Environ. Sci. Technol. 1987, 21, 898-903
Elemental Characterization of Particle Size-Density Separated Coal Fly Ash by Spectrophotometry, Inductively Coupled Plasma Emission Spectrometry, and Scanning Electron Microscopy-Energy Dispersive X-ray Analysis Kellchl Furuya,* Yoshlhiro Miyajlma, Tohru Chiba, and Tadashl Klkuchl Department gf Applied Chemistry, Faculty Japan
of Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162,
Coal fly ash was separated into 54 size-density fractions (149-5 pm, 1.6-3.2 g/cm3) by supersonic sieving with a methanol dispersion medium coupled with density separation with heavy liquids. Scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX), spectrophotometry, inductively coupled plasma emission spectrometry (ICP),and X-ray diffraction analysis (XRD) were applied to analyze these fractions. The characterization of separated coal fly ash in morphologies and compositions is discussed. On the basis of the size and density distribution, elements are grouped into three groups: group I (Si, Al, Fe, Ca, Mg, and Mn), elements whose concentration is dependent on particle size and dependent on density; group 11 (Zn and Cu), elements whose concentration is dependent on particle size and independent of density; group I11 (V, Cr, and Ti), elements whose concentration is dependent on particle size only in the higher density fractions. The elements of group I11 were found to concentrate selectively on particles in higher density fractions. Chemical affinity between condewed elements and substrate composition has been suggested to be dominant. Introduction Coal fly ash, emitted from a coal-burning power plant, consists of many kinds of particles, ranging from submicron to several hundred microns in diameter with various densities and compositions. Recently, researchers have found that certain elements in coal are more likely to concentrate in finer fly ash particles, which have a greater tendency to escape from collection systems into the atmosphere (1-6). This means that the public is likely to be exposed to these finer particles because of their longer atmospheric residence time (compared to larger particles) and their eventual deep lung deposition (1). The aerodynamic particle size separation method, which is a practical measurement method in relation to inhalation and deposition in human respiratory systems, has been generally utilized for most of the studies on the sizes of fly ash particles ( 4 , 5, 7-9). Among them, Coles et al. (9) rationalized their data by dividing the elements into three classes: group I, elements that show little or no enrichment in smaller particle fractions (lithophilee); group 11, elements whose enrichment increases with decreasing particle size (chalcophiles); group 111, elements whose behavior is intermediate to that of elements in group I and group 11. The mechanism has been explaiqed by the volatilization-condensation mechanism in which vaporized species condense on a surface of the particles in a combustion system after combustion, and thus their concentration linearly correlates to the surface-to-volume ratio of the particles ( 5 ) . There are, however, some inconsistencies of the behaviors of some elements for the enrichment. Cu, Cr, Mn, and V are the elements mostly belonging to an intermediate group in which a little size dependency was often observed (5,9). Table I shows a comparison of those in three related papers. 898
Environ. Sci. Technol., Val. 21, No. 9, 1987
The separation methods based on aerodynamical properties, however, have problems. An aerodynamic particle size is defined as ra = dp0.5 where d = geometric diameter and p = density. Therefore, aerodynamic particle size is dependent on density. And moreover, the larger size fractions obtained from this method often contain numerous smaller particles as secondary particles, in which individual primary particles were gathered together through adhesion and agglomeration. When the density of a particle is large, a geometrically small particle is evaluated as a larger size as shown in the above equation. The aerodynamical estimation for secondary particles is not always in agreement with the geometrical particle size characteristics. This might be the reason for some of the scatter data among previous reports. If the particle size is independent of its density, we can easily estimate the compositions and chemical properties. This not only provides information about elemental concentration behavior but also suggests an accurate estimation method for secondary particles, fractionated into their primary size fractions. Hopke et al. (10) tried to analyze roadside dust with five particle size-four density separation in the range of 500-20 pm and 2.9 g/cm3. We have already reported (11, 12) an effective size separation method for smaller particles up to 5-pm size using supersonic sieving with a methanol dispersion medium coupled with a density separation with heavy liquids up to >3.2 pm. The particle size-density separation method was found effective to estimate the correlation between particle characteristics and the composition. This paper presents the result of elemental analysis on the representative five major and seven minor elements in 54 particle-density separated fractions up to 5 pm for coal fly ash obtained from a bag filter in a pulverized coal and heavy oil mixed combustion furnace system and discusses their enrichment behaviors. This paper is a preliminary one. The process of the development for further finer particle ranges, which have more importance for environmental concerns, is in progress. Size separation reported herein covers only the two size ranges of greatest environmental interest. However, the enrichment behaviors of elements can be sufficiently demonstrated. Experimental Section Sample. Pulverized Japanese brown coal (Taiheiyo coal) of 6300 kcal/kg of high heat value was burnt at 1500 "C (maximum temperature) as a mixture with 72-62% (in daytime) and 4&33% (in night) heavy oil of 10500 kcallkg ash. The nominal composition of coal used with 5 X was as follows: ash, 16%, volatile matters; 44%; fixed carbon, 40%. Sample coal fly ash was taken from a bag filter attached to a combustion furnace after NH3 dry-type NO, reduction equipment. Being a test furnace experiment, this system was inserted between an exit of the combustion furnace and an electric precipitator.
0013-936X/87/0921-0898$01.50/0
0 1987 American Chemical Society
Table I. Inconsistent Classifications of Elemental Concentration Behaviors in Fly Ash Particlesn
Block et al. ( 4 ) Coles et al. (14) Hansen et al. ( 5 )
Si
A1
Fe
Ca
Mg
element cu
Zn
V
Ti
Cr
Mn
I
I I I
I I I
I I I
I I I
I1 111 I11
I1 I1 I1
I I11 I1
I I I
I I11 I11
I I I11
I, group I; 11, group 11; 111, group 111. Table 11. Density and Composition of Heavy Liquid compn (vol ratio)
temp, "C
density, g/cm3
CC14:CH2Br2(96:4) CC1,:CH2Br2(50:50) CCl4:CH2Br2(11:89) CC14:CHJz ( 3 0 7 0 ) CC1,:CHoI9 (7:93)
26.2 26.2 24.0 20.5 20.5
1.61 2.02 2.39 2.81 3.20
Table 111. Operating Conditions and Wavelength of ICP Analysis Plasma reflected rf power, W coolant argon flow rate, L/min sample argon flow rate, L/min plasma argon flow rate, L/min
10
20 0.5 1.2
Measurement
Particle Size-Density Separation. The details of the separation procedure have been described elsewhere (12). A total of 10 g of sample was dispersed in methanol as a medium by means of a supersonic wave of 19 kHz, 100 W, and 30 W/cm2 and separated into nine size fractions by six brass mesh testing sieves with opening sizes of 149, 74, 44, 37, 25, and 20 pm and two nylon mesh testing sieves with opening sizes of 10 and 5 pm, which were mounted on a supersonic separation apparatus. No breakage of particles by supersonic treatment was observed. Nine size fractions were subjected to density separation with five heavy liquids of 1.6, 2.0, 2.4, 2.8, and 3.4 g/cm3, which were mixtures of carbon tetrachloride, methylene bromide, and methylene iodide. The compositions of the five heavy liquids are listed in Table 11. As a result, 54 size-density fractions were obtained. Analysis. The elements determined were chosen by their characteristic enrichment behaviors in the concerned particle size-density range. The 54 particle size-density fractions were analyzed by emission spectroscopy (ICP) for Al, Ca, Mg, Cu, V, Fe, Ti, Cr, Mn, and Zn, by X-ray diffraction analysis (XRD), and by scanning electron microscopy-energy dispersive X-ray analysis (SEM-EDX). For quantitative reference material, NBS 1366a coal fly ash was used. The water used in all experiments was back-percolated, and the standard solutions were prepared from the commercial stock solutions for atomic absorption spectrometry. Spectrophotometry. The concentration of Si in each separated fraction was spectrophotometrically determined. Less than 10 mg of coal fly ash from a separated fraction was digested with 1 g of sodium hydroxide and then filtered. The filtrate was used for spectrophotometric determination of Si. Ammonium molybdate was used as a coloring agent. When less than 2 mg of the sample was digested with 1 g of sodium hydroxide, Si concentration could be determined with an error of 3% in coefficient of variation. ICP. The concentration of 10 other elements (Al, Fe, Ca, Mg, Ti, Mn, Cr, V, Zn, and Cu) of 54 separated fractions was determined on an Atomcomp 975 (Jarrell Ash Corp.) calibrated with multielement standard solutions prepared from NBS 1633a coal fly ash sample. The method used to decompose 10 mg of sample was basically the same as that reported by Haraguchi (13). A total of 10 mg of sample was decomposed with 1 mL of 60% perchloric acid and 1 mL of hydrochloric acid, and the final volume of sample solution was made up to 10.0 mL for the determination of 10 elements by ICP. In cases of full or partial overlap of an interfering line with an analyte line,
wavelength, nm A1 Fe Ca Ti Mg Cr cu Mn V Zn (I
396.1 259.9 317.9 334.9 279.5 276.6 327.4 257.6 292.4 213.8
detection limit, 10-3 pg/L
mcc' 1000
4.5 7.2 2.2 8.3
400 64 64 36 0.8 0.5 0.8 1.0 0.8
mcc, maximum calibrated concentration in pg/L.
correction factors were determined for the analyte line by using standard solutions of major elements. The operating conditions employed in ICP analysis are listed in Table 111. The sample solutions were diluted to the ranges for calibration curves after the acid concentration was adjusted. For Zn and Cu, a blank test for brass sieving was carried out and compared with the case of stainless steel sieves, and the data were corrected. The reproducibility of repeated analysis was better than 10%. XRD. X-ray diffraction patterns were obtained on a Geigerflex Rad-A X-ray diffractometer (Rigaku Denki) with Cu Ka radiation. Specimens were mounted on plastic stubs after being ground in a mortar before measurement. Diffraction patterns were measured a t 20'-80' (20) with a current at 150 mA at 50 keV. a Quartz was used for 28 calibration. SEM-EDX. Fifty-four fractions were examined by a scanning electron microscope (Hitachi S-550 electron microscope), equipped with an energy dispersive X-ray spectrometer (p-7000, Kevex Corp.). Particles from each fraction were mounted on double-adhesive tape affixed to carbon stubs and then evaporatively coated with carbon to help reduce charging effects. Results and Discussion Particle Size-Density Distribution. Table IV shows a result of particle size-density separation carried out by the above-mentioned procedures. As shown in Table IV, particles are mainly distributed in the diagonal fractions in the range of the lower density region rather than that of 2.8 g/cm3. The amounts of lower density fractions (C2.8 g/cm3) increased with decreasing particle size as shown in Figure 1. We have already reported that the contents of Si, Al, and other major components of this fly ash sample were almost independent of their particle size (22). The distribution of density ranges of the same composition Environ. Sci. Technol., Vol. 21, No. 9, 1987
899
Table IV. Particle Size-Density Distribution of Coal Fly Ash (wt %) (12)
density,
particle size, pm
g/cm3
>149
149-74
74-44
44-37
37-25
25-20
2C-10
10-5
11.6 1.6-2.0 2.0-2.4 2.4-2.8 2.8-3.2 >3.2
0.32 0.61 0.47 0.07 0.01 0.01
1.77 5.57 1.94 0.41 0.14 0.02
0.75 8.51 4.06 1.31 0.31 0.06
0.92 2.49 1.59 1.31 0.36 0.03
0.77 1.76 2.61 2.57 0.84 0.05
0.54 1.76 2.65 2.83 0.86 0.06
0.36 0.97 1.90 2.15 0.62 0.03
1.49
9.85
15.00
6.70
8.60
8.70
6.03
total
‘
i
c
i
,
I
LmmmiBA,A1
nd nd
I”,
i /CJ
m e 3. SEM microgaphs of Wm highest density fiyash (25-20 pm, >3.2 gtcm’) indicating (A) overview of Wm fraction and (E and C) variety of observed crystal-precipitated particles. Magnification: (A) 135X: (E) 1125X; (C) 1125X.
Muu 2. SEM mlcroaaohs of Vw lowest densitv Rv ash 125-20 um.
4 . 6 g/cm3) indicati& (A) OVeNieW of the &ion’, (E) typical & s i smooth prUcle. and (C)potabke particle. MagniRcatbn: (A) 135X:
(E) 9OOX; (C) 1350X.
might be caused from the difference of their inner morphology. Many plerosphere particles were observed in smaller size fractions, while cenosphere particles were observed in larger fractions (12). The density fraction of 2.4-2.8 d c m 3 comprises apDroximately 90% of the size .. fractions’less t h a n 6 pm. Morvholotv. From SEM-EDX observation. various characteristic-morphologies were observed in these fractions: Char particles were observed in larger sizelower density fractions (149-74 pm, 1.6-2.4 g/cm3). Many sharp-edged particles that did not melt during combustion at a flame temperature less than 1500 “C were observed in the fractions of 44-37 pm and 4 . 4 g/cm3. Fused and coagulated particles with smaller spheres and stick-shaped particles were observed in smaller size regions (3.2g/cm’): (E) X-ray image 01 S K a : (C) X-ray image of Ca Kn: (D) X-ray image of Fe Kn.
result, the supersonical method appears to be effective for the separation of particles without a change in morphology. Major Components. Coal fly ash particles mainly consist of Si, Al, Fe, Ca, and Mg in percent order concentrations. The concentrations of these five elements in 54 fractions were dependent on density but not so much on particle size. The density distribution of their concentration in 25-20-pm particle size fractions is shown as an example in Figure 5. The characteristic density distribution of these elements suggests that particles in lower density fractions mainly consist of Si and Al, while particles in higher density fractions mainly consist of Fe, Ca, and AI. This is in good agreement with XRD results in Figure 6. Mullite- and u quartz like silicates were detected in lower density fractions, and magnetite was detected in higher density
32
1 .6
2.4
2.0
3.2
2.8
Density (glcrn”
Figure 5. Density distributions of major components in 25-20-pm fraction.
1.6-2.0 g/cm3
2.0-2.4 g/cm3 Y,,IYIUM?
2o08T i‘
2.4-2.8 g/cm3
’
f
1000
r
S
s
s
s
s
2.8-3.2 g/cm3
1008 Ma
- %%
Ma
30
Ma
60
50
40
Ma
70
Mu:Mullite, S:Siiicate ( a - O u a r t r ) , L : L i m e , Ma:Magnetite
XRD spectra of coal fly ash.
Flgure 8.
a
0
0: -1.6 0:1.6-2.0 A:2.0-2.4
m
v:2.4-2.a
0 0.
0 :2.8-3.2 .:3.2-
i
rc
%
-
4
-
z
, I
where [Mnfraction] is the Mn concentration of a fraction and [Mn,] is the total average concentration with equal weight for each fraction, for the purpose of the correlation of enrichment in each particle size-density fraction being well expressed. Mn distribution is independent of particle size for all density fractions. This is in good agreement with the data of Coles et al. (9). At the same time, Mn concentration increases with increasing density as well as in the cases of Fe in Figure 7B and Ca and Mg in Figure 5. Gluskoter (16) reported that Mg, Fe, and Mn were often associated with sedimentary carbonate minerals and thermally decomposed during combustion and, subsequently, Mg and Mn may be associated with iron oxides in coal fly ash. This behavior also coincides with the reports of Lauf (17) and Quan (18). Zn and Cu. Figure 8 shows the particle size-density distribution of Zn and Cu. In spite of a lack of sufficient data caused by a scarce quantity of separated fractions,
Fe
A
.
0:
I - 1 .6
0:1 .6-2.0
A:2.0-2.4 v:2.4-2.8 0:2.8-3.2 .:3.2(g/crn3
.
3
(g/cm
C
6
.
- Mn
-
28
fractions as well as in the previous studies (5, 11, 15). The concentration of Ca increases with increasing density, but that of Ca slightly decreases in the fractions of >3.2 g/cm3 rather than that of particles in the fraction of 2.8-3.2 g/cm3 as shown in Figure 5. We have already mentioned (12) that CaS04 precipitated on the surface of a particle in the fractions of >3.2 g/cm3 rather than on the surface of particles in the fractions of C3.2 g/cm3 from the SEM-EDX observation as shown in Figure 4. It can be expected that the precipitates of CaS04 are a product of SO, and calcium oxide on the surface of a particle that has been produced by thermal decomposition of carbonates in coal. The state of Ca in the fractions of C3.2 g/cm3 might be different from that in the fraction of >3.2 g/cm3. In the fractions of C3.2 g/cm3, Ca exists in the bulk of the particle as a solid solution to aluminosilicate matrix or other matrices. Minor Elements. The concentrations of these elements in the unseparated coal fly ash were in the range of 5-800 PPm. Mn. Figure 7A shows the particle size-density distribution of Mn. The broken line in Figure 7A shows
)
3 -
U
U
m
--
0
u-i
c
0
0
0
0
2 -
0
0
O
5
10
20
25
37
44
74
149
P a r t i c l e s i z e ( wrn )
5
10
20
0
25
0
37
44
14
149
Particle size ( urn )
Figure 7. Particle size-density distributions of (A) manganese and (8) iron in coal fly ash against enrichment ratio. Environ. Sci. Technol., Vol. 21, No. 9, 1987
901
6tA
0.
5
-
4
-
> ri
u m
-
0
4J
I
UU
0: -1.6 *:I . 6 - 2 . 0 A :2.0-2.4 T :2.4-2.8 0 :2.8-3.2 W:3.23 (glcrn )
m
t
I
Zn
0: -1.6 e : 1 .6-2.0 A :2.0-2,4 T :2.4-2.8 0 :2.8-3.2 m:3.2( g/cm3 1
N C
8
T
I
T
\
6
3
.ri
A
Y
0
m
-
w
2
8
-
N
O
0
I
T
:.U
1
O
T
_.---._. Q...*
G
m
a:
0
a *
m *
A
I
s
b
O
-3...
~
e
1
-
~~
5
10
20
25
37
44
74
149
Flgure 8. Particle size-density distributions of (A) zinc and (B) copper in coal fly ash against enrichment ratio.
6tA
0:
3
1
Ti
Cr
-1.6
.:1.6-2.0 A :2 . 0 - 2 . 4 V:2.4-2.8 0:2.8-3.2 m:3.2ig/crn3)
..
C
B
V
*:1 . 6 - 2 . 0 A: 2 . 0 - 2 . 4 .:2.4-2.8 0:2.8-3.2 m:3.2(gIcrn3)
-1.6 .6-2.0 A: 2 .O-2.4 v:2.4-2.8 0:2.8-3.2 m:3.2( g / c m 3I 0:
.:I
T
o 0
-1.6
0:
0
m
I v
2
1
tA
. ..... . T .... .
.. .v
n
0
0
0
v Y ... .T.. ... .........
._.___... .. ...
,...
.T.....
4
.
*.. .&
1....L.... x .... x... ..
8
9
q
.
.
.....*.
~~
5
10
20
25
37
74
44
Particle size
(
149
urn
5
10
20
25
37
74
44
Particle size
(
dm
149 )
Figure 9. Particle size-density distributions of (A) vanadium, (B) chromium, and (C) titanium in coal fly ash against enrichment ratio.
the concentrations of these elements increase with decreasing particle size (i.e., with increasing surface-to-volume ratio). This suggests that the distribution behavior of these elements is dependent on the vaporization-condensation mechanism. There can be observed, at the same time, a slight difference of enrichment to the different density fractions. Figure 8 shows that these elements condensed more in higher density fractions in every density region. Hulett et al. (15) reported that the first row transition elements are more concentrated in the highest density fractions mainly consisting of magnetite. Therefore, these results suggest that Zn and Cu condensed on the surface but more preferentially on the higher density fractions. Ti, Cr, and V. Ti has been classified into group I and V and Cr have been classified into Group I11 by Coles et a1 (9). In this study, however, these elements showed similar behaviors to each other. The particle size-density distributions of Ti, Cr, and V are illustrated in Figure 9. The concentration of Ti, Cr, and V increased with de902
Environ. Sci. Technol., Vol. 21, No. 9, 1987
creasing particle size in higher density fractions (2.8-3.2 and >3.2 g/cm3) but is independent of particle size in the lower density fractions (
