Environ. Sci. Technol. 1884, 18, 544-547
Comparison of Micron and Submicron Fly Ash Particles Using Scanning Electron Microscopy and X-ray Elemental Analysis N. Kaufherrt and David Llchtman' Department of Physics and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201
Fly ash particles from the Bull Run Power Station of TVA were studied in the submicron range (average size 0.6 pm) and in the micron range (2-7 pm) by means of a scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer for X-ray elemental analysis (EDX). It was found, for the fly ash studied, that the submicron particles were very similar to the particles in the micron range. Both types of particles were spherical and contained Si, AI, K, Fe, Ti, and S as the main components. The submicron particles were distinguished only in some enrichment in sulfur content.
Introduction The particulate residue from coal-burning power stations, its nature, and its impact on environment and health have been of particular interest in recent years due to the increased significance of coal as a power source and short-term solution for the energy crisis. Presently, the general and broader earlier studies are giving place to a more subtle approach. At this stage, the aim is to obtain a better understanding of the particles in view of burning conditions, type of coal, and other characteristics of the power station on one hand and the relations between inherent properties of the particles and their chemistry on the other. Methods of particle collection and the means by which they are studied are becoming more and more sophisticated. In this study we concentrate on the question as to the similarities or differences between micron and submicron particles in view of the belief that submicron particles are major contributors to the hazards of fly ash (1). The following results are a summary of the SEM-EDX studies on micron and submicron particles. It should be remembered that EDX of particles in this size range provides essentially semiquantitative bulk composition analysis. An attempt was made to understand the uniqueness of submicron particles compared to larger particles, as well as their common features. Data were critically evaluated in view of the capabilities of EDX analysis. Experimental Section The fly ash particles studied were collected at the Bull Run Steam Plant of TVA, Oak Ridge, TN. The coal used in the plant contained 7.1% moisture, 15.0% ash, and 0.9% sulfur. Particles were collected on stainless steel substrates in the stack at 138 OC by means of impact collectors, according to their aerodynamic size. Particles studied were from stages 2, 5, and 8 with a calculated aerodynamic diameter of 4.8, 1.4, and 0.2 pm, respectively (assumed density 2.5; see also section 1under Results). Particles were assigned as FA-C-8, FA-C-5, or FA-C-2 if originating from stage 8,5, or 2, respectively, and FA-C-2,5 if originating from both stages 2 and 5. The 'Permanent address: I.M.I. Institute for Research and Development, Haifa 31002, Israel. 544
Environ. Sci. Technol., Vol. 18, No. 7, 1984
diameter of each particle analyzed was measured prior to analysis. The total number of particles that was studied in detail was 66. In an attempt to randomize data, a number of samples were analyzed from each stage. The particles were studied by means of SEM-EDX, carried out in a JEOL Model JSM U3 scanning electron microscope (SEM), and energy dispersive X-ray analysis data were obtained from single particles by means of a Kevex 7000 unit. For analysis, particles were transferred from the stainless steel support by means of a graphite tip to a highly polished carbon stub. This method of transfer was found to be especially convenient for submicron particles. The soft graphite also assured no transfer of support material. Particles were slightly carbon coated prior to analysis. X-ray analysis was performed in spot mode with a current of 150-300 pA at 25 kV. Data were accumulated for 200-300 s. X-ray analysis data were processed by a standardless semiquantitative routine supplied by Kevex, and results were normalized to 100% on the basis of oxide composition. Carbon content was analyzed in a JEOL 35C SEM with a 35FCS four crystal wavelength dispersive spectrometer. Analysis was done at a 15-kV accelerating voltage and a current of 0.14 PA. Sodium carbonate was used for a standard, and zero corrections were made for stainless steel substrate carbon content.
Results (1) Particle Size and Morphology. In addition to the size measurement of the particles analyzed and the calculated aerodynamic size, a particle size distribution was carried out for stages 2 and 4 to verify the degree of accuracy of the calculated diameter and efficiency of collections. It was found that the major part of the particles were 5 and 2 pm, respectively, in agreement with calculations, although some larger and smaller particles were observed as well. The submicron particles which were examined were 0.6 pm, on the average (Table I). Examination of the morphology of particles of stages 2,4, and 8 showed the major part of be spherical (see Figures 1and 2) with only a small fraction of the particles of different shape. For the submicron particles in stage 8, it was found that, from time to time, two particles were connected by what was believed to be a partial fusion. Irregular shapes were observed for particles in the micron range. Some of them were reminiscent of a crystal; others were perforated. The particles of irregular shape were analyzed, and their composition was given separately as certain shapes seemed to be connected with a high SiOz content (Table 11). (2) Chemical Analysis. (a) Main Group of Particles. Si, Al, K, Fe, Ti, Mg, and S were the most common elements analyzed both in the micron and submicron particles (Tables I and 111) (in the latter are also given, for comparison, the composition of fly ash from the literature as cited in ref 2, 3, and 4). Ca, P, Na, C1, and Ni were detected in a small number of particles while Cr, Mn, V, and Cu could be identified from time to time, but the
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0 1984 American Chemical Society
Table I. Average Composition of Main Group of Particles from EDX Analysis on Individual Particles (Weight Percent Oxide) type of particle'
particle size, pm
SiOz
AIZO3
Si/AIb
KzO
FeO
TiOz
MgO
so8
0.6 0.2 19
53.0 2.8 19
31.3 2.4 19
1.4 0.1 19
4.3 1.5 19
4.3 0.9 19
1.3 0.7 10
0.9 0.9 5
7.2 3.0 15
2
54.7 2.7 18
34.3 2.5 18
1.4 0.1 18
4.2 0.6 18
2.9 0.6 17
1.2 0.6 15
1.5 0.3 13
1.6 0.9 10
53.8 2.2 10
35.0 1.9 10
1.3 0.1 10
4.7 0.5 10
2.3 0.6
0.9 0.5 10
1.7
0.4 8
1.2 0.8 8
53.1 0.4 6
34.5 1.2 6
1.3 0.1 6
4.9 0.7 6
1.7 0.7 6
1.9 0.3 6
0.9 0.5 4
FAG8 i 0
N FA-C-2.5 i
0 18
0
N FA-C-2 ~~~. ~
5 0
i Ll
N
10
10
FAG2 c
6.6 0.7
N
6
i
'i, average value; 0 , standard
2.6 0.4 6
deviation; N , number of particles. bAtomicratio.
Table 11. Composition of Nonspherical Particles from Stage 2 (FA-C-2) (Weight Percent Oxide) SiOz
AIZO3
Si/Al
KzO
FeO
51.5 53.8 41.1 60.1 88.4 80.9 81.7
33.4 34.5 32.9 34.2 7.2 10.8 1.2
1.3 1.3 1.2 1.5 10.4 6.4 10.3
5.5 4.9 6.0 3.6 1.3 5.1 1.8
2.8 3.3 4.7 0.9
TiOZ 2.7
MgO
SO3
1.5
2.5 1.4 5.6 0.9 1.1 0.9 2.1
0.4
2.7 0.4 0.6 0.5 0.5
0.6 1.8 0.7
0.7
remarks two fused particles center of one particle center of other particle junction of the two particles perforated particle crystalline-likeparticle crystalline-likeparticle crystalline-likeparticle
Table 111. Comparison of the Concentration of Main Constituents of Different F l y Ashes in Weight Percent of Elements type of fly ash and size
Si
AI
K
Fe
Ti
Mg
s
0
FA-C-8,
