃O₄ was also present, but in a much smaller amount.

FeS₂ and FeCO₃ decayed when coal was burned in the Ekibastuz basin at T ≈ 500 °C. There was a gradual increase of the oxidation state of iron: first, formation of FeO, then Fe₃O₄ and finally, α-Fe₂O₃ through the intermediate γ-Fe₂O₃ phase. Oxidation to higher oxide continued to the temperature of T = 1388 °C. At T ≥ 1400 °C α-Fe₂O₃ was converted to Fe₃O₄ in carbon atmosphere. Formation of magnetically ordered minerals in the furnace unit can be represented as follows:

FeS₂ + FeCO₃ + MO₂ → FeO → Fe₃O₄ → Fe₃O₄ + γ-Fe₂O₃ → α-Fe₂O₃ + Fe₃O₄ + γ-Fe₂O₃ → Fe₃O₄;

where M is the amount of O₂, depending on the operation mode of the furnace unit.

Two competing processes can occur while burning coal: the reactions of thermo-chemical transformation in the reducing carbon atmosphere and the reactions occurring in the oxidizing atmosphere. As a result, the mineral components of coal being located in the flare flame (3–5 s) at the temperature of 1400–1900 °C turn into fly ash. It is well known from the reactions, occurring in the blast furnace [41], that a mixture of oxide CO and carbon dioxide CO₂ reduces ore to metallic iron. This happens when the CO₂/CO ratio is maintained below a certain value, which depends on the temperature. If this value is higher, the iron will oxidize. In normal combustion of coal, a gas mixture was formed, which contained sufficient CO₂ to oxidize the iron. In contrast to the authors of [42], no signs of pure iron presence were found in this study, i.e., no traces of the reducing atmosphere were detected. This gives the right to assert that the processes occur in the oxidizing atmosphere.

According to the results of chemical analysis, the content of iron oxides in the fly ash of the Ekibastuz coal was 5.56%. Chemical analysis of elemental composition of fly ashes was carried out by the X-ray fluorescence method. The obtained iron content (5.56%) is close to the value indicated in [4]. Therefore, the amount of iron, localized in the magnetically ordered and paramagnetic matrix, was determined from the relative areas of certain components of the Mössbauer spectra, and then the iron content was calculated in certain components of the studied samples (

Table 4. Content of iron oxides (%) in the samples of fly ashes FA1 and FA2.

Sample	Hematite	Maghemite	Magnetite	Hercynite	Mullite	Silicate
FA1	0.19	-	3.61	0.63	0.71	0.37
FA2	0.76	0.48	1.77	0.97	1.05	0.54

Error is not above 0.04.

). Table 4 shows that iron was concentrated mainly in the structure of magnetite in the fly ash FA1. Maghemite was present only in the fly ash FA2. The lower content of hercynite in fly ash FA1 could be explained by the fact that at high temperatures it decomposed with the formation of magnetite. Indeed, hercynite was present in the fly ash in the form of oriented intergrowths with magnetite. Above 858 °C, the mutual solubility of FeAl₂O₄ and FeFe₂O₄ was unlimited [31]. In addition, this mineral melted incongruently (with decomposition) at T = 1440 °C. At T = 1300 °C, the maximum content of iron (10%) was reached in mullite, but at T = 1400 °C its content decreased (to 6.5%) [33,36]. Thus, at high temperatures due to decay of hematite, maghemite and hercynite and a decrease in the amount of iron in mullite, the fraction of magnetite increased in the fly ash FA1. Consequently, the analysis of the fly ash phase composition enabled us to make the conclusion that the amount of iron, contained in magnetite, was a temperature indicator of fly ashes formation.

3.2. X-ray Diffraction

The use of the X-ray diffraction method complements the information on the phase composition of the studied materials [42,43]. Figure 5 shows X-ray diffraction patterns of the samples FA1 and FA2. Against the background of halo, indicating the presence of the amorphous phase, the reflections of quartz SiO₂ (Q) and mullite Al₆Si₂O₁₃ (Mu) were confidently identified. In addition, there were the reflections of magnetite Fe₃O₄ (Mg), maghemite γ-Fe₂O₃ (Mh), hematite α-Fe₂O₃ (Hm) and hercynite FeAl₂O₄ (He). Differentiation of the reflections responsible for Fe₃O₄ and γ-Fe₂O₃ was difficult since the diffraction peaks in the X-ray images were overlapping. The reflections of maghemite γ-Fe₂O₃ (Mg) were more visible in the X-ray image of the fly ash FA2, which agreed quite well with the Mössbauer results. In addition to the indicated phases, the reflections, close to the reflections of calcite CaCO₃ (Ca), were observed in the X-ray image of the fly ash FA2. Calcite is the most stable of the three modifications of calcium carbonate. During annealing at the temperatures above 1500 °C in the atmosphere of carbon excess pressure, it is transferred to calcium carbide CaC₂ [44]. However, there are no characteristic reflections of this mineral in the X-ray diffraction pattern of the sample FA1.

4. Conclusions

Mössbauer spectroscopy was used to study samples of fly ashes formed after the combustion of coal from the Ekibastuz basin at the thermal power plants TPP-2 and TPP-3 in Almaty. It was established that the fractions of fly ashes contained iron in the form of magnetite Fe₃O₄ and hematite α-Fe₂O₃. The mixed valence of iron Fe³⁺ ↔ Fe²⁺ in the octahedral sublattice of magnetite was destroyed by isostructural substitution impurities. Maghemite γ-Fe₂O₃ was additionally present in the fly ash of TPP-3 as a product of magnetite slow oxidation. It was shown that at T ≥ 1400 °C, the proportion of magnetite increased in fly ash due to decay of hematite, maghemite, hercynite and a decrease in the iron content in mullite. It was concluded that the amount of iron in magnetite was a temperature indicator of fly ashes formation. The parameters of hyperfine interactions were determined in the iron-containing minerals of fly ash. It was identified that formation of the fly ashes structure occurred in the oxidizing atmosphere, since no traces were revealed of reducing environment effect on the phase composition.

The X-ray diffraction method revealed the amorphous state of minerals, quartz, mullite and calcite. In addition, there were the reflections of magnetite, maghemite, hematite α-Fe₂O₃ and hercynite.