Petrology, Mineralogy, and Chemistry of Size-Fractioned Fly Ash from

Article pubs.acs.org/EF

Petrology, Mineralogy, and Chemistry of Size-Fractioned Fly Ash from the Jungar Power Plant, Inner Mongolia, China, with Emphasis on the Distribution of Rare Earth Elements Shifeng Dai,† Lei Zhao,† James C. Hower,*,‡ Michelle N. Johnston,‡ Weijiao Song,† Peipei Wang,† and Songfeng Zhang† †

State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology (Beijing), Beijing 100083, China ‡ University of Kentucky, Center for Applied Energy Research, 2540 Research Park Drive, Lexington, Kentucky 40511, United States ABSTRACT: Previous studies proved the existence of the coal-hosted Al (Ga and rare earth elements) ore deposit in the Jungar Coalfield, Inner Mongolia, northern China, and the fly ash derived from these metallic coals is the source for Al and Ga extraction. Six sized fractions from plus-120 mesh to minus-500 mesh (125 to 25 μm) coal combustion fly ash from the 200-MW Jungar power plant, Inner Mongolia, China, were studied for their mineralogical, chemical, and petrographic properties. The parent sample of the fly ash represents one time and temperature (not specifically known, but certainly in excess of 200 °C) in the ash-collection system. X-ray diffraction (XRD) data showed that the glass content of the fly ash decreased from the coarse to fine fractions. However, quantitative data under an optical microscope showed that glass content of the fly ash increased from the coarse to fine fractions; portions of what appeared to be glass under microscope actually contained mullite and corundum. Rock fragments, indicative of incomplete melting, comprise significant portions of the plus-300 mesh (about 50 μm) fractions. Compared to the upper continental crust, the size-fractioned fly ashes show enrichment in the light rare earth + Y (REY) and have negative Eu, Ce, and Y anomalies. Relative to raw fly ash (unsized), Eu and Ce in the size-fractioned ashes respectively exhibit positive and negative anomalies, and the finer fly ashes (minus-300 mesh) are enriched in light REY. In contrast to bottom ash, all the size-fractioned fly ashes show distinctly negative Ce anomalies. Lanthanum, Ce, Pr, and Nd were detected in minerals within the glassy phases in the fly ash. Mercury was detected in higher concentrations in the relatively carbon-rich coarse fraction than in the carbon-depleted fine fractions. Fluorine, V, Zn, and Pb have relatively high concentrations in the high-surfacearea fine fly ash fractions.

1. INTRODUCTION Fly ash, a coal combustion product (CCP), is a processed resource that has potential to be an attractive source of critical rare earth elements,1 perhaps at levels within an order of magnitude of mined rare earth ores (carbonatites and weathering crust ore deposits).1 Seredin2 and Seredin and Dai3 discussed the occurrence of rare earth elements and yttrium (REY; or REE if Y is not included) in coals, describing three enrichment patterns: LREY- (LaN/LuN > 1), MREY- (LaN/SmN < 1, GdN/LuN > 1), and HREY- (LaN/LuN < 1), in comparison with the upper continental crust (UCC). They divided the REY into groups according to industrial need: critical (Nd, Eu, Tb, Dy, Y, and Er), uncritical (La, Pr, Sm, and Gd), and excessive (Ce, Ho, Tm, Yb, and Lu) groups. The high volatile bituminous Pennsylvanian No. 6 coal in the Jungar Coalfield in Inner Mongolia of northern China hosts an Al (Ga and REE) ore deposit,4 for which the pilot plant for Al and Ga extraction from fly ash was successfully built in 2011.5 The 200-MW Jungar Power Plant produces about 380 kt of fly and bottom ashes per year. The fly ash collected by water-film ash concentration units accounts for 90% of total combustion residues, which are piped into a landfill 3-km northeast of the power plant. The power plant uses 30-m-thick6 No. 6 coal of the Jungar Coalfield as the feedstock. Although the feedstock is low-sulfur coal (125 μm) and 160 × 300 mesh (95 × 50 μm) size fractions account for large proportions, 23.9 and 37.7%, respectively (Table 1). The petrology, mineralogy, and chemistry results are presented on Tables 2−4, respectively. On the basis of the

Table 1. Weight Percentage (%) of the Size-Fractioned Fly Ashes (Mesh) size fractions

plus-120

120 × 160

160 × 300

300 × 360

360 × 500

minus-500

percentage

23.9

9.6

37.7

6.3

12.5

10.0

Table 2. Petrologic Composition (vol %) of the Size-Fractioned (Mesh) Ash Samples Determined under a Microscope size fractions

plus-120

120 × 160

160 × 300

300 × 360

360 × 500

minus-500

glass mullite spinel quartz rock fragment isotropic coke anisotropic coke inertinite

77.2 0.0 trace 4.0 7.6 1.2 7.2 2.8

86.8 0.0 0.0 5.6 7.2 0.0 trace 0.4

86.8 0.0 trace 8.4 4.4 0.0 trace 0.4

92.4 0.0 trace 5.2 1.6 trace 0.4 trace

95.6 0.0 0.8 2.8 0.8 0.0 trace trace

97.2 0.0 trace 0.4 1.6 0.0 0.4 0.4

Table 3. Mineral Composition (wt %) of the Size-Fractioned (Mesh) Ash Samples Determined by XRD and Siroquant sample

quartz

mullite

corundum

plus-120 120 × 160 160 × 300 300 × 360 360 × 500 minus-500

0.5 0.8 2.0 0.5 2.8 3.1

35.5 35.6 36.7 38.0 38.0 42.4

1.1 2.8 4.3 5.9 7.7 10.5 1503

gypsum

amorphous

orthoclase

0.4 0.1 0.3 4.5

62.9 60.5 56.9 55.3 46.9 43.8

0.1

dx.doi.org/10.1021/ef402184t | Energy Fuels 2014, 28, 1502−1514

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Figure 2. Petrologic compositions of size-fractioned fly ash. (A) Glass rim surrounding rock core (r). Image Jungar plus- 120 mesh 05. (B) Glassy fly ash. Image Jungar plus-120 mesh 15. (C) Anisotropic coke. Image Jungar plus-120 mesh 10. (D) Glassy fly ash. Image Jungar 120 × 160 mesh 03. (E) Mix of glassy fly ash with particle with glass rim surrounding rock core (r). Image Jungar 160 × 300 mesh) 03. (F) Mix of glassy fly ash with particle with glass rim surrounding rock core (r). Image Jungar 300 × 360 mesh 01. (G) Glassy fly ash. Image Jungar 360 × 500 mesh 01. (H) Glassy fly ash. Image Jungar minus-500 mesh 01.

sum of Si, Al, and Fe oxides, the fly ash investigated in this study has aluminosilicate composition and belongs to Class F according to the ASTM chemical classification.18 As simple and often wrong assumptions, Class F ashes are often considered to be a combustion product of higher (bituminous and higher) rank coals, and Class C ashes are considered to result from the

combustion of lower (subbituminous and lower) rank coals.10 As noted above, the Jungar Class F fly ash was derived from high volatile bituminous coal.6,7 According to the microscopic analysis, the fly ash sized fractions are dominated by glass (Table 2), with the glass percentage increasing with increasing fineness of the size fraction. 1504


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Figure 3. SEM backscattered electron images of mullite, spinel, and glass in the size-fractioned fly ashes. (A), (B), and (D) 120 × 160 mesh (125 × 95 μm) sample. (C) 160 × 300 mesh (95 × 50 μm) sample. Elemental data in Table 5.

larger ones.21−23 It seems that the smaller size of fly ash from the Jungar Power Plant is more favorable for the mullite and corundum formation. This is consistent with the report by Goodarzi24,25 that the mullite content of fly ash collected by the baghouse, which has a greater ability to capture fine fly ash particles than electrostatic precipitators (ESP), is greater than the ESP ash from the same station. The higher mullite and corundum contents in finer than in coarser fly ashes indicate that the finer fly ashes experienced a relatively higher overall temperature in the boiler. In other words, the finer particles attained a more uniform temperature than the coarser particles, as seen in the greater abundance of partially vitrified rock fragments in the coarser fly ashes. Needle mullite crystallites on glass spheres (Figure 3A) indicate that mullite crystallizes from the amorphous aluminosilicate melt in the postcombustion cooling process. Corundum is seldom found not only in coal but also in combustion residues, although it can be found in a few coals as an accessory minor component,26 high-temperature ashes,27 and coal combustion residues.28,29 Corundum was not observed in the coals of the Jungar coalfield6,7,30 and feed coals of Jungar Power Plant.8 Corundum has three modes of occurrence in the fly ash: (1) mainly as cryptocrystalline lumps (Figure 4A,B) and, to a lesser extent, as (2) cryptocrystalline cell-fillings (Figure 4D), with only a very small proportion as (3) well-crystallized crystals (Figure 4C,E,F). Boehmite in the Jungar coals consistently occurs as both lumps and cell-fillings.4,6,7 Though boehmite is considered to be a thermally inactive mineral during coal combustion,31

Incomplete melting of the rocks in the feed coal, expressed in the “rock fragments” category, contributes to an important constituent of the fly ash, particularly for the plus-300 mesh (plus 50 μm) fractions. The rock fragments, as seen in Figure 2a and, less prominently, in Figure 2e,f, are generally present as a partially melted core, with or without thermally altered carbons. The rock fragments are usually surrounded by a rim of glass, particularly evident in Figure 2a. The variety of fly ash glass can be seen on Figure 2b,d−h, collectively representing the size fractions. Carbons are an important constituent only in the plus-120 mesh (plus 125 μm) fraction (Table 2; Figure 2c). XRD + Siroquant and SEM-EDS analysis provide a different perspective on the inorganic constituents of the fly ash (Table 3; Figures 3−6). Obviously, some of what is identified as glass (amorphous) is really mineral phases, with minerals such as mullite and corundum present in grain sizes too small to be distinguished optically. The latter mineral assemblage, coupled with the absence of anorthite, is consistent with the near-equal amounts of Al2O3 and SiO2 and low CaO in the fly ash. Within the Al2O3−SiO2−CaO composition field, mullite and corundum crystallize at about 1600 °C.19,20 In consideration of the rapid cooling of fly ash in the flue gas stream, it is likely that any mineralization does not continue to completion, as indicated by the high percentages of glass. The percentages of mullite and corundum increase but amorphous matter decreases with increasing fineness of the size fractions (Table 3). However, some studies showed the smaller size fractions usually contain a higher content of glass than the 1505


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Figure 4. SEM backscattered electron images of corundum, Ca-aluminosilicate, glass, and (Fe,Al,Si)Ox in the size-fractioned fly ashes. (A) 300 × 360 mesh (50 × 40 μm) sample. (B), (C), and (D) plus-120 mesh sample. (E) and (F) 300 × 360 mesh (50 × 40 μm) sample. (F) is the enlargement of the specified area in (E). Elemental data in Table 5.

corundum in the fly ashes present in this study was probably formed in the boiler from boehmite in feed coal during coal combustion and, thus, is a characteristic mineral formed during the combustion if its feedstock is Al-oxyhydroxide-rich (e.g., boehmite-rich coal). A Si-depleted coal otherwise would be expected to form glass or mullite. Although Ca is relatively lower in the feed coals of the Jungar power plant relative to the Chinese coals32 (Table 4) and Ca-bearing minerals in the size-fractioned ashes were, if present, under XRD and Siroquant detection limits (Table 3), a trace proportion of Ca-bearing materials was identified under FE-SEM-EDS (Figure 5), these including Ca-bearing aluminate,

Ca-bearing aluminosilicate (possibly gehlenite), and minor Ca-bearing silicate (possibly wollastonite). The formation of the above Ca-bearing mineral series in the fly ash may depend on the relative abundance of Al and Si in the combustion system. The Ca required for above Ca-bearing mineral formation may be derived from calcite, organic matter, or feedstock lime. Calcite may decompose independently to produce lime; the lime may in turn react with clay (e.g., metakaolin) to form calcium aluminosilicates (e.g., gehlenite), with boehmite to form Ca-bearing aluminate, or with free silicon to form Ca-bearing silicate (e.g., wollastonite). Calcium occurring in the organic matter, on the other hand, may be released into the furnace 1506


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Table 4. Major Element Oxides (%), Total Sulfur (%), Loss on Ignition (LOI, %), and Trace Elements (μg/g unless indicated) in the Raw Fly Ash, Bottom Ash, Feed Coal, and Size-Fractioned Fly Ashesa element SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 St LOI Li Be F Sc V Cr Co Ni Cu Zn Ga Ge Rb Sr Y Zr Nb Mo Cd In Sn Sb Cs Ba Hf Ta W Hg (ng/g) Tl Pb Bi Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu LREE

plus-120

120 × 160

160 × 300

300 × 360

360 × 500

minus-500

raw fly ash

bottom ash

feed coal

43.45 1.23 43.72 1.00 0.007 0.12 0.73 0.05 0.38 0.060 0.03 9.02 296 2.60 347 13.6 41.8 22.0 1.12 7.40 23.1 11.5 24.6 0.66 9.25 108 20.1 376 40.1 3.33 0.41 0.051 3.98 0.22 0.82 29.9 11.6 2.99 3.95 155 0.15 14.6 bdl 36.6 6.89 37.3 78.2 8.04 27.1 4.71 0.79 4.95 0.7 4.26 0.79 2.26 0.32 2.24 0.29 156

45.49 1.33 47.29 1.27 0.009 0.12 1.08 0.04 0.42 0.050 0.06 2.69 271 2.57 275 15.7 45.1 12.6 1.27 8.24 29.9 13.8 25.0 0.69 10.3 86 23.7 411 46.3 3.26 0.45 0.063 4.21 0.24 0.88 31.3 12.9 3.34 4.09 50 0.19 20.8 0.12 37.2 7.70 43.8 94.7 9.6 32.8 5.67 0.97 5.94 0.83 4.95 0.9 2.61 0.36 2.5 0.34 188

46.03 1.59 46.99 1.47 0.011 0.15 1.24 0.04 0.48 0.060 0.05 1.64 254 3.45 256 19.8 57.3 15.2 1.58 9.16 42.9 22.8 24.9 0.78 10.4 107 30.3 517 50.7 3.39 0.59 0.079 5.26 0.31 0.90 46.9 15.6 3.68 3.93 42 0.27 34.3 bdl 44.5 9.14 56.5 123 11.7 41.3 6.9 1.21 7.18 1.03 6.07 1.13 3.2 0.45 3.08 0.41 241

43.86 1.83 48.37 1.79 0.014 0.17 1.66 0.04 0.54 0.070 0.10 1.30 240 4.30 278 23.7 70.8 21.1 2.10 10.2 53.5 35.5 27.0 0.95 12.3 171 35.2 646 57.5 3.73 0.79 0.104 5.73 0.42 0.92 43.5 18.5 4.15 4.13 28 0.33 46.5 bdl 53.8 10.1 76.2 152 16.1 55.0 9.08 1.56 9.33 1.27 7.47 1.36 3.89 0.54 3.72 0.5 310

43.14 1.93 48.78 2.00 0.015 0.18 1.82 0.04 0.54 0.075 0.04 1.08 225 4.82 292 25.4 77.3 21.6 2.15 12.1 59.2 31.5 26.8 1.01 12.1 186 39.9 732 58.6 4.05 0.81 0.120 5.75 0.53 0.93 26.3 20.5 4.17 4.06 27 0.37 54.6 1.44 59.4 10.9 84.9 165 17.9 60.6 10.4 1.78 10.71 1.44 8.38 1.52 4.32 0.61 4.19 0.56 341

40.72 2.24 48.78 2.20 0.018 0.19 2.58 0.04 0.56 0.108 0.06 1.89 240 6.36 695 29.4 97.0 30.0 2.98 13.7 51.2 42.0 37.6 1.62 11.9 322 54.4 904 65.1 5.95 1.03 0.170 8.52 0.75 0.90 44.0 24.7 4.62 4.68 31 0.46 77.2 0.50 66.9 12.7 112 222 22.3 81.7 13.8 2.38 13.8 1.91 10.9 2.01 5.57 0.79 5.25 0.72 454

41.74 0.67 49.88 0.80 0.012 0.17 2.03 0.12 0.41 0.079 0.05 4.12 408 5.23 1034 17.0 68.3 90.2 2.92 12.3 37.7 48.1 43.4 1.46 8.49 347 34.25 713 81.9 3.51 0.10 0.124 2.70 0.45 0.69 150.0 24.4 4.23 4.82 50 0.47 55.5 0.89 37.0 10.6 62.2 107 12.4 43.4 8.02 1.43 7.20 1.17 7.21 1.44 4.07 0.6 4.07 0.58 235

28.9 0.93 36.24 1.10 0.013 0.24 2.19 0.28 0.33 0.097 0.08 30.05 316 4.36 229 16.4 63.8 25.7 21.4 65.5 27.3 32.9 35.4 2.26 8.82 412 38.3 706 59.6 3.30 0.09 0.067 3.99 0.32 0.67 91.1 20.6 3.69 242 10 0.19 23.9 0.31 44.8 10.9 75.4 118 14.7 49.4 9.58 1.62 8.46 1.30 7.86 1.53 4.49 0.62 4.43 0.62 269

16.3 0.75 17.47 0.94 0.007 0.05 0.23 0.04 0.31 0.109 0.41 64.36 164 2.90 534 8.50 34.6 11.8 1.37 4.25 20.0 127 26.7 1.20 5.19 254 20.1 423 29.2 2.31 0.11 0.094 3.16 0.28 0.31 43.8 11.5 1.88 4.45 500 0.19 36.7 0.74 26.2 5.39 46.2 80.1 8.90 29.7 5.45 0.96 4.79 0.71 4.16 0.82 2.42 0.35 2.25 0.32 171

1507


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Table 4. continued element

plus-120

120 × 160

160 × 300

300 × 360

360 × 500

minus-500

raw fly ash

bottom ash

feed coal

HREE REE LREY MREY HREY REY critical uncritical excessive La/Yb REYcritical/REYtotal

15.8 172 155 30.8 5.9 192 55.2 55.0 81.8 16.7 28.7

18.4 206 187 36.4 6.71 230 65.9 65.0 98.8 17.5 28.7

22.6 263 239 45.8 8.27 293 83.1 82.3 128 18.3 28.3

28.1 338 308 54.8 10.0 373 104 111 158 20.5 28.0

31.7 372 339 62.2 11.2 412 116 124 172 20.3 28.2

41.0 495 452 83.4 14.3 550 157 162 231 21.3 28.5

26.3 261 233 51.3 10.8 295 91.5 89.8 114 15.3 31.0

29.3 298 267 57.5 11.7 336 103 108 125 17.0 30.6

15.8 187 170 30.7 6.16 207 58.1 65.3 83.8 20.5 28.0

a

LOI, loss on ignition; St, total sulfur. LREE, the sum of La to Eu; HREE, the sum of Gd to Lu; LREY, the sum of La to Sm; MREY, the sum of Eu to Dy plus Y; HREY, the sum of Ho to Lu; REY, the sum of the lanthanides and yttrium. bdl, below detection limit.

Figure 5. SEM backscattered electron images of Ca-bearing aluminate, aluminosilicate, and silicate in the size-fractioned fly ashes. (A), (C), and (D) are for the 120 × 160 mesh (125 × 95 μm) fly ash. (B) 360−500 mesh (40 × 25 μm) sample. Elemental data in Table 5.

Ca-bearing and Ca−Mg ferrites may be the results of the reaction between Ca and Mg released from different minerals and inorganic sources during combustion process.35−37 3.2. REY Fractionation in the Size-Fractioned Fly Ashes. Hower et al.9 emphasized the distribution of rare earth elements in their discussion of the fly ash. That newly determined data are listed in Table 4. The latter table also includes a broader representation of the major oxide and minor and trace element chemistry than provided in Hower et al.’s study.9 The concentrations of the REE, the light REE, heavy REE, and the Seredin2 and Seredin and Dai3 classifications (light, medium, and heavy REY; critical, uncritical, and

atmosphere in a more reactive and dispersed elemental form, and, thus, have a greater opportunity to interact with the same Si−Al-, Al-, or Si-residues and produce the latter refractory Ca-bearing minerals during furnace operation. Other minor minerals below XRD and Siroquant detection limits also identified under SEM-EDS include celsian, quartz, Ca-bearing ferrite, Ca−Mg ferrite, REY-bearing CaCO3(F), zircon, rutile, and Barite (Figure 6). Though celsian is rare in coal and CCPs, it has been identified in a few coals28,33,34 and fly ash.28,34 Celsian may be a neo-formed mineral resulting from the reaction between Ba released from Ba-bearing mineral (such as gorceixite) and clay residue (e.g., metakaolin). 1508


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Figure 6. SEM backscattered electron images of minor minerals in the size-fractioned fly ashes. (A), (B), and (C) plus-120 mesh (plus 125 μm) sample. (D) 120 × 160 mesh (125 × 95 μm) sample. (E) 300 × 360 mesh (50 × 40 μm) sample. (F) 360 × 500 mesh (40 × 25 μm) sample. Elemental data in Table 5.

excessive) all increase from the plus-120 mesh (plus 125 μm) to the minus-500 mesh (minus 25 μm) fractions. Because the individual concentrations and sums of concentrations are all increasing at similar rates, the ratios, such as the LREE/ HREE and the critical REY/total REY do not exhibit large variations. In general, the size fractions (Figure 7A), as well as the raw economizer fly ash, and the bottom ash (Figure 7B) show enrichment in the light REY and have negative Eu, Ce, and Y anomalies, as compared to the Upper Continental Crust;38 this is similar to the trends seen in the feed coals (Figure 7B) and the raw Jungar coals as reported by Dai et al.6,7

However, in contrast to the normal laboratory ash, the raw (unsized-fractioned) fly ash and the bottom ash, the REY in the size-fractioned fly ashed show some fractionation (Figure 8): (1) Relative to the normal laboratory ash, almost all the REY in each size-fractioned ash are depleted, which is attributed to the incomplete correspondence between the feed coal and the fly ash, although they were both collected at the same time. The fly ash sample, however, just represents one part of the total fly ash stream, and it does not represent any REY that might have partitioned to the bottom ash. The REY in each size fraction presented an enrichment pattern of MREY type (LaN/SmN < 1, GdN/LuN > 1), though the MREY enrichment is not distinct. 1509


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Figure 7. REY distribution patterns for size-fractioned fly ashes (A) raw fly ash, bottom ash, feed coal, and laboratory ash (B) normalized by Upper Continental Crust.38

Cerium and Eu respectively showed positive and negative anomalies in all sized fractions, but yttrium did not present distinct anomaly (with the exception of plus-500 mesh (plus 25 μm) sample). (2) Compared to the raw fly ash (unsized fractions), the finer fly ashes (minus-300 mesh (minus 50 μm)) are enriched in LREY and have higher REY concentrations (with exceptions of Ho−Lu in 300 × 360 mesh (50 × 40 μm) fraction), but REYs do not show distinct fractionations in the coarser ashes (plus300 mesh (plus 50 μm)). Similar to the laboratory ash, Eu and Ce in the size-fractioned ashes relative to raw fly ash show positive and negative anomalies. (3) In comparison with bottom ash, the light-REYs are slightly enriched in the finer fly ashes (minus-300 mesh (minus 50 μm)), but REYs do not show distinct fractions in the coarser fly ashes (plus-300 mesh (plus 50 μm)). All the size-fractioned fly ashes show distinctly negative Ce anomalies, and Ce anomalies become more distinct with decreasing particle size. In addition, all the size-fractioned fly ashes show negative and weak Eu anomalies. However, with the exception of the sample minus-500 mesh (minus 25 μm) ash, yttrium was not fractioned. Hower et al.39 used Ce, the most abundant of the REE, as a proxy for other REE in their wavelength-dispersive spectrometry electron microprobe study of the associations of REE in fly ash. Cerium appeared to be dispersed in the glass; therefore, the increase in Ce concentration as the glass content of the fly ash increases in the finer sizes makes sense. Rare earth elements are not at sufficiently high concentrations to be measured in all of the Jungar glass, as seen by its absence (or below-detectionlimit abundance) in the glass-spinel association shown in Figure 3A. Similarly, REE were not seen in the phases shown on Figures 4 and 5. Yttrium, La, Ce, Pr, and Nd were detected in REY-bearing calcite and CaCO3(F) (probably parisite) in the Figure 6C fly ash particle (the elemental composition at sites in the particles is shown in Table 5). In contrast to the apparent

Figure 8. REY distribution patterns for size-fractioned fly ashes, normalized by normal laboratory ash (A), raw fly ash (B), and bottom ash (C).

REE-glass association seen by Hower et al.,39 the FE-SEMEDAX detected mineral associations with a broader range of REY. 3.3. Behavior of Other Trace Elements during Coal Combustion. Although other modes of occurrence of Hg in fly ash have been found,40 mercury has a known association with fly ash carbons,41 accounting for the greater concentration (155 ng/g) in the relatively carbon-rich plus-120 mesh (plus 125 μm) fraction compared to 27−31 ng/g in the carbondepleted minus-300 mesh (minus 50 μm) fractions. The Hg concentration is highly correlated with loss on ignition (r = 0.995; Figure 9). The unburned carbon dominantly contributes to the loss on ignition, as there is no or extraordinarily low carbonate and clay minerals were identified in the sizefractioned ashes as mentioned in Tables 2 and 3. In contrast to the Hg distribution, almost all the trace elements (with a few exceptions of Li, Cr, Ba, W, and Bi) in the boiler are captured on the surface of the glass and have higher concentrations in the finer sized-fractions than in the coarser ones (Table 4). Owing to the relatively high temperatures in the economizer, the economizer fly ash does exhibit some 1510


a

1511

23.99 6.07

40.98

24.97

8.23 14.11

52.70

31.37

68.63

6.37 22.59

49.58 5C-2

17.62 4.08

7.12 21.6

3B-1

25.30 38.64

36.06

5C-3

30.27 34.68

35.05

3B-2

76.12

3.16 1.25

0.85 3.37 14.59

51.28

19.47

1.51 1.17 12.92 5D-1

19.62 23.83

8.67 32.15

3C-2

7.97 21.95

39.19 5C-5

24.61 3.90

6.54 25.76

3C-1

45.93

43.99

0.96

2.57

9.31 16.16

6B-1

25.23 1.15

23.97 12.19

4.81 32.65

4A-1

6.25

30.48 37.22

32.3

6A-2

31.05 36.64

32.31

3D-3

26.99

14.48 14.19 1.02

7.65 16.72

59.84 6A-1

12.34 1.78

7.11 18.93

3D-2

54.27

1.06 10.04 4.55

23.83

4.81 1.64 6.27 5D-2

22.09 31.74

33.45

3D-1

67.18

03.15

6.37

9.11 13.05

6B-2

59.67 3.31

37.03

4A-2

61.31

38.69

6B-3

64.18

35.82

4B-1

7.85 26.85 3.86 14.20

2.08

6.37

0.97 3.87

27.86 6.09

6C-1

3.45

54.04 5.84

36.67

4C-1

7.82 24.85 3.23 11.82 2.29

2.71

6.34

3.09

23.46 10.32 4.08

6C-2

11.37

28.53 19.42

7.72 32.95

4C-2

2.60

42.93

01.56

1.87 14.85

13.31 21.56

6D-1

60.11 10.92

28.97

4D-1

24.63 32.01

9.01 34.34

6D-2

26.0 29.75

12.64 30.57

4D-2

1.31

65.73

0.99

33.28

15.98 6-E1

45.18 6.19

32.65

4F-1

61.37 4.36

32.25

1.27

0.74

7.04 0.34 2.03 1.26

19.14 25.28

10.28 43.29

6F-2

23.61

27.37 21.52

27.51

5A-2

5.21 11.64

23.33 16.90

6F-1

0.25 0.4 0.91 32.94 6 × 10−2 34.27

27.56 38.34

34.11

5A-1

22.72 8.71

5.40 28.67

4-F2

The test spots are indicated in Figures 3−6. Note: With exceptions of test spots 6C-1 and 6C-2, the carbon content of the test spot is due to the conductive carbon coating for SEM experiment.

C O F Mg Al Si P S K Ca Ti Fe Y Zr Ba La Ce Pr Nd Th U

20.63 5A-3

54.60 5C-1

14.98

13.11 23.52

3.51

5.88 24.54

7.59 31.65

C O F Mg Al Si P S K Ca Ti Fe spot

3A-2

3A-1

spot

Table 5. SEM-EDS Data for the Inorganic Matter in Size-Fractioned Fly Ashes of Jungar Power Plant (wt %)a

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from previous investigations that the rare earth elements showed an increase in concentration from the coarse to the fine fly ash and the light REE/heavy REE passed through a minimum in the 160 × 300 mesh (95 × 50 μm) fraction. Some of the particle-size distribution of REY could be due to the greater amount of carbon and partially vitrified rock fragment in the coarser fly ash versus the greater amount of glass in the finer fragments. It would be useful to study a wider range of fly ashes in order to determine if the trends observed here hold true for other coal and fly ash sources. Six fly ash size fractions from plus-120 mesh (plus 125 μm) to minus-500 mesh (minus 25 μm) were separated for this study. The parent sample of the fly ash represents one time and temperature in the ash-collection system. The glass content of the fly ash determined by the optical microscope increased from the coarse to fine fractions, with “rock fragments” representing incomplete melting representing significant portions of the plus300 mesh (plus 50 μm) fractions. Mineralogic analysis, though, demonstrated that some of what appeared to be glass under the optical microscope actually contained mullite and corundum. No one technique is going to be optimal for every situation. Optical techniques are limited by the optical resolution but have advantages in being able to screen a wide range of samples relatively rapidly without relying on expensive electron-beam techniques. The electron microprobe39 and FE-SEM-EDS (this study) can determine the siting of some elements, but are limited, aside from the expense noted above, by the small area covered by each analysis. Relative to raw unsized fly ash, Eu and Ce in the size-fractioned ashes respectively exhibit positive and negative anomalies, and the finer fly ashes (minus-300 mesh; minus 50 μm) are enriched in LREY. In contrast to bottom ash, all the size-fractioned fly ashes show distinctly negative Ce anomalies. Field emission-scanning electron microscopy with an energy-dispersive X-ray spectrometer demonstrated that La, Ce, Pr, and Nd could be detected in minerals within some of the glassy phases in the fly ash. The concentrations of the critical REY (Nd, Eu, Tb, Dy, Y, and Er) are higher in the finer size-fractioned fly ash than in the coarser ones. However, the concentrations of uncritical (La, Pr, Sm, and Gd) and excessive (Ce, Ho, Tm, Yb, and Lu) REY have similar increasing rates with those of the critical-group REY, leading to the enrichment of all REY in the finer fly ash. In consideration of the weight percentage of the size-fractioned fly ashes (Table 1), highest in 160 × 360 mesh (37.7%) and followed by plus-120 mesh (23.9%), not only the finer but also the coarse fly ashes should be paid much attention for their potential economic significance of REY extraction. Mercury, known to be adsorbed on fly ash carbons, was present in higher concentrations in the relatively C-rich coarse

Figure 9. Relation between Hg concentration and loss on ignition of size-fractioned ashes.

capture of trace elements. For example, the F concentration in the minus-500 mesh (minus 25 μm) fraction is significantly higher than the concentration in any of the other fractions. Similarly, metals such as V, Zn, and Pb increase in concentration in the finer fractions, although V was not fractioned too much between raw unsized fly ash and bottom ash (Figure 10). The enrichment of Pb in both finer sized-fractions and in unsized raw fly ash (relative to bottom ash) is attributed to its volatility during coal combustion process.42 Elements Co, Ni, and W are enriched in the bottom ash relative to the raw unsized fly ash (Figure 10), in accordance with the studies of the trace-element fate during coal combustion by Clarke and Sloss.42 However, the volatility of Co and Ni, V, and REE indicated by its enrichment in finer-sized fractions could be attributed not only to their organic-bound modes of occurrence but also to the larger surface area of the finer sized-fractions, although Co, Ni, and REEs were enriched in bottom ash (relative to raw unsized fly ash; Figure 10). For example, studies by Dai et al. showed that a large proportion of REE in the Jugar coals is organically bound.6,7 In the three-dimensional arrays of fly ash collection devices, such as ESPs, the trace element distribution is attributed to the decrease in both T and particle size (therefore, surface area) toward the back rows of the ESP array.43−45 In this study, the fly ash represents one temperature, so, between T and surface area, trace element distribution is, putting aside the petrographic and mineralogic factors, a function of the surface area variations between the fractions.

4. SUMMARY Fly ash from the 200-MW Jungar power plant, Inner Mongolia, China, burning the Pennsylvanian No. 6 coal, was studied for its chemical and petrographic properties. In particular, we knew

Figure 10. Trace-element ratios between raw unsized fly ash and bottom ash from Jungar power plant. 1512


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fraction than in the C-depleted fine fractions. Other volatile trace elements, such as F, V, Zn, and Pb, have relatively high concentrations in the fine fractions owing to the high surface area in those fly ash particles.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Key Basic Research Program of China (No. 2014CB238902), the National Natural Science Foundation of China (Nos. 41272182 and 40930420), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13099).

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