Morphology and Composition of Microspheres in Fly Ash from the Luohuang Power Plant, Chongqing, Southwestern China

In order to effectively raise both utilization rate and additional value of fly ash, X-Ray diffraction (XRD), scanning electron microscope (SEM) and energy-dispersive X-Ray spectrometer (EDS) were used to investigate the morphology, and chemical and mineral composition of the microspheres in fly ash from the Luohuang coal-fired power plant, Chongqing, southwestern China. The majority of fly ash particles are various types of microspheres, including porous microsphere, plerospheres (hollow microspheres surrounding sub-microspheres or mineral fragments) and magnetic ferrospheres. Maghemite (γ-Fe 2 O 3) crystals with spinel octahedron structure regularly distribute on the surfaces of ferrospheres, which explained the source of their strong magnetism that would facilitate the separation and classification of these magnetic ferrospheres from the fly ash. Microspheres in Luohuang fly ash generally are characterized by an elemental transition through their cross-section: the inner layer consists of Si and O; the chemical component of the middle layer is Si, Al, Fe, Ti, Ca and O; and the Fe-O mass (maghemite or hematite) composes the outer layer (ferrosphere). Studies on composition and morphological characteristics of microspheres in fly ash would provide important information on the utilization of fly ash, especially in the field of materials.


Introduction
Coal accounts for over 74% of the present energy consumption of China and will be the primary energy for the foreseeable future [1].With the rapid economic growth and enormous energy demand in China over the past thirty years, both the coal consumption and the emission of coal-fired fly ash have increased continuously.Environmental pollution and induced human health problems relevant to coal combustion, such as endemic arsenosis, fluorosis, selenosis, and lung cancer, are serious hazards in some districts of China [1][2][3][4][5][6][7][8].There is no doubt that the fly ash stock in China will keep growing for decades to come.Thus, effective utilization of fly ash is urgently needed.
A number of studies showed that some coals are enriched in rare metals (such as Ge, Ga, Nb, Zr, Au and rare earth elements) that can be potentially utilized from coal combustion products [9][10][11][12][13][14]. Research on coal combustion byproducts is of great significance not only environmentally but also economically.The Luohuang power plant, one of the largest thermal power plants in southwestern China, consumes about six million tons of coal each year, with an annual fly ash output of about two million tons.Nearly 90% of this fly ash was sold at a fairly low price as raw material for concrete and cement production after a rough particle size classification.Along with recent deceleration of hydropower and real estate construction and reduced consumption of fly ash in China, numerous coal-fired power plants including the Luohuang plant have been facing increasing environmental and economic pressure.
In addition to valuable rare metals that could be potentially extracted from coal combustion by-production, the microspheres separated from fly ash of power plants are valuable industrial products, owing to their particular chemical and mechanical properties including density, hydrophobicity, thermo conductivity and stability [15][16][17][18].Fly ash microspheres have been widely used to create functional materials, such as thermoset plastics, special concrete, nylon, coating material, high-density polyethylene (HDPE), and others [19][20][21].After a high-temperature (1200-1700 ˝C) thermochemical transformation of the organic matter and mineral constituents in coal during combustion, several morphological types of microspheres may form [22], such as cenospheres, porous microspheres and plerospheres.Plerospheres, as identified and described by Fisher et al. [23] and Goodarzi et al. [24], are hollow microspheres filled with finer microspheres or mineral particles.Meanwhile, the probable element differentiations in fly ash would result in the formation of microspheres with various chemical compositions, such as iron-rich or alumino-silicate microspheres [25,26].Based on the magnetic difference or density variation, several types of fly ash microspheres can be extracted out for the varying application scenarios mentioned above.
Fly ash samples from 15 Chinese coal-fired power plants contained between 10% and 80% microspheres [27].A primary particle count under optical microscope indicated that the percentage of microspheres in Luohuang fly ash exceeds 80% (Table S1).Enrichment of microspheres may allow Luohuang fly ash to be used efficiently and have higher economic value.Chemical and mineral composition and the interior microstructure characteristics of fly ash microspheres were investigated in the present study.Detailed information revealed by this study may not only benefit the eventual extraction of microspheres, but also expand or deepen the utilization of fly ash.

Samples Collection and Analysis
The Luohuang power plant, affiliated with the Huaneng Group, one of the largest coal-fired thermal power enterprises in China, is located in the town of Luohuang, Jiangjin district, approximately 40 km SW of the center of Chongqing city.This plant is mainly fueled with the high ash, medium-high to high sulfur anthracite from the Songzao Coalfield in Chongqing, and other provinces (e.g., Ningxia) occasionally.All six subcritical W-type flame pulverized coal furnaces are applied in this plant.NO x selective catalytic reduction (SCR) technology is adopted, with liquid ammonia as the reducing agent, using imported catalysts.The total flue gas denitration efficiency is above 85%, namely a NO x emission concentration below 200 mg/N¨m 3 .Limestone-gypsum wet flue gas desulfurization (WFGD) system is also applied to control the emission of SO 2 , with a desulfurization efficiency of 96%.The annual output of the desulfurization gypsum reaches one million tons.Six sets of double-room four-field horizontal electrostatic precipitators (ESP) are used to capture the fly ash from the flue gas, with a collection efficiency of 99.7%.Fly ash captured by all four electric fields of each ESP system is transported by a pneumatic ash pipeline to an ash silo, without separation of fine and coarse ash.Fly ash from silos is subsequently classified into three levels according to particle size for sale.
Sampling continued for 13 days by collecting one approximately one-kilogram fly ash sample each day.All 13 fly ash samples were collected through a tap on the ash pipeline connecting to the ESP system.Considering the possible particle size differentiation of fly ash along with the pneumatic transportation distance, sampling point was set close to ash buckets of the ESP.
XRD (X-Ray diffraction) analysis of all these fly ash samples was then performed by a D/max-2500/PC powder diffractometer (Rigaku, Tokyo, Japan), equipped with a Ni-filtered Cu-Kα radiation source and a scintillation detector.All samples were scanned within a 2θ interval of 2.6 ˝-70 ˝, with a step size of 0.01 ˝.Based on the X-Ray diffractograms acquired, JADE 6.5 (MDI, Burbank, CA, USA) and Siroquant™ (Canberra, Australia) were applied to identify and quantify the mineral phases in the sample, respectively.Siroquant™ was developed by Taylor [30] on the basis of diffractogram profiling principles presented by Rietveld [31].Detailed practices of this technique on coal-related materials were further described by Ward et al. [32] and Dai et al. [33,34].Metakaolin and tridymite were consistent in representing the amorphous or glassy material in fly ash in the Siroquant quantitative analysis [35].In this study, however, tridymite was preferred in Siroquant quantitative analysis, considering its better-fitted value compared to that of metakaolin.
XRF (Table S2) and XRD (Table S3) analyses indicate that the chemical and mineral compositions of these samples are quite stable.Thus, one sample whose major elements concentration belongs to the medium level of the 13 fly ash samples was chosen for further study.
The particle size distribution of the selected fly ash sample was analyzed by laser particle analyzer (Malvern Mastersizer 2000, Malvern Instruments, Malvern, UK) in conjunction with a dispersal device (Hydro G, Malvern Instruments).
After sample splitting, approximately one gram of fly ash was made into pellet, polished, and then coated with carbon in a sputtering coater (Q150T ES, Quorum Technologies, Lewes, UK).A Field Emission-Scanning Electron Microscope (FE-SEM, FEI Quanta™ 650 FEG, FEI, Hillsboro, OR, USA), equipped with an energy-dispersive X-Ray spectrometer (EDS, Genesis Apex 4, EDAX Inc., Mahwah, NJ, USA), was applied to observe the microstructure of the microspheres in the polished section, as well as to evaluate the distribution of some elements.The working distance of the SEM-EDS was 10 mm, with beam voltage 20.0 kV, aperture 6, and spot sizes 3-5.5 nm.Images were acquired through a backscatter electron detector or a secondary electron detector.For more details on the FE SEM-EDS working conditions, see Dai et al. [33,34,36,37].

The Particle Size Distribution of the Fly Ash
As shown in Figure 1, the particle size of the Luohuang fly ash is between 0. lithium borate flux (50% Li2B4O7 + 50% LiBO2), and then was fully fused in an automated fusion furnace (CLAISSE TheBee-10, Claisse, Quebec, QC, Canada).Finally, a glass-like disk (diameters 35 mm) was obtained for the XRF analysis.
XRD (X-Ray diffraction) analysis of all these fly ash samples was then performed by a D/max-2500/PC powder diffractometer (Rigaku, Tokyo, Japan), equipped with a Ni-filtered Cu-Kα radiation source and a scintillation detector.All samples were scanned within a 2θ interval of 2.6°-70°, with a step size of 0.01°.Based on the X-Ray diffractograms acquired, JADE 6.5 (MDI, Burbank, CA, USA) and Siroquant™ (Canberra, Australia) were applied to identify and quantify the mineral phases in the sample, respectively.Siroquant™ was developed by Taylor [30] on the basis of diffractogram profiling principles presented by Rietveld [31].Detailed practices of this technique on coal-related materials were further described by Ward et al. [32] and Dai et al. [33,34].Metakaolin and tridymite were consistent in representing the amorphous or glassy material in fly ash in the Siroquant quantitative analysis [35].In this study, however, tridymite was preferred in Siroquant quantitative analysis, considering its better-fitted value compared to that of metakaolin.
XRF (Table S2) and XRD (Table S3) analyses indicate that the chemical and mineral compositions of these samples are quite stable.Thus, one sample whose major elements concentration belongs to the medium level of the 13 fly ash samples was chosen for further study.
The particle size distribution of the selected fly ash sample was analyzed by laser particle analyzer (Malvern Mastersizer 2000, Malvern Instruments, Malvern, UK) in conjunction with a dispersal device (Hydro G, Malvern Instruments).
After sample splitting, approximately one gram of fly ash was made into pellet, polished, and then coated with carbon in a sputtering coater (Q150T ES, Quorum Technologies, Lewes, UK).A Field Emission-Scanning Electron Microscope (FE-SEM, FEI Quanta™ 650 FEG, FEI, Hillsboro, OR, USA), equipped with an energy-dispersive X-Ray spectrometer (EDS, Genesis Apex 4, EDAX Inc., Mahwah, NJ, USA), was applied to observe the microstructure of the microspheres in the polished section, as well as to evaluate the distribution of some elements.The working distance of the SEM-EDS was 10 mm, with beam voltage 20.0 kV, aperture 6, and spot sizes 3-5.5 nm.Images were acquired through a backscatter electron detector or a secondary electron detector.For more details on the FE SEM-EDS working conditions, see Dai et al. [33,34,36,37].

The Particle Size Distribution of the Fly Ash
As shown in Figure 1, the particle size of the Luohuang fly ash is between 0.5 and 400 μm, with D50 (medium diameter) of 50.7 μm, D10 of 6.10 μm and D90 of 178.38 μm, similar to those previously reported by Mardon et al [38], Vassilev et al [39] and Dai et al [40].

Major Elements of the Fly Ash
An obvious characteristic of the chemical composition of the Luohuang fly ash is the enrichment (14.09%) of Fe2O3, which can be attributed to the relatively high percentage of pyrite in the feed coal.As reported by Zhao et al. [41] and Dai et al. [42], the Songzao coals contain 2.15-10.65 wt % Fe2O3, with an average of 7.63%; however, the average Fe2O3 value in common Chinese coals is 5.78% [43].The value of loss on ignition (LOI) was used to represent the content of unburned carbon (4.01%).

Major Elements of the Fly Ash
An obvious characteristic of the chemical composition of the Luohuang fly ash is the enrichment (14.09%) of Fe 2 O 3 , which can be attributed to the relatively high percentage of pyrite in the feed coal.As reported by Zhao et al. [41] and Dai et al. [42], the Songzao coals contain 2.15-10.65 wt % Fe 2 O 3 , with an average of 7.63%; however, the average Fe 2 O 3 value in common Chinese coals is 5.78% [43].The value of loss on ignition (LOI) was used to represent the content of unburned carbon (4.01%).Concentration of major elements in fly ash was given in the form of oxides by XRF.Oxides of major elements, including SiO 2 (48.27%),Al 2 O 3 (21.59%),Fe 2 O 3 (14.09%),and CaO (5.72%), account for 93.4% of the inorganic matter of the fly ash (Table 1).

Mineral Composition of the Fly Ash
During the high-temperature (over than 1400 ˝C) combustion in the pulverized coal furnaces of the Luohuang plant, minerals in coal including kaolinite, illite-montmorillonite mixed layer, pyrite, calcite, siderite, and even anatase and quartz (to an extent at least) have melted.Some newly-formed minerals such as mullite, hematite, maghemite, anhydrite, and maybe a proportion of quartz, were formed by recrystallization as the molten mass cooled down.However, there are still large percentages of major elements existing in fly ash as an amorphous substance, or so-called glass (Table 2).In the XRD pattern (Figure 2), the section where the baseline of the curve is raised up (2θ from 13 ˝to 38 indicates the existence of glass.Percentage of active silicon and aluminum in glass is one of the most important factors affecting the pozzolanic activity of fly ash [44].High percentage of glass (79.5%) made the use of large quantities of the Luohuang fly ash in cement and concrete production possible.Ferromagnetic matter separated from coal fly ash could be utilized in coal cleaning circuits as a dense medium [45], or be used for special concrete [46].Maghemite (γ-Fe 2 O 3 , strong magnetic) accounts for 70.8% of the iron-bearing independent minerals, maghemite plus hematite (α-Fe 2 O 3 , weak magnetic), which will facilitate the separation of ferromagnetic matter from the Luohuang fly ash.Concentration of major elements in fly ash was given in the form of oxides by XRF.Oxides of major elements, including SiO2 (48.27%),Al2O3 (21.59%),Fe2O3 (14.09%), and CaO (5.72%), account for 93.4% of the inorganic matter of the fly ash (Table 1).

Mineral Composition of the Fly Ash
During the high-temperature (over than 1400 °C) combustion in the pulverized coal furnaces of the Luohuang plant, minerals in coal including kaolinite, illite-montmorillonite mixed layer, pyrite, calcite, siderite, and even anatase and quartz (to an extent at least) have melted.Some newly-formed minerals such as mullite, hematite, maghemite, anhydrite, and maybe a proportion of quartz, were formed by recrystallization as the molten mass cooled down.However, there are still large percentages of major elements existing in fly ash as an amorphous substance, or so-called glass (Table 2).In the XRD pattern (Figure 2), the section where the baseline of the curve is raised up (2θ from 13° to 38°) indicates the existence of glass.Percentage of active silicon and aluminum in glass is one of the most important factors affecting the pozzolanic activity of fly ash [44].High percentage of glass (79.5%) made the use of large quantities of the Luohuang fly ash in cement and concrete production possible.Ferromagnetic matter separated from coal fly ash could be utilized in coal cleaning circuits as a dense medium [45], or be used for special concrete [46].Maghemite (γ-Fe2O3, strong magnetic) accounts for 70.8% of the iron-bearing independent minerals, maghemite plus hematite (α-Fe2O3, weak magnetic), which will facilitate the separation of ferromagnetic matter from the Luohuang fly ash.The content of quartz (5.8%) in the fly ash is much lower than that in the feed coal (18.8%, on ash basis), which indicates that quartz, at least partially, melted during the coal combustion at around 1400 ˝C.FeS 2 (pyrite), CaO (calcite) and some alkali metals (e.g., K and Na) in coal are likely to react with the clay, quartz and other minerals in coal to form a low-temperature eutectic mixture [47,48], which can melt at a temperature much lower than the melting points of single minerals (e.g., 1750 ˝C of quartz).Anatase (melting point of 1850 ˝C) existing in feed coal is not detected in the fly ash, which can be attributed to the same reason.

Morphology and Composition of Microspheres in the Fly Ash
Under the scanning electron microscope (SEM), a few irregular mineral fragments, cohesive bodies and debris of microspheres can be observed in the Luohuang fly ash (Figure 3A); the majority are spherical particles called "microspheres" (Figure 3A,B,D,E).Microspheres smaller than 10 µm are uniformly spherical (Figure 3E,F).Porous microspheres (Figure 3D) are found in the Luohuang fly ash.The presence of these pores further increases the specific surface area of the fly ash and enhances its adsorption ability.During the cooling of the molten drop, trapped gas was emitted and gave rise to pores on the surfaces of the porous microspheres.Plerospheres (Figure 3E,F) with a larger diameter (e.g., 100 µm) enclosing sub-microspheres or mineral fragments (mostly <10 µm) are rather common.Fine spheres have better mechanical properties and a higher chemical reactivity [49][50][51][52].The value of Luohuang fly ash might be increased by crushing plerospheres to release the sub-microspheres.The content of quartz (5.8%) in the fly ash is much lower than that in the feed coal (18.8%, on ash basis), which indicates that quartz, at least partially, melted during the coal combustion at around 1400 °C.FeS2 (pyrite), CaO (calcite) and some alkali metals (e.g., K and Na) in coal are likely to react with the clay, quartz and other minerals in coal to form a low-temperature eutectic mixture [47,48], which can melt at a temperature much lower than the melting points of single minerals (e.g., 1750 °C of quartz).Anatase (melting point of 1850 °C) existing in feed coal is not detected in the fly ash, which can be attributed to the same reason.

Morphology and Composition of Microspheres in the Fly Ash
Under the scanning electron microscope (SEM), a few irregular mineral fragments, cohesive bodies and debris of microspheres can be observed in the Luohuang fly ash (Figure 3A); the majority are spherical particles called "microspheres" (Figure 3A,B,D,E).Microspheres smaller than 10 μm are uniformly spherical (Figure 3E,F).Porous microspheres (Figure 3D) are found in the Luohuang fly ash.The presence of these pores further increases the specific surface area of the fly ash and enhances its adsorption ability.During the cooling of the molten drop, trapped gas was emitted and gave rise to pores on the surfaces of the porous microspheres.Plerospheres (Figure 3E,F) with a larger diameter (e.g., 100 μm) enclosing sub-microspheres or mineral fragments (mostly <10 μm) are rather common.Fine spheres have better mechanical properties and a higher chemical reactivity [49][50][51][52].The value of Luohuang fly ash might be increased by crushing plerospheres to release the sub-microspheres.A total of 11 individual microspheres with various particle sizes (30-250 μm) of different types (ferrospheres, porous and plerospheres) and 26 detection points were analyzed using SEM and EDS.Backscattered electron images of several typical microspheres and their EDS analysis data are listed in Figure 3 and Table 3, respectively.In the SEM backscattered electron images, the higher the atomic number of an element, the brighter it appears.As shown in Figure 3A, the majority of the widespread bright microspheres or spots are ferrospheres (Figure 3B,C,F), microspheres coated with iron oxides.According to Figure 3B and the EDS analysis results (Table 3), iron oxides are common on the surfaces of ferrospheres and have a dendritic form [53].In Figure 3C, an enlargement of the area marked in Figure 3B, further shows a spinel octahedron structure of the iron oxide crystals.Additionally, as discussed in Section 3.3., maghemite is the primary iron-bearing mineral in the Luohuang fly ash.It is therefore concluded that these iron oxide crystals are maghemite.Likewise, these ferrospheres are magnetic microspheres.In view of the promising applications of magnetic microspheres in the composites of magnetic materials, magnetic media, adsorbents, catalysts and ion exchangers [22], separating and classifying magnetic microspheres from the fly ash will bring additional value.3B, D3 means the 3rd detection spot in Figure 3D; and so on for the other spots.** The value of oxygen is obtained through calculation rather than physical detection, and is therefore not fully credible.A total of 11 individual microspheres with various particle sizes (30-250 µm) of different types (ferrospheres, porous and plerospheres) and 26 detection points were analyzed using SEM and EDS.Backscattered electron images of several typical microspheres and their EDS analysis data are listed in Figure 3 and Table 3, respectively.In the SEM backscattered electron images, the higher the atomic number of an element, the brighter it appears.As shown in Figure 3A, the majority of the widespread bright microspheres or spots are ferrospheres (Figure 3B,C,F), microspheres coated with iron oxides.According to Figure 3B and the EDS analysis results (Table 3), iron oxides are common on the surfaces of ferrospheres and have a dendritic form [53].In Figure 3C, an enlargement of the area marked in Figure 3B, further shows a spinel octahedron structure of the iron oxide crystals.Additionally, as discussed in Section 3.3., maghemite is the primary iron-bearing mineral in the Luohuang fly ash.It is therefore concluded that these iron oxide crystals are maghemite.Likewise, these ferrospheres are magnetic microspheres.In view of the promising applications of magnetic microspheres in the composites of magnetic materials, magnetic media, adsorbents, catalysts and ion exchangers [22], separating and classifying magnetic microspheres from the fly ash will bring additional value.3B, D3 means the 3rd detection spot in Figure 3D; and so on for the other spots.** The value of oxygen is obtained through calculation rather than physical detection, and is therefore not fully credible.
Additionally, metallic iron is detected in a cracked plerosphere (Figure 3F).It may be generated from the disoxidation of Fe 2+ or Fe 3+ in a strong reducing environment before the burst of plerospheres.
As discussed above, variance of brightness in the backscattered electron images can reflect the chemical change on the whole.Brightness of zones at different depths (surface or inside) of microspheres varies obviously.In EDS data, detection spots distributed at different depths of microspheres (Figure 3D) reveal that these microspheres show the characteristic of elemental transition through their cross-section.The inner layer with the lowest brightness consists of Si and O (Detection spot B1, D1 and D2, as shown in Figure 3 and Table 3).The chemical component of the middle layer is Si, Al, Fe, Ti, Ca and O; this layer appears brighter than the inner one but darker than the iron-bearing minerals.For ferrospheres, the Fe-O mass (maghemite or hematite) would compose the outer layer.This indicates that an elemental differentiation may have occurred during the formation process of these microspheres.
The majority of particles in the fly ash from the Luohuang plant are various types of microspheres: porous microspheres, plerospheres (hollow microspheres surrounding sub-microspheres or mineral fragments) and magnetic ferrospheres.Maghemite (γ-Fe 2 O 3 ) crystals with spinel octahedron structure occur on the surfaces of microspheres, displaying a dendritic or fabric framework.Separating and classifying these magnetic ferrospheres from the Luohuang fly ash would generate considerable additional value.Microspheres in Luohuang fly ash generally show a characteristic of elemental transition through their cross-section: the inner layer consists of Si and O; the chemical component of the middle layer is Si, Al, Fe, Ti, Ca and O; and the Fe-O mass (maghemite or hematite) composes the outer layer (plerospheres).This indicates that an elemental differentiation occurred during the formation process of the microspheres.The above investigations on the composition and morphological characteristics of microspheres in fly ash provide important information on the utilization of the Luohuang fly ash, especially in the field of materials.

Figure 1 .
Figure 1.Particle size distribution of the Luohuang fly ash.

Figure 1 .
Figure 1.Particle size distribution of the Luohuang fly ash.

Figure 3 .
Figure 3. Scanning electron microscope (SEM) backscattered electron images and energy-dispersive X-Ray spectrometer (EDS) analyses of microspheres in the Luohuang fly ash: (A) Overall view of the Luohuang fly ash; (B) Enlargement of the area marked in (A), magnetic microsphere; (C) Enlargement of the area marked in (B), maghemite (γ-Fe2O3) crystals with spinel structure; (D) Porous microsphere; (E) Plerosphere; and (F) Cracked plerosphere containing a particle of metallic iron.

Figure 3 .
Figure 3. Scanning electron microscope (SEM) backscattered electron images and energy-dispersive X-Ray spectrometer (EDS) analyses of microspheres in the Luohuang fly ash: (A) Overall view of the Luohuang fly ash; (B) Enlargement of the area marked in (A), magnetic microsphere; (C) Enlargement of the area marked in (B), maghemite (γ-Fe 2 O 3 ) crystals with spinel structure; (D) Porous microsphere; (E) Plerosphere; and (F) Cracked plerosphere containing a particle of metallic iron.

Table 1 .
Loss on ignition (LOI, %) and the concentrations of major elements (%) in the Luohuang fly ash.

Table 1 .
Loss on ignition (LOI, %) and the concentrations of major elements (%) in the Luohuang fly ash.

Table 2 .
Mineral compositions of the Luohuang fly ash determined by X-Ray diffraction and Siroquant technologies.

Table 2 .
Mineral compositions of the Luohuang fly ash determined by X-Ray diffraction and Siroquant technologies.

Table 3 .
Data from energy-dispersive X-Ray spectrometer (EDS) analyses of the Luohuang microspheres shown in Figure3.

Table 3 .
Data from energy-dispersive X-Ray spectrometer (EDS) analyses of the Luohuang microspheres shown in Figure3.
* Detection spot B1 means the 1st detection spot in Figure